Biology 102

Biology 102
Study Notes Exam 2
Chapter 24: Evolution & Diversity of Plants
Evolutionary History of Plants
Characteristics of Plants (kingdom Plantae)
plants are multicellular photosynthetic eukaryotes adapted to life on land
plants are believed to have evolved from a freshwater green algal ancestor (possibly
stoneworts) over 500 million years ago (Paleozoic era)
o
both utilize chlorophylls a and b and various accessory pigments, store food as starch
& have cell walls containing cellulose
plants, from nonvascular to vascular, nourish a multicellular embryo within the body of
the female plant; this distinguishes them from green algae
vascular plants have vascular tissues, specialized elongated cells that conduct water and
solutes through the plant
the cone-bearing gymnosperms and flowering angiosperms both produce seeds
o
seeds are mature ovules and stored food within protective seed coat
o
seeds are resistant to drought and somewhat resistant to predators
Alternation of Generations
plants have a two-generation life cycle called alternation of generations
the sporophyte generation is a diploid (2n) generation producing haploid spores by
meiotic cell division
the
spores
produce the
gametophyte
generation,
a
haploid
generation producing haploid gametes by mitotic division
mitosis occurs as a spore becomes a gametophyte, and also as a zygote becomes a
sporophyte
plants differ in which generation–gametophyte or sporophyte–is dominant
o
in nonvascular plants, the gametophyte is dominant
o
in the vascular plants, the sporophyte is dominant or more conspicuous
o
the shift to sporophyte dominance is an adaptation to life on land; the gametophyte
becomes microscopic and dependent on the sporophyte
Other Adaptations to Terrestrial Environments
sporophyte dominance & adaptation for water transport and conservation
vascular tissues transports water and nutrients in the body of the plant
leaves and stems covered by a waxy cuticle that retains water & limits gas exchange
leaves & other tissues have openings (stomata) that regulate gas and water exchange
Nonvascular Plants (≥Bryophytes≤)
nonvascular plants lack true roots, stems, and leaves, although they have rootlike,
stemlike, or leaflike structures
the gametophyte is the dominant generation recognized in bryophytes
o
the gametophyte produces eggs in archegonia, flagellated sperm in antheridia
nonvascular plants are quite small because of lack of vascular tissue and the need for
sperm to swim to the archegonia in water
o
because sexual reproduction involves flagellated sperm, they are usually found
in moist habitats
o
mosses compete well in harsh environments because the gametophyte can reproduce
asexually, allowing them to spread into stressful habitats

-

Hornworts (phylum Anthocerophyta)
the small sporophytes look like tiny green broom handles and are attached to a filmy
gametophyte that is less than two cm in diameter
Liverworts (phylum Hepatophyta)
o
the thallus or body appears similar to lobes of the liver
o
Marchantia has a flat, lobed thallus about a centimeter in length
o
the upper surface of thallus is smooth; lower surface bears numerous rhizoids (rootlike extensions) projecting into soil (absorb water & minerals)
o
asexual reproduction involves gemmae in gemmae cups on upper surface of the
thallus; gemmae can start a new plant
o
sexual reproduction: antheridia are on disk-headed stalks and produce flagellated
sperm; archegonia are on umbrella-headed stalks and produce eggs
ß
the zygote develops into a tiny sporophyte with a foot, short stalk, & capsule
ß
spores produced within the gametophyte capsule are dispersed by wind
Mosses (phylum Bryophyta)
o
mosses are found from the Arctic through the tropics to parts of the Antarctic
o
moss prefers damp, shaded localities but some survive in deserts, bogs & streams
o
some ≥mosses≤ are not true mosses:
ß
Irish moss is a red alga; Reindeer moss is a lichen; Club mosses are vascular
plants; Spanish moss is a flowering plant related to pineapple
o
most mosses can reproduce asexually by fragmentation
o
the moss life cycle begins with algalike protonema developing from the germination of
a haploid spore
ß
upright shoots develop covered with leafy structures & anchored by rhizoids
ß
the shoots bear antheridia and archegonia at their tips
ß
the antheridia produce flagellated sperm which need external water to reach eggs
in archegonia
ß
the archegonium looks like a vase with a long neck; it has an outer layer of sterile
cells with a single egg at the base
ß
fertilization results in a diploid zygote that undergoes mitotic division to develop a
sporophyte
o
the sporophyte consists of a foot (which grows down into the gametophyte tissue
starting at the former archegonium), a stalk, and an upper capsule (sporangium) where
spores are produced
Uses of Bryophytes
o
sphagnum (bog or peat moss) has tremendous ability to absorb water and is
important in gardening
o
sphagnum does not decay in some acidic bogs; the accumulated dried peat can be
used as fuel
o

-

-

-

Vascular Plants
vascular tissue
o
xylem is vascular tissue that conducts water & minerals upward from the roots
o
phloem is vascular tissue that transports sucrose & hormones throughout the plant
o
lignin strengthens the walls of conducting cells in xylem
o
the cuticle and stomata are also characteristics of a dominant sporophyte
o
seedless plants are mostly homosporous, using spores for dispersal
o
all seed plants are heterosporous, using pollen grain and seeds
Seedless Vascular Plants
Club Mosses (division Lycopodophyta)

o
o
o

-

-

-

common in temperate woodlands where they are called ≥ground pine.≤
a branching rhizome sends up aerial stems less than 30 cm tall
tightly packed, scalelike microphylls cover stems and branches; each contains one
strand of vascular tissue
o
sporangia are borne on the surface of leaves called sporophylls which are grouped in
club-shaped stroboli; spores germinate into independent gametophytes
o
most club mosses live in tropics or subtropics as epiphytes, plants that live on trees
without harming them
o
closely related are spike mosses (Selaginella) and quillworts (Isoetes)
Ferns and Allies
o
Phylum Sphenophyta today contains one genus, Equisetum (horsetails)
o
a rhizome produces aerial stems that stand about 1.3 meters tall
o
whorls of slender side branches & small scalelike leaves encircle nodes of a stem,
resembling a horseπs tail
o
the tough, rigid stems have silica in the cell walls; early Americans used them as
≥scouring brushes.≤
Whisk Ferns (phylum Psilotophyta)
o
whisk ferns occur in the southern United States and in the tropics
o
whisk ferns have no leaves or roots; a branched rhizome with rhizoids and a
mycorrhizal fungus helps gather nutrients
o
other genera including Tmesipteris have true leaves that are microphylls
Ferns (phylum Pterophyta)
o
ferns are widespread, and especially abundant in warm, moist tropical regions
o
ferns range in size from low-growing mosslike forms to tall trees
o
fronds are leaves that are variable in size and shape
o
ferns are the only group of seedless plants to have well-developed megaphylls;
megaphylls may have evolved by fusion or branching of stems
o
life cycle of a fern: spores produced by meiotic cell division within sporangia, located
in sori on underside of leaflets
ß
spores are released and disperse largely by wind
ß
a spore germinates into a prothallus which grows to develop antheridia and
archegonia underneath
ß
fertilization occurs if water is present; flagellated sperm swim from antheridia to
archegonium; the resulting zygote begins its development inside archegonium but
embryo soon outgrows the space
ß
a sporophyte becomes visible as the first leaf grows above and as roots develop
below the prothallus
ß
the young sporophyte develops a root-bearing rhizome from which fronds project.

Seed Plants
seeds are mature ovules containing embryonic sporophyte and stored food enclosed in a
protective seed coat
seeds disperse the sporophytes
seeds are resistant to adverse conditions (dryness and temperature extremes)
food reserve supports the emerging seedling until it can exist on its own
there are separate male female gametophytes
pollen grains are drought resistant & become multicellular male gametophytes
pollination is the transfer of pollen to the vicinity of the female gametophyte
o
the whole male gametophyte, not just the sperm, moves to the female gametophyte
o
sperm is delivered to an egg through a pollen tube; no external water is required for
fertilization

-

the female gametophyte develops within an ovule which, after fertilization, becomes an
embryonic plant or ≥seed.≤
in gymnosperms, the ovules are not completely enclosed by sporophyte tissue at
pollination
in angiosperms, the ovules are completely enclosed within diploid sporophyte tissues
which becomes a fruit
Gymnosperms
o
the Gymnosperms include the conifers, cycads, ginkgo, and gnetophytes
o
all have ovules exposed on the surface of sporophylls or similar structures
o
Conifers: phylum Coniferophyta (~ 575 species)
ß
conifers are cone-bearing trees and shrubs: pines, hemlocks, and spruces
ß
conifers usually have evergreen needlelike leaves well adapted to withstand
extremes in climate
ß
pine needles have a thick cuticle and recessed stomata
ß
pine uses: construction wood; resin is insect & fungal deterrent (turpentine)
ß
the pine life cycle:
ß
the sporophyte is dominant and its sporangia are borne in cones
ß
two types of cones are pollen cones (small and near the tips of lower branches)
and seed cones
ß
each scalelike sporophyll of a pollen cone has two or more microsporangia on the
underside
ß
within the sporangia, each microsporocyte undergoes meiosis and produces
four microspores
ß
each microspore develops into a male gametophyte which is the pollen grain
ß
a megasporangium is within an ovule; a megasporocyte undergoes meiosis
producing four megaspores
ß
only one spore develops into a female gametophyte with 2–6 archegonia, each
containing a single large egg
ß
once a pollen grain is enclosed within the seed cone, it develops a pollen
tube that digests its way toward a female gametophyte and discharges
twononflagellated sperm
ß
fertilization takes place one year after pollination
ß
the ovule matures and becomes the seed, composed of embryo, reserve food and
seed coat
ß
the woody seed cone opens to release winged seeds in the fall of 2nd season
o
Cycads: phylum Cycadophyta (~100 species)
ß
the trunk is stout and unbranched; the large leaves are compound giving a
palmlike appearance
ß
cycads have pollen and seed cones on separate plants, pollinated by insects
ß
the pollen tube bursts in the vicinity of the archegonium and multiflagellated sperm
swim to reach an egg
ß
today, cycads are endangered because of their very slow growth
o
Ginkgoes (maidenhair trees): phylum Ginkgophyta (1 species)
ß
it is called the maidenhair trees because its forked-veined, fan-shaped leaves
resemble the maidenhair fern
ß
ginkgo ovules are at the end of short, paired stalks; female trees produce seeds
with a fleshy covering and foul odor
ß
similar to cycads, the pollen tube of Gingko bursts to release multiflagellated
sperm that swim to the egg produced by the female gametophyte in an ovule
o
Gnetophytes: phylum Gnetophyta (~70 species)

ß
ß
ß
ß
-

gnetum consists of trees and climbing vines with broad leaves; they live mainly in
the tropics (Ephedra & Welwitschia in deserts of US & Africa)
the xylem and stroboli are uniform across genera, and all lack archegonia
angiosperms also lack archegonia, suggesting that gnetophytes are the
gymnosperms most closely related to angiosperms
some gnetophytes produce nectar in their reproductive structures, recruiting
insects in pollination

Angiosperms (flowering plants): phylum Anthophyta (240,000 known species)
o
unlike gymnosperms, angiosperms enclose their ovules within diploid tissues
o
flowering plants became the dominant plants in the late Cretaceous and early Tertiary
periods, and probably arose ~ 200 million years ago
o
Monocots and Eudicots
ß
most flowering plants belong to one of two classes: Monocotyledones (65,000
species) or the Eudicotyledones (175,000 species)
ß
the term eudicots is preferred to the earlier dicots; some former dicots are now
know to have split off before the rise of these two major classes
ß
monocot produce one cotyledon (seed leaf) at germination and have flower parts
mostly in threes or multiples of threes
ß
dicots produce two cotyledons (seed leaves) at germination and have flower
parts mostly in fours or fives, or multiples of these numbers
o
The Flower
ß
flowers have several kinds of highly modified leaves arranged in rings and
attached to a receptacle
ß
receptacle is a modified stem tip to which flower parts are attached
ß
sepals are outer ring of modified leaves of flowers; usually green, they enclose
flower before it opens
ß
petals (collectively a corolla) are a ring of modified leaves inside of sepals; large
and colorful, they help attract pollinators
ß
stamens form a whorl inside the petals and around a pistil; each slender filament
has an anther at its tip
ß
the anther produces pollen
ß
the pistil contains one or more fused carpels; it consists of a stigma, style, and
ovary
∑
carpels are modified sporophylls that contain ovules in which megasporangia
are located
∑
a stigma is a landing platform for pollen and the site where the pollen tube
enters the style
∑
the style is a slender column that holds up the stigma to receive pollen
∑
pollen grains develop a pollen tube that takes sperm to the female
gametophyte in the ovule
∑
glands located in the region of the ovary produce nectar, a nutrient gathered by
pollinators as they go flower to flower
o
the angiosperm life cycle:
ß
a megaspore located in an ovule within an ovary of a carpal develops into an eggbearing female gametophyte called the embryo sac
ß
usually, the embryo sac has seven cells; one is an egg and one contains two
polar nuclei
ß
microspores produced
in anthers become pollen
grains which
mature
into sperm-bearing male gametophytes

ß

o

the mature male gametophyte consists of three cells; the tube cell and two
sperm cells
ß
pollination brings the male gametophyte to the stigma where it germinates
ß
during germination, the tube cell produces a pollen tube that carries the two
sperm to the micropyle opening of an ovule
ß
in double fertilization, one sperm fertilizes egg and one sperm unites with
polar nuclei to form the triploid endosperm
ß
the ovule becomes the seed and contains the embryo (the sporophyte of the next
generation) and stored food enclosed within a seed coat
ß
A fruit is derived from an ovary and possibly accessory parts of the flower; some
fruits are fleshy and some are dry
Flowers and Diversification
ß
inconspicuous flowers disperse pollen by wind; colorful flowers attract specific
pollinators (e.g., bees, wasps, flies, butterflies, moths, and even bats) which carry
only a particular pollen
ß
flowers promote efficient cross pollination; they also aid in dispersal through
production of fruits
ß
there are fruits that utilize wind, gravity, water, and animals for dispersal

Chapter 25: Structure And Organization Of Plants
Plant Organs
Diverse Flowering Plant Structure
o
structures of flowering plants are well-adapted to varied environments including water
o
flowering plants usually have three vegetative organs: root, stem and leaf
o
the flower itself contains a number of organs
Roots (root systems)
o
the root system is the main root plus its lateral (side) branches; it is generally equal in
size to the shoot system, the part above ground
o
roots anchor a plant in soil and give support
o
roots absorb water and minerals from soil; root hairs are central to this process
ß
root hair cells are in a zone near root tip
ß
root hairs are numerous to increase absorptive surface of a root
ß
transplanting plants damages a plant when the root hairs are torn off
ß
roots produce hormones that are distributed along with water and nutrients to the
rest of the plant
o
perennials ≥die back≤ to regrow the next season; roots of herbaceous perennials store
food (e.g., carrots, sweet potatoes)
Stems
o
the shoot system of a plant consists of the stem, the branches, & leaves
o
the stem forms the main axis of the plant, along with lateral branches
o
upright stems produce leaves and array them to be exposed to as much sun as
possible
o
a node occurs where a leaf attaches to the stem and an internode is the region
between nodes; nodes and internodes identify a stem even if it is underground
o
the stem has vascular tissue to transport water and minerals from roots and sugar from
leaves
o
nonliving cells form a continuous pipeline through vascular tissue
o
a cylindrical stem expands in girth and length; trees use woody tissue to strengthen
stems
o
stems function in storage: cactus stems store water and tubers are horizontal stems
that store nutrients
Leaves
o
a leaf is the major organ of photosynthesis in most plants
o
leaves receive water from roots by way of the stem
o
broad, thin leaves have a maximum surface area to absorb CO2 and collect solar
energy
o
a blade is the wide portion of a leaf with most photosynthetic tissue
o
petiole is a stalk that attaches a leaf blade to stem
o
the leaf axil is the upper acute angle between petiole and stem where an axillary
(lateral) bud originates
o
some leaves protect buds, attach to objects (tendrils), store food (bulbs), or capture
insects
Monocot Versus Eudicot Plants
cotyledons are embryonic seed leaves providing nutrition from the endosperm before the
mature leaves begin photosynthesis
Monocots: **
o
1 cotyledon in seed
o
root xylem & phloem in a ring

o
o
o
o
o
-

vascular bundles scattered in stem
parallel leaf veins
flower parts in multiples of 3
usually one aperture in pollen grain
includes grasses, lilies, orchids, rice, wheat, corn
Eudicots: **
o
2 cotyledons in seed
o
root xylem & phloem in a cross or star-shaped pattern
o
vascular bundles arranged in a ring in stem
o
net pattern to leaf veins
o
flower parts in multiples of 4 & 5
o
usually 3 aperture in pollen grain
o
includes dandelions to oak trees

Plant Tissues
Meristem Produces Tissue: plants continually grow due to meristem (embryonic tissue)
in stem and root tips (apexes)
3 types of primary meristem continually produce three types of specialized tissue
o
protoderm is outermost primary meristem giving rise to epidermis
o
ground meristem is inner meristem producing ground tissue
o
procambium produces vascular tissue
3 specialized tissues are produced
o
epidermis forms outer protective covering
o
ground tissue fills the interior
o
vascular tissue transports water and nutrients and provides support
Epidermal Tissue
o
epidermis is an outer protective covering tissue of plant roots, leaves, and stems of
nonwoody plants containing closely packed epidermal cells
o
waxy cuticle covers the walls of epidermal cells, minimizing water loss and protecting
against bacteria
o
in roots, certain epidermal cells are modified into root hairs that increase surface area
of the root for absorption of water and minerals and help to anchor plants in the soil
o
epidermal cells are modified as glands to secrete protective substances
o
on the lower epidermis of eudicot leaves, and both surfaces of monocot leaves, special
guard cells form microscopic pores called stomata that regulate gas exchange and
water loss
o
in older woody plants, the epidermis of the stem is replaced by cork tissue
ß
cork is outer covering of the bark of trees; composed of dead cork cells that may
be sloughed off
ß
cork cambium is lateral meristem that produces new cork cells
ß
mature cork cells produce the lipid suberin making them waterproof & inert
ß
cork protects a plant and is resistant to attack by fungi, bacteria, and animals
Ground Tissue
o
ground tissue fills the inside of plants with parenchyma, collenchyma and
sclerenchyma cells
o
parenchyma are the least specialized of all plant cell types
ß
contain plastids (e.g., chloroplasts or colorless storage plastids)
ß
found in all organs of a plant & divide to form more specialized cells (e.g., roots
develop from stem cuttings in water)
o
collenchyma resemble parenchyma but has thicker primary cell walls & are uneven in
the corners

ß
ß

-

usually occur as bundles of cells just beneath epidermis
give flexible support to immature regions of plants (e.g., a celery stalk is mostly
collenchyma)
o
sclerenchyma cells are mostly nonliving & have thick secondary cell walls
ß
they are impregnated with lignin that makes the walls tough and hard
ß
provide strong support to mature regions of plants
ß
form fibers (used in linen and rope) and shorter sclereids (found in seed coats, nut
shells, and gritty pears)
Vascular Tissue
o
xylem passively conducts water and mineral solutes upward through a plant from
roots to leaves
ß
xylem contains tracheids and vessel elements
ß
both are hollow, nonliving cells
ß
vessel elements form a continuous pipeline for water and mineral transport
ß
vascular rays conduct water and minerals across the width of the plant
o
phloem is vascular tissue that conducts the organic solutes in plants, from the leaves
to the roots; it contains sieve-tube cells and companion cells
ß
sieve-tube cells contain cytoplasm but no nucleus
∑
they are arranged end to end & have channels in their end walls (≥sievetubes≤), through which plasmodesmata extend from one cell to another
ß
companion cells are connected to sieve-tube cells by many plasmodesmata
∑
they are smaller and more generalized than sieve-tube cells & have
a nucleus which may control & maintain the function of both cells
∑
they are also thought to be involved in the transport function of phloem
o
vascular tissue extends from root to leaves as vascular cylinder (roots), vascular
bundles (stem) and leaf veins

Organization of Roots
Eudicot Root Tip: the eudicot root tip, a site of primary growth, is organized into zones of
cells in various stages of differentiation
o
cells are continuously added to a root cap below and zone of elongation above by
contributions from the zone of cell division
o
the root cap is a protective cover; its cells are replaced constantly because they are
soon ground off
o
the zone of elongation is above the zone of cell division where cells become longer
and more specialized
o
the zone of cell division contains meristematic tissue and adds cells to root tip and
zone of elongation
o
the zone of maturation is above the zone of elongation; cells are mature and
differentiated with root hairs
Tissues of a Eudicot Root
o
epidermis is a single layer of thin-walled, rectangular cells that forms the protective
outer layer of the root
ß
root hairs in the region of maturation project as far as 5–8 mm into the soil
o
cortex is a layer of large, thin-walled, irregularly shaped parenchyma cells
ß
these cells contain starch granules; the cortex functions in food storage
ß
the cells are loosely packed; water and minerals can diffuse through the cortex
o
endodermis is single layer of rectangular cells that forms the boundary between cortex
and inner vascular cylinder
ß
it regulates the entrance of minerals into the vascular cylinder

ß

-

-

the Casparian strip is an impermeable lignin and suberin layer that excludes water
and mineral ions
o
vascular cylinder is an arrangement of vascular tissues as a cylinder
ß
the pericycle is the first layer of cells within vascular cylinder
ß
its cells have retained the capacity to divide; it can start the development of branch
or secondary roots
ß
vascular tissue forms main portion of a vascular cylinder
∑
it is composed of xylem, whose cells are arranged in a star-shaped pattern;
and phloem, whose cells are located in regions between arms of xylem
Organization of Monocot Roots: monocot roots have the same zones as a eudicot root
but do not undergo secondary growth
o
the monocot root has a ring of vascular tissue where alternating bundles of xylem
and phloem surround pith
o
monocot roots also have pericycle, endodermis, cortex, and epidermis
Root Diversity
o
roots have adaptations to help anchor plants, absorb water and minerals, and store
carbohydrates
o
there are three general root types
ß
taproot is common in eudicots; root adapted to store food (e.g., carrots, beets)
ß
the fibrous root system of monocots is a mass of slender roots and lateral
branches that hold the plant secure in the soil
ß
adventitious roots develop from underground stems or from the base of aboveground stems
∑
prop roots (corn and mangrove plants) anchor the plant
∑
ivy has holdfast roots to anchor aerial shoots
o
haustoria are rootlike projections from stems on parasitic plants (e.g., dodders and
broomrapes)
o
mycorrhizae are an association between fungus and roots
ß
in this mutualism, fungus receives sugars and amino acids from plant & plant
receives water and minerals from the fungus
o
legumes (e.g., peas and beans) have root nodules containing nitrogen-fixing bacteria
ß
bacteria extract nitrogen from air and reduce it to a form that can be used by plant
tissues

Organization of Stems
Primary Growth
o
the stem tip is the site of primary growth where cell division extends length of stems or
roots
o
shoot apical meristem produces new leaves and primary meristems, increasing stem
length & is protected within a terminal bud of leaf primordia (immature leaves)
o
bud scales are scalelike coverings protecting terminal buds during winters when bud
growth stops
Herbaceous Stems
o
herbaceous stems are mature nonwoody stems that exhibit only primary growth
o
the outermost tissue of herbaceous stems is epidermis covered by a waxy cuticle to
prevent water loss.
o
xylem and phloem are in distinctive vascular bundles
ß
in each bundle, xylem is found to the inside of the stem; phloem is found to
the outside
ß
in the eudicot herbaceous stem, vascular bundles are arranged in a ring towards
outside of the stem and separating the cortex from the central pith

ß

-

-

in monocot stem, vascular bundles are scattered throughout the stem; there is no
well-defined cortex or pith
ß
cortex sometimes carries on photosynthesis; pith may function as storage site
Woody Stems
o
woody plants have both primary and secondary tissues
o
primary tissues are new and form each year from primary meristem right behind the
apical meristem
o
secondary tissues develop from second year onward from lateral meristem growth
o
primary growth increases length of a plant; secondary growth increases its girth
o
as secondary growth continues, it is not possible to distinguish individual vascular
bundles
o
the woody eudicot stem has a different organization with three distinct areas: bark,
wood, and pith
o
bark of a tree contains cork, cork cambium, and phloem
o
cork cambium is meristem beneath the epidermis that produces new cork cells when
needed
ß
cork cells become impregnated with suberin, causing them to die but making
them waterproof
ß
consequently, cork forms an impervious barrier, even to gas exchange, except at
lenticels
o
wood is secondary xylem which builds up each year; the vascular cambium is
dormant during the winter
ß
spring wood is composed of wide xylem vessel elements with thin walls, necessary
to conduct sufficient water and nutrients to supply abundant growth that occurs
during spring
ß
summer wood forms when moisture is scarce; composed of a lower proportion of
vessels, it contains thick-walled tracheids and numerous fibers
ß
an annual ring is one ring of spring wood followed by a ring of summer wood; this
equals one yearπs growth
ß
sapwood is outer annual rings where transport occurs
ß
heartwood is inner annual rings of older trees & help to support a tree
∑
vessels no longer function in transport; they become plugged
with resins and gums that inhibit growth of bacteria and fungi
o
Woody Plants
ß
the first flowering plants were probably woody shrubs; herbaceous plants evolved
later
ß
woody plants have an advantage when there is adequate rainfall; they can grow
taller and have adequate tissue to support and service leaves
Stem Diversity
o
stolons are stems that grow along the ground; new plants grow where the nodes
contact the soil (e.g.: strawberries)
o
the succulent stems of cacti are modified for water storage
o
tendrils of grapes and morning glories are stems adapted for wrapping around support
structures
o
rhizomes are underground horizontal stems
ß
rhizomes are long and thin in grasses and thick and fleshy in irises
ß
some rhizomes have tubers that function in food storage (e.g.: potatoes)
o
corms are bulbous underground stems that lie dormant during winter, like rhizomes
(e.g.: crocus, gladiolus)
o
humans use stems: sugarcane is primary source of table sugar, cinnamon and quinine
are from bark, wood is from paper, etc.

Organization of Leaves
Leaf Structure
o
leaves are organs of photosynthesis in plants; they have a flattened blade and
a petiole
o
the leaf veins contain vascular tissues that transport water and nutrients
o
leaf veins have a net pattern in eudicots and a parallel pattern in monocots
o
a petiole is a stalk that attaches a leaf blade to the plant stem
o
epidermis is the layer of cells that covers the top and bottom sides of a leaf
ß
epidermis often bears protective hairs or glands that irritating substances
ß
epidermis is covered by a waxy cuticle that keeps the leaf from drying out
ß
the epidermis, particularly lower epidermis, contains stomata that allow gases to
move into and out of the leaf
o
mesophyll is the inner body of a leaf and the site of most of photosynthesis
ß
palisade mesophyll is the layer of mesophyll containing elongated parenchyma
cells with many chloroplasts
ß
spongy mesophyll contains loosely packed parenchyma cells that increase the
surface area for gas exchange
Leaf Diversity
o
simple leaves have margins not deeply lobed or divided into smaller leaflets
o
compound leaves are divided into smaller leaflets, and each leaflet may have its own
stalk
o
leaves are variously modified
ß
shade plants have broad leaves while desert plants have reduced leaves with
sunken stomata
ß
cactus spines are modified leaves; succulents have fleshy leaves to hold moisture
ß
onion bulbs have leaves surrounding a short stem
ß
the tendrils of peas and cucumbers are leaves
ß
the Venusπs-flytrap has leaves to trap and digest insects

Chapter 26: Nutrition And Transport In Plants
Plant Nutrition and Soil
Early Views
o
ancient Greeks believed plants converted soil into plant tissues
o
the 17th Century Dutchman Jean-Baptiste Van Helmont conducted an experiment from
which he concluded the increase in tree weight came from water; he was unaware of
substances in air
Essential Inorganic Nutrients
o
essential inorganic nutrients (e.g., carbon, hydrogen, oxygen) comprise 96% of plant
dry weight
ß
carbon dioxide is the source of carbon for a plant
ß
water is the source of hydrogen
ß
oxygen can come from either atmospheric oxygen, carbon dioxide, or water
o
essential nutrients must fulfill the following criteria
ß
they have an identifiable nutritional role
ß
no other element can substitute and fulfill the same role
ß
a deficiency of the element causes the plant to die
o
beneficial nutrients: elements required or that improve growth of a particular plant
Determination of Essential Nutrients
o
when a plant is burned, most mineral elements (except nitrogen) remain in the ash
o
hydroponics is the preferred method for determining plant mineral requirements
ß
hydroponics is cultivation of plants in water
ß
nutrient requirements of plants are determined by omitting a mineral and observing
the effects
ß
if plant growth suffers, it can be concluded that the omitted mineral is a required
nutrient
Soil Formation
o
soil formation begins with weathering of rock by freezing, glacier flow, stream flow, and
chemicals
o
lichens and mosses grow on barren rock and trap particles and leave decaying tissues
o
decayed organic matter (humus) takes time to accumulate; its acidity leaches minerals
from rocks
o
depending on parent material and weathering, a centimeter of soil may develop within
15 years
The Nutritional Function of Soil
o
soil consists of soil particles, decaying organic matter, living organisms, air and water
o
the best soil includes particles of different sizes; this provides critical air spaces
o
soil particles include sand, silt & clay
ß
soil particles vary by size: sand particles are largest (0.05–2.0 mm in
diameter); silt particles
are
medium
sized
(0.002–0.05
mm
in
diameter); clayparticles are smallest: below 0.002 mm in diameter
ß
sandy soils lose water too readily; clay packs tight to hold water and clumps
ß
clay particles are negatively charged and attract positively charged ions (e.g.,
calcium [Ca2+] and potassium [K+])
ß
in acidic soils, hydrogen ions replace positively charged nutrients and the nutrient
ions float free and are leached; this is why acid rain kills trees
ß
clay cannot retain negatively charged NO 3-, and the nitrogen content of clay soil is
low
ß
loam (a mixture of the three soil particles) retains water and nutrients; roots take
up oxygen in the air spaces

o

o

o

humus: a mixture of 10-20% humus mixed with a top layer of soil particles is best for
plants
ß
humus keeps soil loose and crumbly, decreases runoff and aerates soil
ß
humus is acidic and retains positively charged minerals for plants to use later
ß
bacteria and fungi break down organic matter in humus and return inorganic
nutrients to plants
living organisms in soil
ß
small plants play a major role in formation of soil from rock and in succession
ß
roots of larger plants penetrate the soil and weather rocks
ß
larger moles and badgers and smaller earthworms help turn over the soil
ß
soil animals, from mites to millipedes help break down leaves and other plant
remains
ß
fungi, protozoa, algae and bacteria complete decomposition
ß
soil bacteria make nitrate available to plants
ß
some soil organisms (roundworms and insects) are crop pests that feed on roots
soil erosion is caused by water or wind carrying away soil
ß
erosion removes 25 billion tons of topsoil worldwide annually
ß
deforestation and desertification contribute to erosion

Uptake of Water and Minerals
Pathways
o
minerals follow the path of water uptake
ß
some mineral ions diffuse in between the cells
ß
because of the impermeable Casparian strip, water must eventually enter the
cytoplasm of endodermal cells
ß
water can move directly into the cytoplasm of root hair epidermal cells and is
transported across the cortex and endodermis of a root
ß
in contrast to water, minerals are actively taken up by plant cells
o
mineral ions cross plasma membranes by a chemiosmotic mechanism
ß
plants absorb minerals in ionic form: nitrate (NO3-), phosphate (HPO4-),
and potassium ions (K+) all have difficulty crossing a charged plasma membrane
ß
it has long been known plants expend energy to actively take up and concentrate
mineral ions
ß
a plasma membrane pump called a proton pump hydrolyzes ATP to transport
+
H ions out of cell; this sets up an electrochemical gradient that causes positive
ions to flow into cells
ß
negative ions are carried across the plasma membrane in conjunction with H + ions
as H+ ions diffuse down their concentration gradient
Adaptations of Roots for Mineral Uptake
o
two symbiotic relationships are known to assist roots in acquiring nutrients
o
legumes have nodules infected with the bacterium Rhizobium
ß
Rhizobium makes nitrogen compounds available to plants in exchange for
carbohydrates by reducing atmospheric nitrogen (N2) to ammonium (NH4+)
(nitrogen-fixation)
o
most plants have mycorrhizae; those lacking mycorrhizae are limited in where they
can grow
ß
mycorrhizae are a mutualistic symbiotic relationship between soil fungi and
plant roots
ß
the fungus increases the surface area for mineral and water uptake and breaks
down organic matter
ß
in return the root furnishes the fungus with sugars and amino acids

o

some plants have poorly developed roots or no roots; other mechanisms supply
minerals and water
ß
epiphytes take nourishment from air; their attachment to other plants gives them
support
ß
parasitic
plants
(e.g.,
dodders,
broomrapes,
pinedrops)
send
out haustoria (rootlike projections) that grow into host and tap into xylem and
phloem of host
ß
venus flytrap and sundew obtains nitrogen and minerals as leaves capture and
digest insects

Transport Mechanisms in Plants
Transport Tissues
o
vascular plants have transport tissues as an adaptation to living on land
o
xylem passively conducts water and minerals from roots to leaves; it contains two
types of conducting cells: tracheids and vessel elements
o
phloem conducts organic solutes in plants mainly from leaves to roots; contains sievetube cells and companion cells
o
these transport systems rely on the mechanical properties of water
ß
diffusion moves molecules from higher to lower concentrations
ß
water potential considers both water pressure and osmotic pressure
The Concept of Water Potential
o
water flows from a region of higher water potential (the potential energy of water) to a
region of lower water potential
o
water potential is a measure of the capacity to release or take up water; in cells, water
potential includes the following:
ß
pressure potential, the effect that pressure has on water potential; water will
move from a region of higher pressure to a region of lower pressure; and
ß
osmotic potential, the effect that solutes have on water potential; water tends to
move by osmosis from an area of lower solute concentration to area of higher
solute concentration
o
water flows by osmosis into a plant cell with greater solute concentration than a
surrounding solution
ß
as water enters, pressure increases inside the cell; the strong plant cell wall allows
water pressure to build up
ß
pressure potential inside the cell increases and balances the osmotic potential
outside cell; water stops entering
ß
turgor pressure is the pressure potential that increases due to process of
osmosis; it is critical to plants, since plants depend on it to maintain the turgidity of
their bodies
ß
wilted plant cells have insufficient turgor pressure and the plant droops
Water Transport
o
water entering root cells creates a positive pressure called root pressure
ß
root pressure (primarily at night) tends to push xylem sap upward in a plant
ß
guttation is the appearance of drops of water along the edge of leaves, as a result
of water being forced out of leaf vein endings; it is the result of root pressure
ß
root pressure is not a sufficient mechanism for water to rise to the tops of trees
Cohesion-Tension Model of Xylem Transport
o
water and dissolved minerals must be transported upward from roots to xylem, perhaps
as high as 90 meters
o
transpiration is a plantπs loss of water to atmosphere through evaporation at leaf
stomata

o

-

-

-

the cohesion-tension model states that transpiration creates a tension (i.e., a negative
pressure) that pulls water upward in xylem
o
water molecules are cohesive with one another, adhesive with xylem walls
o
cohesion is the tendency of water molecules to cling together due to their forming
hydrogen bonds
o
adhesion is the ability of water (a polar molecule) to interact with molecules comprising
the walls of xylem vessels; adhesion gives a water column extra strength and prevents
it from slipping back down
o
at least 90% of the water taken up by roots is lost through stomata by transpiration
o
with plenty of water, stomata will remain open, allowing CO2 to enter the leaf and
photosynthesis to occur
o
transpiration exerts a tension that draws the water column up in vessels
o
under water stress, more water is lost through a leaf than can be brought up and the
stomata close
o
photosynthesis requires CO2 to enter the leaf; there must be sufficient water so
stomata can remain open and allow CO2 to enter
Opening and Closing of Stomata
o
each stoma has two guard cells with a pore between them
o
stomata open from turgor pressure when guard cells take up water; when they lose
water, turgor pressure decreases and stomata close
o
guard cells are attached to each other at their ends; the inner walls are thicker than
outer walls
o
as they take up water, they buckle out, thereby creating an opening between cells
o
since 1968, it has been known that when stomata open, there is accumulation of
K+ ions in guard cells
o
a proton pump run by breakdown of ATP to ADP and P transports H + outside the cell;
this establishes an electrochemical gradient allowing K+ to enter by way of a channel
protein
o
the blue-light component of sunlight is a signal that can cause stomata to open
ß
there is evidence that flavin pigments absorb blue light
ß
this pigment sets in motion a cytoplasmic response activating the proton pump that
causes K+ ions to accumulate in guard cells
o
abscisic acid (ABA) produced by cells in wilting leaves, also causes stomata to close;
photosynthesis cannot occur but water is conserved
o
in plants kept in dark, stomata open and close on a 24-hour basis as if responding to
sunlight in daytime and the absence of sunlight at night; some sort of internal biological
clock must keep time
Organic Nutrient Transport
o
Marcello Malpighi (1679) suggested bark transferred sugars from leaves to roots
ß
today, we know phloem was removed but xylem remained; therefore, phloem does
transport sugars
o
radioactive tracer studies using 14C confirmed phloem transports organic nutrients
ß
similar studies confirm phloem transports amino acids, hormones, and mineral
ions
Pressure-Flow Model of Phloem Transport
o
the pressure-flow model explains the transport of sap through sieve tubes by a
positive pressure potential
o
the buildup of water creates a positive pressure potential within the sieve tubes that
moves water and sucrose to a sink (e.g., at the roots)
o
pressure exists from the leaves to the roots; at the roots, sucrose is transported out and
water also flows through due to the pressure

o
o
o
o
o
o
o
o

consequently, this pressure gradient causes a flow of water from leaves to roots
the conducting cells of phloem are sieve tubes lined end to end
cytoplasm extends through the sieve plates of adjoining cells to form a continuous tube
system
during the growing season, leaves produce sugar
sucrose is actively transported into phloem by an electrochemical gradient established
by a H+ pump
water flows passively into sieve tubes by osmosis
a sink can be at the roots or any other part of the plant that requires nutrients
because phloem sap flows from source to sink, sap can move any direction along
phloem.

Chapter 27: Control Of Growth And Responses In Plants
Plant Responses
Organisms Respond to Stimuli - a defining characteristic of life
o
adaptive organisms respond to environmental stimuli because it leads to longevity and
survival of the species
o
animals have nerves and muscles; plants respond by growth patterns
Tropisms
o
a tropism is plant growth toward or away from a directional stimulus
o
positive tropism: growth toward a stimulus
o
negative tropism: growth away from a stimulus
o
by differential growth, one side elongates faster; the result is a curving toward or away
from a stimulus
o
reception of the stimulus Æ transduction of the stimulus into a form meaningful to the
organism Æ response by the organism
o
phototropism, gravitropism, & thigmotropism
Phototropism: growth of plants in response to light
o
stems show positive phototropism
o
occurs because cells on shady side of stems elongate
o
a yellow pigment related to riboflavin appears to act as a photoreceptor for light
o
following reception, the plant hormone auxin migrates from the bright side to the shady
side of a stem
o
auxin is also involved in gravitropism, apical dominance, and root and seed
development
Gravitropism: response to earthπs gravity
o
roots demonstrate positive gravitropism; stems demonstrate negative gravitropism
o
an upright plant placed on its side displays negative gravitropism; it grows upward
opposite gravity
o
the hormone auxin underlies both positive and negative gravitropisms
ß
auxin inhibits the growth of root cells; cells of the upper surface elongate and the
root curves downward
ß
auxin stimulates the growth of stem cells; cells of the lower surface elongate and
the stem curves upward
Thigmotropism: unequal growth due to touch (e.g., coiling of tendrils around a pole)
o
the coiling of morning glory or pea tendrils around posts, etc., is a common example
o
cells in contact with an object grow less while those on the opposite side elongate
o
this process is quite rapid; tendrils can encircle an object in ten minutes
o
response can be delayed; tendrils touched in the dark respond when illuminated
ß
ATP rather than light can cause the response
ß
the hormones auxin and ethylene are involved; they induce curvature of tendrils
in the absence of touch
o
thigmomorphogenesis is a touch response involving the whole plant
ß
an entire plant responds to presence of touch, wind or rain
Nastic Movements: in contrast to tropisms, nastic movements are independent of the
direction of stimulus
o
seismonastic movements result from touch, shaking, or thermal stimulation
o
when a Mimosa pudica leaf is touched, the leaflets fold because the petiole droops
o
rapid response (1-2 sec.) and is due to a loss of turgor pressure within cells
o
Venusπs-flytrap has three sensitive hairs at the base of the trap
ß
when touched by an insect, an impulse-type stimulus triggers the trap to close
ß
turgor pressure in leaf cells then propel the trap

-

Sleep Movements: nastic responses to the daily changes in light level; an example is
the prayer plant that folds its leaves each night
o
biological clocks (maintain Circadian rhythms) are synchronized by external stimuli to
twenty-four-hour rhythms
o
stomata and flowers usually open in the morning, close at night; some plants secrete
nectar at same time of day

Plant Hormones: chemical messengers for communication & coordination of activities in plants;
a response is influenced by several hormones and may require a specific ratio of two or more
hormones
hormones are synthesized in one part of a plant; they travel in the phloem after a plant
receives an appropriate stimulus
auxin is produced in shoot apical meristem and found in young leaves, flowers, and fruits
o
when a terminal bud is removed, the nearest buds grow and the plant branches
o
auxin production by seeds promotes growth of fruit
o
if auxin is concentrated in leaves and fruits rather than stem, they do not fall off
o
auxin-controlled cell elongation is involved in gravitropism and phototropism
o
when gravity is perceived, auxin moves to lower surface of roots and stems
o
auxin mode of action: in a plant exposed to unidirectional light, auxin moves from the
bright side to the shady side of a stem
ß
auxin binds to receptors and activates the ATP-driven proton (H +) pump
ß
as hydrogen ions are pumped out of the cell, the cell wall becomes acidic,
breaking hydrogen bonds
ß
cellulose fibrils are weakened and activated enzymes further degrade cell wall
ß
the electrochemical gradient established causes of uptake of solutes and water
follows by osmosis
ß
the turgid cell presses against the cell wall, stretching it so elongation occurs
Gibberellins: group of 70 plant hormones that promote growth (elongate cells)
o
GA3 is the most common of the natural gibberellins
o
gibberellins are growth promoters that elongate cells
o
mode of action: the hormone GA3 binds to a receptor; a second messenger (Ca2+)
inside cell combines with calmodulin to activate the enzyme amylase, which acts on
starch to release sugars used as a source of energy by the growing embryo
Cytokinins: class of plant hormones derived from adenine that promote cell division
o
a natural cytokinin zeatin is found in corn kernels; kinetin is a synthetic cytokinin
o
may activate oligosaccharins, chemical fragments released from cell wall, also direct
differentiation
Senescence: aging processes; large molecules break down and are transported
elsewhere in the plant
o
cytokinins prevent senescence of leaves and initiate development of leaf growth
o
cytokinins initiate growth of lateral buds despite apical dominance
Abscisic acid (ABA): sometimes called the ≥stress hormone≤; it maintains seed and
bud dormancy and causes closure of stomata
o
dormancy occurs when a plant organ readies itself for adverse conditions by stopping
growth
ß
ABA moves from leaves to vegetative buds in fall; thereafter these buds are
converted to winter buds which are covered by thick, hardened scales
o
abscisic acid brings about closing of stomata when a plant is under water stress
ß
by some unknown mechanism, ABA causes K+ ions to leave guard cells; as a
result, guard cells lose water and the stomata close

o
-

although external application of ABA promotes abscission, the hormone ethylene is
considered to have this natural function
Ethylene is involved in abscission, the dropping of leaves, fruits, or flowers
o
lower levels of auxin in these areas (compared to stem) probably initiate abscission
o
once abscission begins, ethylene stimulates production of enzymes such
as cellulase (breaks down cellulose in cell walls) that cause leaf, fruit, or flower drop
o
it was an early practice to prepare citrus fruit for market by storage in a room with a
kerosene stove; later work revealed incomplete combustion of kerosene
produced ethylene which ripens fruit
o
ethylene is a gaseous plant hormone; it ripens fruit by increasing the activity of
enzymes that soften fruit

Photoperiodism: a physiological response to relative lengths of daylight and darkness
many physiological changes in plants (e.g., seed germination, the breaking of bud
dormancy, and the onset of senescence) are related to a seasonal change in day length
in some plants, photoperiodism also affects flowering
plants can be divided into three groups, based on photoperiodism
o
short-day plants: flower when day length was shorter than a critical length
o
long-day plants: flower when the day length is longer than a critical length
o
day-neutral plants: flowering is not dependent on day length
a long-day and a short-day plant can have the same critical length
Phytochrome and Plant Flowering
o
U.S.D.A. scientists discovered phytochrome, a blue-green leaf pigment that exists in
two forms
o
Pr (phytochrome red) absorbs red light (wavelength 660 nm); it is converted to Pfr
o
Pfr (phytochrome far-red) absorbs far-red light (wavelength 730 nm); it is converted to
Pr
o
during a 24-hour period, there is a shift in ratio of these two pigments
o
phytochrome conversion may be a first step in reception-transduction-response
pathway resulting in flowering
Other Functions of Phytochrome: the Pr Æ Pfr conversion cycle controls other growth
functions in plants
o
in addition to being involved in flowering, Pfr promotes seed germination and inhibits
stem elongation

Chapter 28: Reproduction In Plants
Reproductive Strategies
Life Cycles
o
in contrast to animals with one type of adult generation, flowering plants exhibit
an alternation of generations life cycle that includes a diploid and a haploid generation
o
the sporophyte is a diploid generation in an alternation of generations life cycle
ß
a sporophyte produces haploid spores by meiotic division
ß
spores develop into a haploid gametophyte
o
a gametophyte is a haploid generation in an alternation of generations life cycle
ß
a gametophyte produces haploid gametes by mitotic division; gametes fuse to
form diploid zygote
ß
the zygote undergoes mitotic cell division to develop into the sporophyte
o
a flower produces two types of spores, microspores and megaspores
ß
a microspore is a plant spore that develops into a microgametophyte
∑
the male gametophyte is a pollen grain; wind or animals carry it to
megagametophyte
∑
when mature, its nonflagellated sperm cells travel down pollen tube to
megagametophyte
ß
a megaspore is a plant spore that develops into a female gametophyte, the
embryo sac which remains within a sporophyte plant
o
in flowering plants, the diploid sporophyte is dominant (longer lasting); it is what we
commonly recognize
o
the sporophyte is the generation that contains vascular tissue and has other
adaptations suitable to living on land, including production of flowers
o
flowers are unique to angiosperms; aside from producing the spores and protecting
gametophytes, flowers attract pollinators and produce fruits to enclose the seeds
Flowers
o
a flower is the reproductive organ of a flowering plant; it develops in response to
environmental signals
o
the shoot apical meristem stops forming leaves to form flowers; axillary buds can
become flowers directly
o
monocot flower parts are in threes or multiples; eudicot flower parts are in fours or fives
or multiples
o
sepals are leaflike, usually green; this outermost whorl protects the bud as a flower
develops within
o
petals are interior to sepals; coloration accounts for attractiveness of many flowers
ß
the size, shape, and color of a flower are attractive to a specific pollinator
ß
wind-pollinated flowers often have no petals at all
o
grouped about a pistil are stamens, stalked structures that have two parts
ß
the anther is a saclike container within which pollen grains develop
ß
a filament is a slender stalk that supports the anther
o
the carpel is the vaselike structure located at the center of a flower (a carpel is a
simple pistil or 1 element of a compound pistil); carpels usually has three parts
ß
the stigma is an enlarged sticky knob on end of a style; stigma serves to receive
pollen grains
ß
the style is a slender stalk that connects stigma with the ovary
ß
the ovary is an enlarged base of a carpel that contains a number of ovules
o
not all flowers have sepals, petals, stamens, and a pistil
ß
complete flowers have sepals, petals, stamens, and a pistil; incomplete
flowers do not

-

ß
bisexual flowers have both stamens and a pistil
ß
staminate flowers have only stamens
ß
carpellate flowers have only carpels
o
if staminate and carpellate flowers are on same plant, the plant is monoecious
o
if staminate and carpellate flowers are on different plants, the plant is dioecious
From Spores to Gametes
o
in plants, the sporophyte produces haploid spores by meiosis; in animals, meiosis
produces gametes
o
flowering plants are heterosporous, producing microspores and megaspores that
become spermbearing pollen grains and egg-bearing embryo sacs, respectively
o
production of the male gametophyte
ß
microspores are produced in the anthers of flowers
ß
microspores develop into pollen grains containing a tube cell and two sperm
ß
the pollen grain is the mature male gametophyte
o
pollination release of pollen as walls of pollen sacs within the anther break down
ß
self-pollination is transfer of pollen from anther to stigma of the same plant
ß
cross pollination is transfer of pollen from anther of one plant to stigma of
another plant; plants often have mechanisms that promote cross pollination
ß
using a pollinator to carry pollen from flower to flower of only one species
increases the efficiency
ß
secretion of nectar is one way to attract certain pollinators, and they may be
adapted to reach only one type of flower
o
production of the female gametophyte
ß
the ovary contains one or more ovules
ß
the female gametophyte (or embryo sac) consists of 7 cells: 1 egg cell, 2
synergid cells, 1 central cell with two polar nuclei, and 3 antipodal cells
o
fertilization
ß
when a pollen grain lands on a stigma, it germinates, forming a pollen tube
ß
as a pollen tube grows, it passes between the cells of the stigma and the style to
reach the micropyle of an ovule
ß
double fertilization occurs after the release of both sperm cells into the ovule
ß
one sperm nucleus unites with the egg nucleus, forming a 2n zygote
ß
the other sperm nucleus migrates and unites with the polar nuclei of the central
cell, forming a 3n endosperm nucleus
ß
the zygote divides mitotically to become the embryo; the endosperm nucleus
divides mitotically to become the endosperm
ß
the embryo, in most plants, is a young sporophyte
ß
the endosperm is tissue that will nourish the embryo and seedling as they
undergo development

Seed Development
Stages
o
development of the seed is the next event
o
plant growth and development involves cell division, cell elongation, and differentiation
of cells into tissues and then organs
Development of the Eudicot Embryo
o
immediately after double fertilization, the endosperm nucleus divides to produce a
mass of endosperm surrounding the embryo
o
the single-celled zygote also divides, but asymmetrically, forming two parts: embryo
and suspensor, which anchors the embryo and transfers nutrients to it from the
sporophyte plant

o

-

globular stage: the proembryo is a ball of cells
ß
the root-shoot axis is established; cells near the suspensor will become a root,
those at the opposite end will become a shoot
ß
the outermost cells become dermal tissue that prevents dessication and also
has stomata that regulate gas exchange
o
the heart-shaped and torpedo-shaped embryos
ß
the embryo has a heart-shape when the cotyledons appear; it then grows to
a torpedo shape
ß
with elongation, the root and shoot apical meristems are distinguishable
ß
ground meristem responsible for most of the interior of the embryo is also present
now
o
the mature embryo
ß
after differentiation into embryo and suspensor, one or two cotyledons develop
ß
the embryo continues to differentiate into three parts:
∑
the epicotyl is between the cotyledons and first leaves; it contributes to shoot
development
∑
the hypocotyl is below the cotyledon and contributes to stem development
∑
the radicle is below the hypocotyl and contributes to root development
ß
the cotyledons are quite noticeable in a eudicot embryo, and may fold over
Monocots Versus Eudicots
o
monocot embryo only has one cotyledon
ß
in monocots, the cotyledon rarely stores food
ß
it absorbs food molecules from the endosperm and passes them to embryo
o
eudicot embryo has two cotyledons
ß
during development of a eudicot embryo, cotyledons usually store the nutrients the
embryo uses
ß
the endosperm seemingly disappears as the nutrients are consumed

Fruit Types and Seed Dispersal
Fruits
o
a fruit is a mature ovary enclosing seeds; sometimes they retain other flower parts
o
fruits serve to protect and disperse offspring
o
the fruit protects the peach seed well but makes germination difficult; the peas escape
easily but are lightly protected
Simple Fruits
o
simple fruit develops from a single carpel or several united carpels of a compound
ovary
o
legumes (pea pods), nuts and cereal grains are examples of dry fruits; such fruits are
mistaken for seeds because a dry pericarp adheres to the seed within
o
hooks and spines of clover, bur, and cocklebur attach to the fur of animals
o
woolly hairs, plumes, and wings disperse by wind
o
a fleshy fruit has a fleshy pericarp (e.g., peach, plum, olive, grape, tomato, apple, and
pear)
ß
birds and mammals eat fruits, including seeds, and defecate them at a distance
ß
squirrels and other animals gather seeds and fruits and bury them some distance
away
o
an apple is an example of an accessory fruit; the bulk of the fruit is not from the ovary
but from the receptacle
Compound Fruits
o
a compound fruit develops from several individual ovaries
o
an aggregate fruit develops from ovaries from a single flower (e.g., blackberry)

o

-

an aggregate fruit where each ovary becomes a one-seeded fruit is called
an achene (e.g., strawberry)
o
a multiple fruit develops from ovaries from separate flowers fused together (e.g.,
pineapple)
Seed Germination
o
seed germination occurs when growth and metabolic activity resume
o
the embryo forms with both shoot and root apical meristem enclosed in a seed
ß
protoderm gives rise to the epidermis
ß
ground meristem produces the cells of the cortex and pith
ß
procambium produces vascular tissue
o
seeds retain their viability for varying times: maples seeds only last a week while lotus
seeds are viable for hundreds of years
o
some seeds do not germinate until they have been dormant for a period of time
ß
seed dormancy is a time during which no growth occurs even though conditions
are favorable
ß
in temperate zones, seeds may have to be exposed to cold weather before
dormancy is broken
ß
in deserts, germination requires rain
o
germination has environmental requirements
ß
oxygen must be available for increased metabolism
ß
adequate temperature allow enzymes to act
ß
adequate moisture hydrates cells
ß
light may also be required
o
seeds that must be planted near the surface probably require light (e.g., lettuce)
o
when a seedling grows in the dark, it becomes long and spindly (etiolated);
phytochrome induces normal growth in light
o
germination in eudicots and monocots
ß
prior to germination, a eudicot embryo consists of the following:
∑
two cotyledons that supply nutrients to the embryo and seedling, but soon
shrivel and disappear
∑
a plumule—a rudimentary plant consists of an epicotyl bearing young leaves
∑
the hypocotyl, which becomes the stem
∑
the radicle, which develops into roots
ß
in dicot seedling, the shoot is hook-shaped to protect the delicate plumule
ß
in monocots, the endosperm is the food-storage tissue and the cotyledon does
not have a storage role
ß
a monocot ≥seed≤ such as a corn kernel is actually the fruit and the outer covering
is the pericarp
ß
the plumule and radicle are enclosed in protective sheaths, the coleoptile and
the coleorhiza, respectively
ß
the plumule and radicle burst through these coverings when germination occurs

Asexual Reproduction in Plants
Means of Asexual Propagation
o
plants contain nondifferentiated meristem tissue and often reproduce asexually
by vegetative propagation
o
in asexual reproduction, offspring arise from a single parent and inherit genes of that
parent only
o
vegetative propagation utilizes the meristematic tissue of a parent plant
ß
violet plants grow from nodes of rhizomes
ß
the nodes of stolons will produce strawberry plants

-

-

ß
each eye of a potato plant tuber is a bud that produces a new plant
ß
sweet potatoes can be propagated from their modified roots
ß
many trees can be started from small ≥suckers≤
o
stem cuttings have long been used to propagate a wide array of plants (e.g.,
sugarcane, pineapple)
o
the discovery that auxin will cause roots to develop has expanded our ability to use
stem cuttings.
Tissue Culture of Plants
o
tissue culture is the process of growing tissue artificially in a liquid or solid culture
medium
o
in 1902, German botanist Gottleib Haberlandt stated plant cells were totipotent; each
cell has full genetic potential of the organism
o
when cultured cells were provided with sugars, minerals, vitamins, and cytokinin from
coconut milk, the undifferentiated cells divide and initially formed acallus, an
aggregation of undifferentiated cells
o
the callus then differentiated into shoot and roots and developed into a complete plant
o
micropropagation is a commercial method of producing thousands to millions of
identical seedlings, by tissue culture in limited space
o
meristem culture micropropagates many new shoots from a single shoot apex culture
in a medium with correct proportions of auxin and cytokinin
ß
since the shoots are genetically identical, the adult plants that develop are clonal
plants
ß
clonal plants have the same genome and display the same traits
ß
meristem culture generates meristem & plants that are virus-free
o
entire plants can be grown from single plant cells
ß
enzymes can digest cell walls & produce naked plant cells called protoplasts
ß
protoplasts regenerate a cell wall and begin cell division
ß
clumps of cells can be manipulated to form somatic embryos
ß
somatic embryos encapsulated in a hydrated gel (≥artificial seeds≤) can be
shipped anywhere
o
anther culture cultures anthers in a medium of vitamins and growth regulators
o
cell suspension culture uses rapidly growing calluses cut into small pieces and
shaken in a liquid nutrient medium
Genetic Engineering of Plants
o
traditionally hybridization (crossing different varieties or species) was used to produce
new plants
o
transgenic plants carry foreign genes directly introduced into their cells
o
Tissue Culture and Genetic Engineering
ß
genetic engineering alters genes of organisms so they have new and different
traits; protoplasts lend themselves to genetic engineering in tissue culture
ß
high voltage electric pulses create pores in plasma membrane so new DNA can be
introduced
ß
when genes for production of firefly enzyme luciferinase were inserted into
tobacco protoplasts, plants glowed when sprayed with the substrate luciferin
ß
foreign DNA can be inserted into a plasmid of Agrobacterium; this bacterium
infects plant cells and can deliver the recombinant DNA to target cells
ß
a gene gun is now used to bombard a callus with DNA coated metal particles
ß
crops have been engineered to resist frost, fungal and viral infections, insect
predation, and herbicides
ß
future crops could have more protein and require less water and fertilizer
o
Agricultural Plants with Improved Traits

ß

o

corn, potato, soybean, and cotton plants have been engineered to be resistant to
insect predation or herbicides
ß
salt-tolerant Arabidopsis has been developed by cloning a gene for sequestering
sodium ions in a vacuole
ß
genes from Vernonia and castor beans were inserted into soybeans to produce
vernolic acid and ricinoleic acid used as hardeners in paints and plastics
ß
genetic engineering is attempting to improve efficiency of RuBP carboxylase and
introduce C4 photosynthesis to rice
Commercial Products
ß
corn has made antibodies to deliver radioisotopes to tumor cells
ß
soybeans make an antibody to treat genital herpes
ß
human gene can be inserted into tobacco plants using tobacco mosaic virus
ß
tobacco plants produced antigens to treat non-Hodgkinπs lymphoma

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