1. Chapter 21
Plant Organization
and Homeostasis
Lecture Outline
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2. 21.1 Flowering plants typically have
roots, stems, & leaves
 Most flowering plants possess a shoot system &
a root system
 Shoot system – stem, branches, leaves, & flowers
 Root system – main root and its branches
21-2

3. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 21.1A, pg. 432
terminal bud Organization of
a plant body
petiole
axillary bud leaf blade
stem
node
internode
node
vascular tissues
(xylem and phloem)
Shoot system
Root system
lateral
branch
root
root
hairs
primary
root
root tip
21-3

4.  The Stem
 4 main functions
 Supports the leaves & flowers
 Growth of stem
 Transport of water & nutrients between leaves and roots
 Food storage
 Terminal bud in shoot tip
 Produces new leaves and new axillary (lateral) buds
 Axillary buds can produce new branches or flowers
 Node is where a leaf or flower joins the stem
 Internode is the region between the nodes
21-4

5. Figure 21.1B, pg. 433: Modified leaves adapt to a plant’s environment
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
stem
leaves
Spines are the leaves of a cactus 21-5
© Patti Murray/Animals Animals

6.  Leaves
 Usually the primary organs of photosynthesis
 Blade – wide portion of a foliage leaf
 Petiole – stalk that attaches the blade to the
stem
 Examples of modified leaves:
21-6

7. Figure 21.1B Modified leaves adapt to a plant’s environment (Cont.)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
tendril
Tendrils are modified leaves of a cucumber
© Michael Gadomski/Photo Researchers, Inc
21-7

8. Figure 21.1B Modified leaves adapt to a plant’s environment (Cont.)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Leaves of a Venus flytrap capture insects
21-8
© Steven P. Lynch

9.  Roots
 Main functions
1. Anchor plant in soil
2. Absorb water & minerals from soil
3. Produce hormones
4. Some also store food
 Root hairs increase surface-to-volume ratio
 Tap roots vs. fibrous roots
Examples of each type of root system: 21-9

10. Figure 21.1C Taproot system (left) versus fibrous root system (right)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Taproot Fibroous root system
(taproot): © Jonathan Buckley/Getty Images;; (fibrous root): © The McGraw-hill companies Inc./Evelyn Jo Johnson, photographer 21-10

11. 21.2 Flowering plants are either
monocots or eudicots
 Monocots have one cotyledon
 Root vascular tissue rings pith
 Vascular bundles scattered in stem
 Leaf veins are parallel
 Flower parts in multiples of three
 Eudicots have two cotyledons
 Root phloem between areas of xylem
 Vascular bundles in a distinct ring
 Leaf veins form a net pattern
 Flower parts in multiples of four or five 21-11

12. Figure 21.2, pg. 434 Monocots & eudicots are structurally different
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Seed Root Stem Leaf Flower
endosperm phloem
pith
axillary bud
Monocots
xylem
Root xylem and Vascular bundles Leaf veins form Flower parts in threes
One cotyledon in seed phloem in a ring scattered in stem a parallel pattern and multiples of three
xylem
pith
axillary bud
Eudicots
phloem
Root phloem between Vascular bundles Leaf veins form Flower parts in fours or
Two cotyledons in seed arms of xylem in a distinct ring a net pattern fives and their multiples
21-12

13. HOW BIOLOGY IMPACTS OUR LIVES
21.3 Monocots serve humans well
 Although the monocots are a small group
compared to the eudicots, they have great
importance
 Domestication of monocot plants included
selective breeding in order to accumulate certain
desirable traits in crops
 Corn is by far the most important crop plant in
the United States
 Over 50% of the world’s people depend on rice
for about 80% of their calorie requirements
21-13

14. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 21A, pg. 435
grain head Monocot variety
Rice plants, Oryza
grain head
Wheat plants,Triticum
(rice plants): © Corbis RF; (rice grain head): © Dex Image/Getty RF; (wheat): © Earl Roberge/Photo Researchers, Inc 21-14

15. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 21A
Monocot
variety
(Cont.)
ear
Corn plants, Zea Barley
(ear of corn): © Doug Wilson/Corbis; (corn plants): © Adam Hart-Davis/SPL/Photo
Researchers, Inc.; (barley plants): © Sundell Larsen/Getty RF; (barley grains): © C.
Sherburne/Photolink/Getty RF 21-15

16. 21.3 Plants have specialized cells & tissues
 Apical meristem
 Located in the terminal bud of the shoot
system and in the root tip
 Daughter cells differentiate into 1 of 3 primary
meristems
1. Epidermal tissue forms the outer protective
covering of a plant
2. Ground tissue fills the interior of a plant and
serves metabolic functions
3. Vascular tissue contains xylem and phloem
21-16

17. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 21.1A, pg. 432
terminal bud Organization of
a plant body
petiole
axillary bud leaf blade
stem
node
internode
node
vascular tissues
(xylem and phloem)
Shoot system
Root system
lateral
branch
root
root
hairs
primary
root
root tip
21-17

18.  Epidermal Tissue
 Epidermis covers entire body of plant
 Waxy cuticle minimizes water loss
 Leaves contains stomata ringed by guard
cells
 Roots have root hairs
 Epidermis is replaced by cork in tree trunk
New cork cells are made by cork cambium
21-18

19. Figure 21.3A, pg. 436 Modifications of epidermal tissue
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
guard cell chloroplasts
corn seedling
root
hairs
epidermal
cell
stoma nucleus
elongating root tip
a. Stoma of leaf b. Root hairs
a: © J.R. Waaland/Biological Photo Service b: © Runk/Schoenberger/Grant Heilman Photography

20. Figure 21.3A Modifications of epidermal tissue (Cont.)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
lenticel periderm
cork cambium
cork
c. Cork of older stem 20 µm
c: © Kingsley Stern
21-20

21.  Ground Tissue
 Bulk of stems, leaves, and roots
 Contains three types of cells
 Parenchyma cells – the least specialized of the
cell types and are found in all the organs of a plant
 Collenchyma cells have thicker primary walls
than parenchyma cells
 Provide structural support in nonwoody plants, especially
in areas of elongation
 Sclerenchyma cells have thick secondary cell
walls impregnated with lignin
 Makes plant cell walls tough and hard, in areas where
growth has stopped elongation
21-21

22. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 21.3B,
pg. 436 Ground
tissue cells
Parenchyma cells with thin walls 50 µm
Collenchyma cells 50 µm Sclerenchyma cells 50 µm
with thicker walls with very thick walls 21-22
(all): © Biophoto Associates/Photo Researchers, Inc

23.  Vascular Tissue
 Xylem
 Transports water and minerals from roots to leaves
 Contains vessel elements (thicker) & tracheids (thinner)
 Phloem
 Transports sugar, in the form of sucrose, and other organic
compounds, such as hormones, from the leaves to the roots
 Sieve-tube members – conducting cells of phloem are
arranged to form a continuous sieve tube
 Companion cells – nucleated cells
21-23

24. Figure 21.3C, pg. 437 Xylem structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
xylem
parenchyma vessel
cell element tracheid
tracheids
pitted
end wall
walls
vessel
element
pits
a. Xylem micrograph 50 µm c.Tracheids
b.Two types of vessels
(Left): © J.R. Waaland/Biological Photo Service
21-24

25. Figure 21.3D, pg. 437 Phloem structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
sieve plate
sieve-tube
member
sieve plate
nucleus
sieve-tube
member
companion cell
companion cell
phloem
parenchyma cell
20 µm
Phloem micrograph Sieve-tube member and companion cells
(Left): © George Wilder/Visuals Unlimited
21-25

26. 21.4 The three types of plant tissues are
present in each organ
 Leaf
 Upper and lower epidermis has an outer, waxy
cuticle, which prevents water loss
 Stomata located in lower epidermis
 Interior of a leaf is made of mesophyll,
 Ground tissue composed of parenchyma cells
 Contain chloroplasts and carry on photosynthesis
 Palisade (tightly packed, elongated cells) vs spongy
mesophyll (irregular cells bounded by air spaces right next to
the stomata)
 Leaf veins branch and terminate in the mesophyll
21-26

27. Xylem transports cuticle
water and minerals.
upper
Phloem transports sugar. epidermis
mesophyll
lower
xylem epidermis
stoma
leaf vein phloem guard cell
a. Leaf

28.  Stem
 Herbaceous plants have “nonwoody” stems
 Ground tissue consists of cortex and central pith
 Vascular bundles
 Ring in eudicot
 Scattered in monocot
 Vascular tissue supports shoot system and
transport food and water
21-28

29. Figure 21.4B Internal
structure of the leaf,
stem, and root
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
vascular vascular
epidermis bundle bundle
epidermis cortex
cortex
pith
Tissue Types 100 µm
epidermal
ground b. Eudicot stem c. Monocot stem
vascular
b: © Ed Reschke; c: © CABISCO/Phototake;
21-29

30.  Root
 Epidermis usually consists of only a single layer of
cells and many epidermal cells have root hairs
 Large, thin-walled parenchyma cells make up the
cortex, the layer of ground tissue cells located
beneath the epidermis
 Cells contain starch granules, and the cortex functions in
food storage
21-30

31. epidermis
vascular
cylinder endodermis
cortex
Tissue Types phloem
epidermal
ground xylem 50 µm
vascular All tissues d. Eudicot root Vascular cylinder

32. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 21.4A , pg. 438
epidermal
tissue
Arrangement of plant
vascular tissues in the organs of
tissue
ground eudicots
tissue
Leaf
epidermal
tissue
vascular
tissue
ground
tissue
Stem
vascular ground
tissue tissue
epidermal
tissue
Root 21-32

33. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 21.4B Internal
Xylem transports
water and minerals.
cuticle structure of the leaf,
stem, and root
upper
Phloem transports sugar. epidermis
mesophyll
lower
xylem epidermis
stoma
leaf vein phloem guard cell
a. Leaf
vascular vascular
epidermis bundle bundle
epidermis cortex
cortex
epidermal
tissue
pith
100 µm
ground
tissue
b. Eudicot stem c. Monocot stem
vascular tissue
shoot system
root system
epidermis
vascular
cylinder endodermis
cortex
Tissue Types phloem
epidermal
ground xylem 50 µm
vascular All tissues d. Eudicot root Vascular cylinder
21-33
b: © Ed Reschke; c: © CABISCO/Phototake; d: © CABISCO/Phototake

34. 21.5 Primary growth lengthens
the root & shoot systems
 Primary growth
 Causes a plant to grow lengthwise
 Centered in the apex (tip) of the shoot and of
the root
 Meristem is a region of actively dividing cells
21-34

35.  Root System
 Zone of cell division
 Protected by the root cap
 Contains the root apical meristem
21-35

36.  Root System
 Zone of elongation
 Region where the root increases in length due to
elongation of cells
 Cells lengthen but are not fully specialized
 Zone of maturation
 Region that does contain fully differentiated cells
 Recognized by root hairs
21-36

37. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
epidermis
cortex
endodermis
pericycle
phloem
xylem
root hair Figure 21.5A , pg. 440
Cells within a eudicot
root tip
Zone of
maturation
Zone of
elongation
Vascular
cylinder
Zone of procambium
cell division
ground
meristem
protoderm
Root apical meristem
protected by
root cap
Root cap root cap
a. b.
21-37
(Right): Courtesy Ray F. Evert/University of Wisconsin Madison

38.  Shoot System
 Shoot apical meristem produces everything
 Leaves, axillary buds (can develop into stems or flowers),
additional stem and sometimes flowers
 Gives rise to the same primary meristems as in the root
 Protoderm → becomes epidermis of stems and leaves
 Ground mersitem → become cortex, pith, and mesophyll
 Procambium → vascular tissue (xylem and phloem)
 Vascular cambium responsible for secondary growth
(growing wider)
21-38

39. Figure 21.5B, pg. 441 Shoot apical meristem
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
leaf primordium
shoot apical
meristem
protoderm
ground
meristem
procambium
epidermis
internode
cortex
auxillary bud
vascular
cambium
xylem
phloem
© J.R. Waaland/Biological Photo Service 21-39

40.  Shoot System – Ex: Winter Twig
Terminal bud contains the apical meristem and leaf
primordia of the shoot tip protected by terminal bud
scales
Leaf scars and vascular bundle scars mark spot of
abscission (dropoff)
Determine age of a stem by counting terminal bud
scale scars because there is one for each year’s
growth
21-40

41. Figure 21.5C, pg. 441 Winter twig showing stem organization
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
vascular bundle terminal bud
scars scales
Twig during winter
leaf scar terminal bud axillary bud terminal bud
scale scar
Twig during spring
21-41

42. Secondary Plant Growth
 Secondary growth occurs only in woody plants
 Increases the girth (or width) of trunks, stems,
branches, and roots
 Occurs due to the growth of lateral meristems:
vascular cambium and cork cambium
 Woody stem has 3 distinct areas
 Bark, wood, and pith
 Wood is actually secondary xylem that builds up year
after year
21-42

43. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1 primary xylem
secondary xylem
Figure 21.6A, pg. 442
vascular cambium
secondary phloem In a tree, vascular
pith primary phloem
primary xylem cork cambium
cork
cambium produces
primary phloem
cortex secondary xylem and
epidermis
Vascular cambium:
phloem each year
Lateral meristem that will
produce secondary xylem
and secondary phloem in
each succeeding year.
2
primary xylem
secondary xylem
secondary phloem
primary phloem
3
Bark: Includes periderm and also phloem
Wood: Increases each year; includes annual
rings of xylem
Pith: Parenchyma cells that have a
storage function; becomes
squeezed out by heartwood
xylem ray
phloem ray
cork cambium
cork lenticel
21-43

44.  Bark
 Contains periderm and phloem
 Periderm is a secondary growth tissue that
contains cork and cork cambium
 Cork cambium lies beneath the epidermis, but later
it is part of the periderm, which replaces epidermis
 Cork cambium divides and produces the cork cells
that disrupt and replace the epidermis
21-44

45. Figure 21.6B, pg. 442 Heartwood has no transport function and serves
as a depository for various substances such as resins and tannin.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
heartwood
Wood
sapwood
vascular
cambium
phloem
Bark
cork
21-45

46. 21.7 Leaves are organized to carry on
photosynthesis
 Functions of a foliage leaf
 Carry on photosynthesis, regulate water loss,
and be protective against parasites and
predators
 Epidermal tissue on upper and lower
surfaces
 Waxy cuticle to prevent water loss
 Stomata all CO2 gain and water loss
21-46

47.  Mesophyll
 Elongated cells of the palisade mesophyll carry on
most of the photosynthesis
 Loosely packed spongy mesophyll increases the
amount of surface area for CO2 gain and water loss
 Leaf veins
 Bring water and minerals to leaves and distribute
products of photosynthesis to other parts of plant
 Bundle sheaths – layers of cells surrounding
vascular tissue
21-47

48. Figure 21.7, pg. 444 Leaf anatomy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
blade trihome
axillary
bud
petiole
cuticle
Water and minerals upper epidermis
enter leaf through xylem. palisade
mesophyll
Sugar exits leaf bundle sheath cell air space
through phloem.
spongy
mesophyll
lower epidermis
cuticle
leaf vein stoma
central vacuole upper
epidermis
nucleus chloroplast
chloroplast epidermal cell palisade
mesophyll
O2 and H O2
exit leaf
through stoma.
leaf vein
nucleus
guard cell spongy
CO2 enters leaf mesophyll
through stoma.
mitochondrion lower
stoma epidermis
Leaf cell Stoma and guard cells SEM of leaf 100 µm
© Jeremy Burgess/SPL/Photo Researchers, Inc
21-48

49. 21.8 Various mechanisms help
plants maintain homeostasis
 Anatomy of plants allows photosynthesis to
occur
 Vascular tissue (xylem and phloem) brings water and
minerals
 CO2 from stomata
 Exposure to solar energy
 Products of photosynthesis maintains
homeostasis (constancy of the internal
environment)
 Epidermis protects plants from invasion
 Closing stomata prevents water loss
21-49

50. Figure 21.8A, pg. 445 The organization of plants is conducive to maintaining
homeostasis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
leaf palisade upper
vein mesophyll epidermis
vascular
tissues
stoma lower spongy
epidermis mesophyll
(Middle): Courtesy Ray F. Evert/University of Wisconsin Madison
21-50

51. Figure 21.8B Stomata open (left) and close (right) according to water availability
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Stoma open 25 µm Stoma closed 25 µm
© jeremy Burgess/SPL/Photo Researchers, Inc.
21-51

52.  Phloem transport
 Source (where there are abundant sugar or carbohydrates) to
sink (where extra sugar or carbohydrates are needed) transport
 Plant hormones
 Highly specific chemical signals between plant parts and
cells
 Tropism is a growth response toward or away from a
particular stimulus
Examples: if in dark place, grow towards light
 Defense Mechanisms
 Cuticle, epidermal projections, chemical toxins
 Mutualistic relationship of plant roots and fungi
 Increase surface area by which roots absorb water and 21-52
minerals from soil, give fungi carbohydrates

53. Figure 21.8C, pg. 446 Homeostatic mechanisms
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
mature leaf
sugar sun
a. Phloem transports
sugar to areas
of need.
immature leaf b. Hormones cause
plants to bend
toward the light.
pathogenic
microbial
attack
d. Plants practice local cell death c. Plant roots associate
as a defense against attack. with fungi to acquire minerals.
dead cell
living cell
plant cells
21-53
C: © D. H. Marx/Visuals Unlimited

54. Connecting the Concepts:
Chapter 21
 Flowering plants are adapted to living on land
 Prevention of water loss is critical for land plants
 Stomata and cork
 On land, plants had to evolve a way to oppose
the force of gravity
 Sclerenchyma cells, tracheids, and vessel elements.
 Secondary growth
 Means of water uptake and transport
 Root hairs, xylem
21-54