Chap39

39 Reproduction in Flowering Plants
For many people, pollen means sneezing and misery because pol-
len grains of many plant species are potent allergens. However,
pollen is not part of nature simply to annoy human beings. What is a
pollen grain? It is a tiny, haploid male plant. To the stigma (the pollen
“landing pad”) of a flower, pollen grains represent an opportunity
for mate selection. That is, the stigma may allow some pollen grains to germinate,
but not others. If a pollen grain survives the mate selection process and germinates,
it may eventually deliver male gametes to a microscopic, haploid female plant em-
bedded in the flower.
Why do angiosperms expend energy and resources to produce flowers and that
sometimes obnoxious pollen? The answer is simple: Flowers are sexual reproductive
structures, and reproduction is the most important goal in a plant’s—or any organ-
ism’s—life.
In this chapter we will look at several aspects of plant reproduction, including
Key Players in Sexual Reproduction
some that are still not well understood. We will contrast sexual and asexual repro- Each species’ pollen has a characteristic
duction, and we will consider sexual reproduction in detail. In doing so, we will look size, shape, and cell wall structure. These
at angiosperm gametophytes, pollination, double fertilization, embryonic develop- structures are the male gametophytes and
are essential for sexual reproduction in
ment, and the roles of fruits in seed dispersal. The tran- seed plants.
sition from the vegetative state to the flowering state is a
key event in plant development, and we’ll see how
changing seasons trigger flowering in some plants—and
speculate on the existence of a flowering hormone. We
will conclude the chapter with an examination of asex-
ual reproduction in nature and in agriculture.

Many Ways to Reproduce
Plants have many ways of reproducing—and humans
have developed even more ways of reproducing them.
Flowers contain the sex organs of plants; it is thus no sur-
prise that almost all angiosperms reproduce sexually. But
some angiosperms reproduce asexually as well; some
even reproduce asexually most of the time. What are the
advantages and disadvantages of these two kinds of re-
production? The answers to this question involve genetic
recombination. As we have seen, sexual reproduction
produces new combinations of genes and diverse phe-
notypes. Asexual reproduction, in contrast, produces a
clone of genetically identical individuals.

750 CHAPTER THIRT Y-NINE

Both sexual and asexual reproduction are important in porangium, within which a female gametophyte may de-
agriculture. Many important annual crops are grown from velop. The stalk of the pistil is the style, and the end of that
seeds, which are the products of sexual reproduction. Seed- stalk is the stigma. Each stamen is composed of a filament
grown crops include the great grain crops, all of which are bearing a two-lobed anther, which consists of four microspo-
grasses—wheat, rice, corn, sorghum, and millet—as well as rangia fused together. Male gametophytes begin their devel-
plants in other families, such as soybeans and safflower. opment within the microsporangia.
Other crops, such as strawberries, potatoes, and bananas, are The petals and sepals of many flowers are arranged in
produced asexually. whorls (circles) or spirals around the carpels and stamens. To-
Orange trees, which have been under cultivation for cen- gether, the petals constitute the corolla. Below them, the sepals
turies, can be grown from seed—except for the navel orange, constitute the calyx. The petals are often colored, attracting
which has no seeds. This plant apparently arose only once in pollinating animals; the sepals are often green and photosyn-
history. Early in the nineteenth century, on a plantation on thetic. All the parts of the flower are borne on a stem tip, the
the Brazilian coast, a single orange seed gave rise to one tree receptacle. Flower parts are very diverse in form, in contrast to
that had aberrant flowers. Parts of the flowers aborted, and the microscopic gametophytes that develop within them.
seedless fruits formed. Asexual reproduction is the only way
of propagating this plant, and every navel orange in the
world comes from a tree that has been derived asexually Flowering plants have microscopic gametophytes
from that original Brazilian navel orange tree. Before reading this section, you may wish to review the sec-
Unlike navel oranges, strawberries are capable of forming tion in Chapter 29 entitled “Life cycles of plants feature al-
seeds and need not be propagated asexually. Nonetheless, ternation of generations” (pages 571–572). Central to under-
asexual propagation of strawberries is common because vast standing plant reproduction is the concept of alternation of
numbers of plants that are genetically and phenotypically generations, in which a multicellular diploid generation al-
identical to a plant humans find particularly desirable can be ternates with a multicellular haploid generation.
produced in this way. In angiosperms, the diploid sporophyte generation is the
We will treat asexual reproduction in greater detail at the larger and more conspicuous one. The sporophyte generation
end of this chapter. We will begin, however, by considering produces flowers. The flowers produce spores, which develop
sexual reproduction. into tiny gametophytes that begin and, in the case of the
megagametophyte, end their development enclosed by sporo-
phyte tisue.
Sexual Reproduction in Plants The haploid gametophytes—the gamete-producing gen-
Sexual reproduction provides genetic diversity through re- eration—of flowering plants develop from haploid spores in
combination (see Chapter 9). Meiosis and mating between sporangia within the flower (Figure 39.1):
different plants shuffle genes into new combinations, giving
Female gametophytes (megagametophytes), which are
a population a variety of genotypes in each generation, some
called embryo sacs, develop in megasporangia.
of which may be superior to those of their parents. This ge-
Male gametophytes (microgametophytes), which are
netic diversity may serve the population well as the envi-
called pollen grains, develop in microsporangia.
ronment changes or as the population expands into new en-
vironments. The adaptability resulting from genetic diversity Within the ovule, a megasporocyte—a cell within the megas-
is the major advantage of sexual reproduction over asexual porangium—divides meiotically to produce four haploid
reproduction, although sexual reproduction can also break megaspores. In most plants, all but one of these megaspores
up well-adapted combinations of alleles through the same then degenerate. The surviving megaspore usually undergoes
process of recombination. three mitotic divisions, producing eight haploid nuclei, all ini-
tially contained within a single cell—three nuclei at one end,
three at the other, and two in the middle. Subsequent cell wall
The flower is an angiosperm’s device formation leads to an elliptical, seven-celled megagameto-
for sexual reproduction phyte with a total of eight nuclei (see Figure 39.1):
A complete flower consists of four groups of organs that are
modified leaves: the carpels, stamens, petals, and sepals (see At one end of the elliptical megagametophyte are three
Figure 30.7). The carpels and stamens are, respectively, the fe- tiny cells: the egg and two cells called synergids. The
male and male sex organs. A pistil is a structure composed of egg is the female gamete, and the synergids participate
one or more carpels. The base of the pistil, called the ovary, indirectly in fertilization by attracting and accepting the
contains one or more ovules, each of which contains a megas- pollen tube.

REPRODUCTION IN FLOWERING PLANTS 751

Ovary Anther

Ovule

4 The second
sperm nucleus
fuses with the
two polar nuclei.
Microsporocyte
Endosperm Ovary
nucleus (3n) Ovule

Zygote (2n)
3 One sperm Megasporocyte (2n)
nucleus fuses
with the egg. Seed Megasporangium

DIPLOID (2n)
Double Fertilization Meiosis
HAPLOID (n)
Pollen grains
(microgametophytes, n)
Surviving
megaspore (n) Pollen grain
Mega- (microgametophyte)
gametophyte
(n)
Antipodal
Pollen cells (3)
tube
1 In the ovule, three
of the four meiotic
Polar nuclei (2) products degenerate.

Egg
4 The pollen tube grows
toward the embryo Synergids (2)
sac (see Figure 39.5).
2 The embryo sac is the female
gametophyte. After three
mitotic divisions, it contains
3 The pollen grain eight haploid nuclei.
is transferred to
the stigma.

Sperm (2)
Tube cell nucleus
of pollen grain
39.1 Development of Gametophytes and Nuclear Fusion The embryo sac is
the female gametophyte; the pollen grain is the male gametophyte. The male and
female nuclei meet and fuse within the embryo sac. Most angiosperms have dou-
ble fertilization, in which a zygote and an endosperm nucleus form from separate
fusion events—the zygote from one sperm and the egg and the endosperm from the
other sperm and two polar nuclei.

At the opposite end of the megagametophyte are three The pollen grain (microgametophyte) consists of fewer
antipodal cells, which eventually degenerate. cells and nuclei than the embryo sac. The development of a
In the large central cell are two polar nuclei. pollen grain begins when a microsporocyte within the anther
The embryo sac (megagametophyte) is the entire seven-cell, divides meiotically. Each resulting haploid microspore de-
eight-nucleus structure. You can review the development of velops a spore wall, within which it normally undergoes one
the embryo sac in Figure 39.1. mitotic division before the anthers open and release these

752 CHAPTER THIRT Y-NINE Pollen

two-celled pollen grains. The two cells are the tube cell Stigma

and the generative cell. Further development of the
Pollen
pollen grain, which we will describe shortly, is delayed
until the pollen arrives at a stigma. In angiosperms, the S3 S4 S2 S1
Pistil S3 S1
transfer of pollen from the anther to the stigma is re- Style
ferred to as pollination. Pollen
tube

Ovary
Pollination enables fertilization in the absence
of liquid water S1S2 S1S2 S1S2
Gymnosperms and angiosperms do not require external wa-
ter as a medium for gamete travel and fertilization—a free-
dom not shared by other plant groups. The male gametes of
gymnosperms and angiosperms travel within pollen grains. S3 and S4 pollen …but S1 and S2 pollen do
are compatible not germinate. They are
But how do angiosperm pollen grains travel from an anther with an S1S2 pistil… self-incompatible.
to a stigma?
Many different mechanisms have evolved for pollen trans- 39.3 Self-Incompatibility Pollen grains do not germinate normally
port. In some plants, such as peas and their relatives, self-pol- if their S allele matches one of the S alleles of the stigma. Thus, the
egg cannot be fertilized by a sperm from the same plant.
lination is accomplished before the flower bud opens. Pollen
is transferred by the direct contact of anther and stigma
within the same flower, resulting in self-fertilization.
Wind is the vehicle for pollen transport in many species. Some aquatic angiosperms are pollinated by water carrying
Wind-pollinated flowers have sticky or featherlike stigmas, pollen grains from plant to plant. Animals, including insects,
and they produce pollen grains in great numbers (Figure 39.2). birds, and bats, carry pollen among the flowers of many
plants.

Some plants practice “mate selection”
In our discussion of Mendel’s work (see Chapter 10), we saw
that some plants can reproduce sexually either by cross-pol-
lination or by self-pollination. But not all plants have this
flexibility. Many plants reject pollen from their own flowers.
This phenomenon, known as self-incompatibility, promotes
genetic variation.
A single gene, the S gene, is responsible for self-incompati-
bility in most plants. The S gene has dozens of alleles. A pollen
grain is haploid and possesses a single S allele; the recipient pis-
til is diploid. In self-incompatible plants, pollen fails to germi-
nate, or the pollen tube fails to traverse the style, if the S allele of
the pollen matches one of the two S alleles in the pistil (Figure
39.3).
The stigma plays an important role in “mate selection” by
flowering plants. The stigmas of most plants are exposed to the
pollen of many other species as well as their own. Pollen from
the same species binds strongly to the stigma due to cell–cell sig-
naling between the stigma and the cell walls of the pollen grains.
In contrast, foreign pollen falls off readily or fails to germinate.

39.2 Wind Pollination The numerous anthers on these A pollen tube delivers male cells to the embryo sac
inflorescences (groups of flowers) of a hazelnut tree all point
away from the stalk and stand free of the plant, promoting When a pollen grain lands on the stigma of a compatible pis-
dispersal of the pollen by wind. til, it germinates. Germination, for a pollen grain, is the de-

Pollen grain Pollen tube
Stigma
Angiosperms perform double fertilization
In most angiosperm species, the mature pollen grain con-
sists of two cells, the tube cell and the generative cell. The
larger tube cell encloses the much smaller generative cell.
Guided by the tube cell nucleus, the pollen tube eventually
grows through the megasporangial tissue and reaches the
embryo sac. The generative cell meanwhile has undergone
one mitotic division and cytokinesis to produce two hap-
loid sperm cells.
Both of the sperm cells enter the embryo sac, where they
are released into the cytoplasm of one of the synergids. This
synergid degenerates, releasing the sperm cells (Figure 39.5).
39.4 Pollen Tubes Begin to Grow These pollen grains have landed Each sperm cell then fuses with a different cell of the embryo
on hairlike structures on the stigma of an Arabidopsis flower, and sac. One sperm cell fuses with the egg cell, producing the
pollen tubes have penetrated the stigma.
diploid zygote. The other fuses with the central cell, and that
sperm cell nucleus and the two polar nuclei unite to form a
triploid (3n) nucleus. While the zygote nucleus begins mitotic
velopment of a pollen tube (Figure 39.4). The pollen tube ei- division to form the new sporophyte embryo, the triploid nu-
ther traverses the spongy tissue of the style or, if the style is cleus undergoes rapid mitosis to form a specialized nutritive
hollow, grows downward on the inner surface of this female tissue, the endosperm. The endosperm will later be digested
organ until it reaches an ovule. The pollen tube may grow by the developing embryo, as we saw in the previous chap-
millimeters or even centimeters in the process. ter. The antipodal cells and the remaining synergid eventu-
The rapid growth of the pollen tube requires calcium ions, ally degenerate, as does the pollen tube nucleus.
which are taken up by the growing tip of the tube, as well as This process is known as double fertilization because it
cell adhesion proteins. The downward growth of the pollen involves two nuclear fusion events:
tube is believed to be guided by a long-distance chemical sig-
One sperm cell fuses with the egg cell.
nal from the synergids within the ovule. If one synergid is
The other sperm cell fuses with the two polar nuclei.
destroyed, the ovule still attracts pollen tubes, but destruc-
tion of both synergids renders the ovule unable to attract
pollen tubes, and fertilization does not occur.
39.5 Sperm Nuclei and Double Fertilization The sperm nuclei
contribute to the formation of the diploid zygote and the triploid
endosperm. Double fertilization is a characteristic feature of
angiosperm reproduction.

4 The synergid breaks down;
one sperm nucleus unites
with the two polar nuclei,
Tube cell forming the first cell of the
Three antipodal 3n endosperm generation.
cells

Generative
cell

Tube cell
Polar nucleus
nuclei
Egg

Synergids

1 Initially the pollen tube contains 2 The generative cell divides 3 The sperm cells enter the 5 The other sperm nucleus
two haploid cells, the generative mitotically, producing two cytoplasm of a synergid. fertilizes the egg, forming
cell and the tube cell. haploid sperm cells. the zygote, the first cell of
the 2n sporophyte
generation.

39.6 Early Development of a Eudicot The embryo develops through
intermediate stages, including a characteristic heart-shaped stage, to
reach the torpedo stage.

The zygote nucleus divides Torpedo-stage
embryo
mitotically, one daughter cell Heart-stage Endosperm
giving rise to the embryo proper embryo
and the other to the suspensor. Cotyledons
Shoot
apex
Embryo Hypocotyl
Endosperm Root apex
nucleus
Suspensor
Embryo
Suspensor Seed
sac
coat
Zygote
The tissues surrounding
the embryo sac develop
into the seed coat.
The fusion of a sperm cell nucleus with the two polar nuclei
to form endosperm takes place only in angiosperms. The fu-
sion of these three nuclei, the possession of flowers, and the
formation of fruit are the three most definitive characteristics the internal tissues begin to differentiate (see Figure 39.6). Be-
shared by angiosperms. tween the cotyledons is the shoot apex; at the other end is the
root apex. Between the shoot and root apices is the hypocotyl.
Each of the apical regions contains an apical meristem whose
Embryos develop within seeds dividing cells will give rise to the organs of the mature plant.
Shortly after fertilization, highly coordinated growth and de- During seed formation, large amounts of nutrients are
velopment of embryo, endosperm, integuments, and carpel moved in from other parts of the plant, and the endosperm
ensues. The integuments—protective tissue layers immedi- accumulates starch, lipids, and proteins. In many species, the
ately surrounding the megasporangium—develop into the cotyledons absorb the nutrient reserves from the surround-
seed coat, and the carpel ultimately becomes the wall of the ing endosperm and grow very large in relation to the rest of
fruit that encloses the seed. the embryo (Figure 39.7a). In others, the cotyledons remain
The first step in the formation of the embryo is a mitotic thin (Figure 39.7b); they draw on the reserves in the en-
division of the zygote that gives rise to two daughter cells. dosperm as needed when the seed germinates.
These two cells face different fates. An asymmetrical (un-
even) distribution of cytoplasm within the zygote causes one
daughter cell to produce the embryo proper and the other
daughter cell to produce a supporting structure, the suspen- In some eudicots, In other eudicots, the In monocots, the
sor (Figure 39.6). The suspensor pushes the embryo against the cotyledons endosperm remains single cotyledon is
or into the endosperm and provides one route by which nu- absorb much of the separate and the pressed against the
endosperm and fill cotyledons remain thin. endosperm.
trients pass from the endosperm into the embryo. most of the seed.
With the asymmetrical division of the zygote, polarity has Seed coat Seed coat
been established, as has the longitudinal axis of the new
Cotyledon Cotyledon
plant. A long, thin suspensor and a more spherical or globu-
Shoot apex Shoot apex
lar embryo are distinguishable after just four mitotic divi-
sions. The suspensor soon ceases to elongate. However, cell Root apex Root apex
divisions continue, the primary meristems form, and the first Cotyledon
organs begin to form within the embryo. Endosperm Endosperm
In eudicots (monocots are somewhat different), the initially (a) Kidney bean (b) Castor bean (c) Corn
globular embryo takes on a characteristic heart stage form as
the cotyledons (“seed leaves”) start to grow. Further elonga- 39.7 Variety in Angiosperm Seeds In some seeds, such as kidney
beans (a), the nutrient reserves of the endosperm are absorbed by
tion of the cotyledons and of the main axis of the embryo gives the cotyledons. In others, such as castor beans (b) and corn (c), the
rise to what is called the torpedo stage, during which some of reserves in the endosperm will be drawn upon after germination.


In the late stages of embryonic development, the seed (a) 39.8 Dispersal of Fruits
loses water—sometimes as much as 95 percent of its original (a) A samara is a winged fruit
characteristic of the maple family.
water content. In this desiccated state, the embryo is inca- (b) A coconut seed germinates
pable of further development; it remains quiescent until in- where it washed ashore on a
ternal and external conditions are right for germination. (Re- beach in the South Pacific.
call from Chapter 38 that a necessary early step in seed
germination is the massive imbibition of water.) In addition
to embryo and endosperm development, the structures of the
ovary are also undergoing developmental changes to form a
seed and fruit.

Some fruits assist in seed dispersal
After fertilization, the ovary wall of a flowering plant—to-
gether with its seeds—develops into a fruit. A fruit may con-
sist of only the mature ovary and the seeds it contains, or it
may include other parts of the flower or structures that are
closely related to it. In some species, this process produces
fleshy, edible fruits such as peaches and tomatoes, while in
other species the fruits are dry or inedible. Some major vari-
ations on this theme are illustrated in Figure 30.12, which
shows only fleshy, edible fruits. Whatever its form, the fruit (b)
serves to assure seed dispersal.
Some fruits help disperse seeds over substantial distances,
improving the chances that at least a few of the many seeds
produced by a plant will find suitable conditions for germi- pany flowering or resume after flowering is completed. But
nation and growth to sexual maturity. Various trees, includ- whatever the specific pattern, flowering always entails ma-
ing ash, elm, maple, and tree of heaven, produce a dry, jor developmental changes.
winged fruit that may be blown some distance from the par-
ent tree by the wind (Figure 39.8a). Water disperses some
fruits; coconuts have been spread in this way from island to Apical meristems can become inflorescence meristems
island in the Pacific Ocean (Figure 39.8b). Still other fruits The first visible sign of the transition to the flowering state may
travel by hitching rides with animals—either inside or out- be a change in one or more apical meristems in the shoot sys-
side them. Fleshy fruits such as berries provide food for tem. During vegetative growth, an apical meristem continually
mammals or birds; seeds that are swallowed whole travel produces leaves, lateral buds, and internodes (Figure 39.9a).
safely through the animal’s digestive tract and are deposited This unrestricted growth is indeterminate (see Chapter 35).
some distance from the parent plant. In some species, seeds Flowers may appear singly or in an orderly cluster that
must pass through an animal to break dormancy. constitutes an inflorescence. If a vegetative apical meristem
We have now traced the sexual life cycle of angiosperms becomes an inflorescence meristem, it ceases production of
from the flower to the fruit to the dispersal of seeds. Seed ger- leaves, lateral buds, and internodes and produces other struc-
mination and the vegetative development of the seedling tures: smaller leafy structures called bracts, as well as new
were presented in Chapter 38. Now let’s complete the cycle meristems in the angles between the bracts and the intern-
by considering the transition from the vegetative to the flow- odes (Figure 39.9b). These new meristems may also be inflo-
ering state, and how this transition is regulated. rescence meristems, or they may be floral meristems, each
of which gives rise to a flower.
Each floral meristem typically produces four consecutive
The Transition to the Flowering State whorls or spirals of organs—the sepals, petals, stamens, and
If we view a plant as something produced by a seed for the carpels—separated by very short internodes, keeping the
purpose of bearing more seeds, then the act of flowering is flower compact (Figure 39.9c). In contrast to vegetative api-
one of the supreme events in a plant’s life. The transition to cal meristems and some inflorescence meristems, floral
the flowering state marks the end of vegetative growth for meristems are responsible for determinate growth—the lim-
some plants. In other plants, vegetative growth may accom- ited growth of the flower to a particular size and form.


(a) Having seen how flowering occurs, we will now
Vegetative
apical consider how the transition from the vegetative to the
meristem A vegetatively growing flowering state is initiated.
apical meristem
Leaf continues to produce
leaves and internodes.
Internode Photoperiodic Control of Flowering
Environmental cues trigger the transition to the flow-
ering state in many cases, but such environmental con-
Floral trol is also subject to genetic modification. The life cy-
(b)
meristem cles of flowering plants fall into three categories:
Inflorescence annual, biennial, and perennial. Annuals, such as
meristem
Inflorescence A bract is a modified, many food crops, complete their life cycle (seed to
meristems give usually reduced
rise to floral
flower) in one growing season. Biennials, such as car-
leaflike structure.
meristems, rots and cabbage, grow vegetatively for all or part of
bracts, and more one growing season and live on into a second growing
inflorescence
meristems. season, during which they flower, form seeds, and die.
Perennials, such as oak trees, live for a few to many
growing seasons, during which both vegetative growth
Carpel and flowering occur. What control systems give rise to
(c) Floral
meristem these and other differences in flowering behavior?
Stamen Modified
A floral meristem leaflike In 1920, W. W. Garner and H. A. Allard of the U.S.
gives rise to structures Department of Agriculture studied the behavior of a
Petal
a flower. of the flower
Sepal
newly discovered mutant tobacco plant. The mutant,
named ‘Maryland Mammoth,’ had large leaves and ex-
ceptional height. When the other plants in the field
39.9 Flowering and the Apical Meristem A vegetative apical
meristem (a) grows without producing flowers. Once the transition flowered, ‘Maryland Mammoth’ plants continued to grow.
to the flowering state is made, inflorescence meristems (b) give rise Garner and Allard took cuttings of ‘Maryland Mammoth’ into
to bracts and to floral meristems (c), which become the flowers. their greenhouse, and the plants that grew from those cuttings
finally flowered in December.
Garner and Allard guessed that this flowering pattern had
something to do with the mutant’s response to some envi-
A cascade of gene expression leads to flowering ronmental cue. They tested several likely environmental vari-
How do apical meristems become inflorescence meristems, ables, such as temperature, but the key variable proved to be
and how do inflorescence meristems give rise to floral meris- day length. By moving plants between light and dark rooms
tems? How does a floral meristem give rise, in short order, to at different times to vary the day length artificially, they were
four different floral organs? How does each flower come to able to establish a direct link between flowering and day
have the correct number of each of the floral organs? Nu- length. We now know that the key variable is the length of
merous genes collaborate to produce these results. We’ll re- the night, rather than the day, but Garner and Allard did not
fer here to some of the genes whose actions have been most make that distinction.
thoroughly studied in Arabidopsis and snapdragons. The ‘Maryland Mammoth’ plants did not flower if the light
period they were exposed to was longer than 14 hours per day,
Expression of a group of meristem identity genes initiates
but flowering commenced after the days became shorter than
a cascade of further gene expression.
14 hours. Thus, the critical day length for ‘Maryland Mam-
This cascade begins with cadastral genes, which partici-
moth’ tobacco is 14 hours (Figure 39.10). The phenomenon of
pate in pattern formation—the spatial organization of the
control by the length of day or night is called photoperiodism.
whorls of organs.
Cadastral genes trigger the expression of floral organ iden-
tity genes, which work in concert to specify the successive There are short-day, long-day, and day-neutral plants
whorls (see Figure 19.12)
Plants that flower in response to photoperiodic stimuli fall
Floral organ identity genes are homeotic genes, and their into several classes. Poinsettias, chrysanthemums, and ‘Mary-
products are transcription factors that mediate the expression land Mammoth’ tobacco are short-day plants (SDPs), which
of still other genes. flower only when the day is shorter than a critical maximum.


‘Maryland Mammoth’ tobacco Henbane flowers only when
The flowering of some angiosperms, such as corn, roses,
flowers only when days are days are longer than 14 hours, and tomatoes, is not photoperiodic. In fact, there are more of
shorter than 14 hours, its critical its critical day length. these day-neutral plants than there are short-day and long-
day length.
day plants. Some plants are photoperiodically sensitive only
14 hours 14 hours when young and become day-neutral as they grow older.
Light Dark Light Dark Others require specific combinations of day length and other
factors—especially temperature—to flower.

The length of the night determines whether a
photoperiodic plant will flower
‘Maryland Henbane, The terms “short-day plant” and “long-day plant” became
Mammoth’ tobacco Hyoscyamus niger entrenched before scientists learned that photoperiodically
(short-day plant) (long-day plant)
sensitive plants actually measure the length of the night, or
of a period of darkness, rather than the length of the day. This
fact was demonstrated by Karl Hamner of the University of
California at Los Angeles and James Bonner of the California
Institute of Technology (Figure 39.11).
Working with cocklebur, an SDP, Hamner and Bonner ran
a series of experiments using two sets of conditions:
Long days; Short days; Long days; Short days;
plant remains plant flowers plant flowers plant remains
vegetative vegetative
EXPERIMENT
39.10 Day Length and Flowering By artificially varying the length
Question: Do short-day plants measure day length or night length?
of the day, Garner and Allard showed that the flowering of ‘Maryland
Mammoth’ tobacco is initiated when the days become shorter than a METHOD
critical length. ‘Maryland Mammoth’ tobacco is thus called a short-
day plant. Henbane, a long-day plant, shows an inverse pattern of Plants were moved between light and dark rooms
flowering. for specified numbers of hours.

RESULTS

Light constant/Darkness varied
16 6
Spinach and clover are examples of long-day plants (LDPs),
which flower only when the day is longer than a critical min- 16 7 No flowering

imum. Generally, LDPs are triggered to flower in midsummer 16 8
and SDPs in late summer, fall, or sometimes in the spring. Be- 16 9 Only plants given
cause short days occur both before and after midsummer, 9 or more hours
16 10
there is a degree of ambiguity in this signal. Could there be of dark flowered.
a more precise way for plants to regulate flowering? 16 11

Some plants require photoperiodic signals that are more Light varied/8 or 10 hours of darkness
complex than just short or long days. One group, the short- 8 10 Only plants given
long-day plants, must experience first short days and then long 10 10 10 hours of dark
ones in order to flower. Accordingly, white clover and other flowered.
12 10
short-long-day plants flower during the long days before
midsummer. Another group, the long-short-day plants, cannot 8 8

flower until the long days of summer have been followed by 10 8 No flowering
shorter ones, so they bloom only in the fall. Kalanchoe, seen 12 8
in Figure 39.16b, is a long-short-day plant. Time (hours)
Other processes besides flowering are also under pho-
toperiodic control. We have learned, for example, that short Conclusion: Short-day plants measure the length of the night and
could more accurately be called long-night plants.
days trigger the onset of winter dormancy in plants. (Ani-
mals, too, show a variety of photoperiodic behaviors, as we’ll 39.11 Night Length and Flowering The length of the dark period,
see in Chapter 52.) not the length of the light period, determines flowering.


For one group of plants, the light period was kept con- EXPERIMENT A
Question: How does interrupting a long day or night affect flowering?
stant—either shorter or longer than the critical day
length—and the dark period was varied. Short-day Long-day
For another group of plants, the dark period was kept plants Experimental conditions plants
constant and the light period was varied. No flowering Flowering
No flowering Flowering
The plants flowered under all treatments in which the dark
Flowering No flowering
period exceeded 9 hours, regardless of the length of the light
period. Thus, Hamner and Bonner concluded that it is the
length of the night that matters; for cocklebur, the critical night
Conclusion: Photoperiodic plants measure the length of the night, not
length is about 9 hours. Thus, it would be more accurate to the day. Interrupting a long night with a brief period of light inhibits
call cocklebur a “long-night plant” than a short-day plant. flowering in short-day plants. Long-day plants flower when the night
In cocklebur, a single long night is sufficient photoperiodic is short, but interrupting their long day has no effect.
stimulus to trigger full flowering some days later, even if the
intervening nights are short ones. Most plants are less sensi- EXPERIMENT B
tive than cocklebur and require from two to several nights of Question: Does phytochrome participate in the photoperiodic
timing mechanism?
appropriate length to induce flowering. For some plants, a
single shorter night in a series of long ones, even one day be- Short-day Long-day
fore flowering would have commenced, inhibits flowering. plants plants
By means of other experiments, Hamner and Bonner Flowering No flowering
gained some insight into how plants measure night length.
They grew SDPs and LDPs under a variety of light condi-
tions. In some experiments, the dark period was interrupted
No flowering R Flowering
by a brief exposure to light; in others, the light period was in- Flowering FR No flowering
terrupted briefly by darkness. Interruptions of the light pe- Flowering R FR No flowering
riod by darkness had no effect on the flowering of either No flowering R FR R Flowering
short-day or long-day plants. Even a brief interruption of the Flowering R FR R FR No flowering
dark period by light, however, completely nullified the effect
Conclusion: When plants are exposed to red (R) and far-red (FR) light
of a long night (Figure 39.12a). An SDP flowered only if the in alternation, the final treatment determines the effect of the light
long nights were uninterrupted. An LDP experiencing long interruption, suggesting that phytochrome participates in
nights flowered if those nights were interrupted by exposure photoperiodic responses.
to light. Thus, the investigators concluded, these plants must
39.12 The Effect of Interrupted Days and Nights
have a timing mechanism that measures the length of a con- (a) Experiments suggest that plants are able to measure the
tinuous dark period. length of a continuous dark period and use this information
The nature of this timing mechanism has been partially to trigger flowering. (b) Phytochromes seem to be involved in
the photoperiodic timing mechanism.
revealed, beginning with the determination of the effective
wavelengths of light and the identity of the photoreceptors.
In the interrupted-night experiments, the most effective
wavelengths of light were in the red range (Figure 39.12b), this suggestion is inconsistent with many experimental ob-
and the effect of a red-light interruption of the night could be servations, such as the fact that when a plant is subjected to a
fully reversed by a subsequent exposure to far-red light, in- dark period several days in duration, the plant’s sensitivity to
dicating that a phytochrome is the photoreceptor. Phy- a light flash during the long night varies on a roughly 24-hour
tochromes and blue-light receptors, which affect several as- cycle. Such data suggest instead that the phytochrome is only
pects of plant development (see Chapter 38), also participate a photoreceptor, and that the timekeeping role is played by a
in the photoperiodic timing mechanism. biological clock that is linked to the phytochrome (which sets
What might that mechanism consist of? It was once hy- the clock) and also to the production of flowers.
pothesized that the timing mechanism might simply be the
slow conversion of a phytochrome during the night from the
Pfr form—produced during the light hours—to the Pr form. Circadian rhythms are maintained by a biological clock
Such phytochrome conversion would function as an “hour- It is clear that organisms have some way of measuring time,
glass,” and the effect of a night would depend simply upon and that they are well adapted to the 24-hour day–night cy-
whether all the phytochrome had been converted. However, cle of our planet. A biological clock resides within the cells of


all eukaryotes and some prokaryotes. The major outward placed in light on a day–night cycle totaling exactly 24 hours,
manifestations of this clock are known as circadian rhythms it would express a rhythm with a period of exactly 24 hours.
(from the Latin circa, “about,” and dies, “day”). However, if an experimenter used a day–night cycle of, say,
We can characterize circadian rhythms, as well as other 22 hours, then over time the rhythm would change—it
regular biological cycles, in two ways: The period is the would be entrained to a 22-hour period.
length of one cycle, and the amplitude is the magnitude of If an organism is maintained under constant darkness, it
the change over the course of a cycle (Figure 39.13). will express a circadian rhythm with an approximately 24-
The circadian rhythms of cyanobacteria, protists, animals, hour period. However, a brief exposure to light under these
fungi, and plants have been found to share some important circumstances can cause a phase shift—that is, it can make the
characteristics: next peak of activity appear either later or earlier than ex-
pected, depending on when the exposure is given. Moreover,
The period is remarkably insensitive to temperature, al-
the organism does not then return to its old schedule if it re-
though lowering the temperature may drastically reduce
mains in darkness. If the first peak is delayed by 6 hours, the
the amplitude of the rhythmic effect.
subsequent peaks are all 6 hours late. Such phase shifts are per-
Circadian rhythms are highly persistent; they may contin-
manent—until the organism receives more exposures to light.
ue for days even in an environment in which there are no
environmental cues, such as light–dark periods.
Circadian rhythms can be entrained, within limits, by Photoreceptors set the biological clock
light–dark cycles that differ from 24 hours. That is, the
Phytochromes and blue-light receptors are known to affect
period an organism expresses can be made to coincide
the period of the biological clock, with the different pigments
with that of the light–dark cycle to which it is exposed.
reporting on different wavelengths and intensities of light.
A brief exposure to light can shift the peak of the cycle—
This diversity of photoreceptors could be an adaptation to
it can cause a phase shift.
the changes in the light environment that a plant experiences
Plants provide innumerable examples of circadian rhy- in the course of a day or a season. How do these photore-
thms. The leaflets of plants such as clover normally hang ceptors interact with a plant’s biological clock?
down and fold at night and rise and unfold during the day. The biological clock of Arabidopsis is based on the activi-
The flowers of many plants show similar “sleep movements,” ties of at least three “clock genes.” The clock genes encode
closing at night and opening during the day. They continue regulatory proteins that interact to produce a circadian oscil-
to open and close on an approximately 24-hour cycle even lation. How does this oscillating clock interact with photore-
when the light and dark periods are experimentally modified. ceptors and the environment?
The period of circadian rhythms in nature is approxi- Arabidopsis is an LDP. Its clock controls the activity of
mately 24 hours. If a clover plant, for example, were to be CONSTANS (a gene that is not part of the clock mechanism)
in such a way that the CONSTANS product, CO protein, ac-
cumulates in one phase of the clock’s cycle—the phase in
which night falls. Under long nights (short days), CO protein
is found at night. Under short nights (long days), CO is also
Circadian rhythms are characterized on the basis of
time, measured in periods of about 24 hours… relatively abundant at dawn and dusk. When CO protein
levels are high, light absorbed by phytochrome A and the
Period (about 24 hours) blue-light receptor cryptochrome 2 leads to flowering (Fig-
ure 39.14). Thus, Arabidopsis flowering results from the coin-
cidence of light (detected by the two photoreceptors) with a
Effect clock-determined phase of the circadian oscillation.
Amplitude
Where is this coincidence-based photoperiodic mechanism
located in relation to where flowering occurs? Is the timing
device for flowering located in a particular plant part, or are
all parts able to sense the length of the night? This question
Time was resolved by “blindfold” experiments, as described next.
…and on the basis of the magnitude of the rhythmic
effect, measured by the cycle‘s amplitude.
Is there a flowering hormone?
39.13 Features of Circadian Rhythms Circadian rhythms, like all
biological rhythms, can be characterized in two ways: by period and It quickly became apparent that each leaf is capable of tim-
by amplitude. ing the night. If a cocklebur plant—an SDP—is kept under a

Under short days, CO protein level Under long days, CO protein levels are high
remains low throughout the light period, enough at both dawn and dusk so that light
and the plant does not flower. absorption by pigments leads to flowering.
39.14 Photoreceptors and the
Biological Clock Interact in Light Dark Light Dark
Photoperiodic Plants One of

Relative concentration of CO

Relative concentration of CO
the genes regulated by the circa- 1.0 1.0
dian clock in Arabidopsis encodes
the CO protein. Flowering
depends on enough CO being
present when photoreceptors
have light available to them. 0.5 0.5


40 40
0 4 8 12 16 18 24 0 4 8 12 16 18 24
Time (hours) Time (hours)

plant does not flower. If, however, the induced leaf re-
regime of short nights and long days, but a leaf is covered so
mains attached to the plant for several hours, the plant
as to give it the needed long nights, the plant will flower (ex-
flowers. This result suggests that something must be syn-
periment A in Figure 39.15). This type of experiment works
thesized in the leaf in response to the inductive dark peri-
best if only one leaf is left on the plant. If one leaf is given a
od and then move out of the leaf to induce flowering.
photoperiodic treatment conducive to flowering—called an
If two cocklebur plants are grafted together, and if one
inductive treatment—other leaves kept under noninductive
plant is exposed to inductive long nights and its graft
conditions will tend to inhibit flowering.
partner exposed to noninductive short nights, both
Although it is the leaves that sense an inductive photope-
plants flower (experiment B in Figure 39.15).
riod, the flowers form elsewhere on the plant. Thus, some
In at least one species, if an induced leaf from one plant
kind of signal must be sent from the leaf to the site of flower
is grafted onto another, noninduced plant, the host plant
formation. Three lines of evidence suggest that this signal is
flowers.
a chemical substance—a flowering hormone.
If a photoperiodically induced leaf is immediately re-
moved from a plant after the inductive dark period, the 39.15 Evidence for a Flowering Hormone If even a single leaf is
exposed to inductive conditions, a signal travels to the entire plant
(and even to other plants, in grafting experiments), inducing it to
flower.

EXPERIMENT A EXPERIMENT B
Question: What part of the plant measures the dark period? Question: How stable is the flowering hormone?

Cocklebur, a short-day If even one leaf is masked for 1 Five cocklebur plants are grafted together
plant, will not flower if part of the day—thus shifting and kept under long days and short nights,
kept under long days that leaf to short days and long with most leaves removed.
and short nights. nights—the plant will flower;
note the burrs. 3 If a leaf on a plant at one end of the chain is
subjected to long nights, all of the plants will flower.

Graft
Burrs 2 A leaf is induced
by long nights/
short days.
Masked leaf

Masked leaf

Hypothetical flowering hormone
Conclusion: The leaves measure the dark period. Therefore,
some signal must move from the induced leaf to the flowering Conclusion: The very stable flowering signal can even travel across multiple
parts of the plant. grafts.


Jan A. D. Zeevaart, a plant physiologist at Michigan State The implications of this finding were of great agricultural
University, performed this last experiment. He exposed a sin- interest in Russia because winter wheat is a better producer
gle leaf of the SDP Perilla to a short-day/long-night regime, than spring wheat, but it cannot be grown in some parts of
inducing the plant to flower. Then he detached this leaf and Russia because the winters are too cold for its survival. Sev-
grafted it onto another, noninduced, Perilla plant—which re- eral studies performed in Russia during the early 1900s
sponded by flowering. The same leaf grafted onto successive demonstrated that if seeds of winter wheat were premoist-
hosts caused each of them to flower in turn. As long as 3 ened and prechilled, they could be sown in the spring and
months after the leaf was exposed to the short-day/long- would develop and flower normally the same year. Thus,
night regime, it could still cause plants to flower. high-yielding winter wheat could be grown even in previ-
Experiments such as Zeevaart’s led to the conclusion that ously hostile regions.
the photoperiodic induction of a leaf causes a more or less This induction of flowering by low temperatures is called
permanent change in the leaf, causing it to start and continue vernalization. Vernalization may require as many as 50 days
producing a flowering hormone that is transported to other of low temperatures (in the range from about –2° to +12°C).
parts of the plant, where the hormone initiates the develop- Some plant species require both vernalization and long days
ment of reproductive structures. Biologists have named this to flower. There is a long wait from the cold days of winter
hypothetical hormone florigen, even though, after decades to the long days of summer, but because the vernalized state
of active searching, it has not been isolated or characterized. easily lasts at least 200 days, these plants do flower when
An elegant experiment suggested that the florigen of SDPs they experience the appropriate night length.
is identical to that of LDPs, even though SDPs produce it
only under long nights and LDPs only under short nights.
An SDP and an LDP were grafted together, and both flow-
Asexual Reproduction
ered, as long as the photoperiodic conditions were inductive Although sexual reproduction takes up most of the space in
for one of the partners. Either the SDP or the LDP could be this chapter, asexual reproduction is responsible for many of
the one induced, but both would always flower. These results the new plant individuals appearing on Earth. This fact sug-
suggest that a flowering hormone—the elusive florigen—was gests that in some circumstances, asexual reproduction must
being transferred from one plant to the other. be advantageous.
The direct demonstration of florigen activity remains a At the beginning of this chapter, we saw that one of the
cherished goal of plant physiologists. For a long time it was advantages of sexual reproduction is genetic recombination.
thought that florigen could be neither a protein nor an RNA Self-fertilization is a form of sexual reproduction, but when
because those molecules were too large to pass from one liv- a plant self-fertilizes, there are fewer opportunities for genetic
ing plant cell to another. However, we now know that such recombination than there are with cross-fertilization. A
macromolecules can be transferred by way of plasmodes- diploid, self-fertilizing plant that is heterozygous for a cer-
mata, and biologists are reexamining the possibility that an tain locus can produce both kinds of homozygotes for that
RNA or a protein is the long-sought florigen. locus plus the heterozygote among its progeny, but it cannot
We have considered the photoperiodic regulation of flow- produce any progeny that carry alleles that it does not itself
ering, from photoreceptors in a leaf to the biological clock to possess. Yet many plants continue to be self-compatible, un-
the need for a signal that travels from the induced leaf to the dergo self-fertilization, and produce viable offspring.
sites of flower formation. However, light is not the only en- Asexual reproduction goes even further than self-fertiliza-
vironmental variable that affects flowering. In some plants, tion: It eliminates genetic recombination altogether. When a
low temperatures are an essential cue that eventually triggers plant reproduces asexually, it produces a clone of progeny that
flowering. are genetically identical to the parent. If a plant is well adapted
to its environment, asexual reproduction may spread its geno-
type throughout that environment. This ability to exploit a par-
Vernalization and Flowering ticular environment is an advantage of asexual reproduction.
Certain cereal grains serve as classic examples of the control
of flowering by temperature. In both wheat and rye, we dis-
tinguish two categories of flowering behavior. Spring wheat, There are many forms of asexual reproduction
for example, is sown in the spring and flowers in the same We call stems, leaves, and roots vegetative organs to distin-
year. It is an annual plant. Winter wheat is biennial and must guish them from flowers, the reproductive parts of the plant.
be sown in the fall; it flowers in the following summer. If The modification of a vegetative organ is what makes vege-
winter wheat is not exposed to cold after its first year, it will tative reproduction—asexual reproduction in plants—pos-
not flower normally the next year. sible. In many cases, the stem is the organ that is modified.


Strawberries and some grasses, for example, produce hori- with stolons or rhizomes, such as beach grasses, rushes, and
zontal stems, called stolons or runners, that grow along the sand verbena, are common pioneers on coastal sand dunes.
soil surface, form roots at intervals, and establish potentially Rapid vegetative reproduction enables these plants, once in-
independent plants (see Figure 35.4b). Tip layers are upright troduced, not only to multiply but also to survive burial by
branches whose tips sag to the ground and develop roots, as the shifting sand; in addition, the dunes are stabilized by the
in blackberry and forsythia. extensive network of rhizomes or stolons that develops. Veg-
Some plants, such as potatoes, form enlarged fleshy tips etative reproduction is also common in some deserts, where
of underground stems, called tubers (see Figure 35.4a). Rhi- the environment is not often suitable for seed germination
zomes are horizontal underground stems that can give rise to and the establishment of seedlings.
new shoots. Bamboo is a striking example of a plant that re- Dandelions, citrus trees, and some other plants reproduce
produces vegetatively by means of rhizomes. A single bam- by the asexual production of seeds, called apomixis. As we
boo plant can give rise to a stand—even a forest—of plants have seen, meiosis reduces the number of chromosomes in
constituting a single, physically connected entity. gametes, and fertilization restores the sporophytic number
Whereas stolons and rhizomes are horizontal stems, bulbs of chromosomes in the zygote. Some plants can skip over
and corms are short, vertical, underground stems. Lilies and both meiosis and fertilization and still produce seeds.
onions form bulbs (Figure 39.16a), short stems with many Apomixis produces seeds within the ovary without the min-
fleshy, highly modified leaves that store nutrients. These stor- gling and segregation of chromosomes and without the
age leaves make up most of the bulb. Bulbs are thus large un- union of gametes. The ovule simply develops into a seed,
derground buds. They can give rise to new plants by divid- and the ovary wall develops into a fruit. An apomictic em-
ing or by producing new bulbs from lateral buds. Crocuses, bryo has the sporophytic number (2n) of chromosomes. The
gladioli, and many other plants produce corms, underground result of apomixis is a fruit with seeds that are genetically
stems that function very much as bulbs do. Corms are identical to the parent plant.
disclike and consist primarily of stem tissue; they lack the Apomixis sometimes requires pollination. In some
fleshy modified leaves that are characteristic of bulbs. apomictic species, a sperm nucleus must combine with the
Not all vegetative organs modified for reproduction are polar nuclei in order for the endosperm to form. In other
stems. Leaves may also be the source of new plantlets, as in the apomictic species, the pollen provides the signals for embryo
succulent plants of the genus Kalanchoe (Figure 39.16b). Many and endosperm formation, although neither sperm nucleus
kinds of angiosperms, ranging from grasses to trees such as as- participates in fertilization. This observation emphasizes that
pens and poplars, form interconnected, genetically homoge- pollination and fertilization are not the same thing.
neous populations by means of suckers—shoots produced by
roots. What appears to be a whole stand of aspen trees, for ex-
ample, may be a clone derived from a single tree by suckers. Asexual reproduction is important in agriculture
Plants that reproduce vegetatively often grow in physi- Farmers and gardeners take advantage of some natural
cally unstable environments, such as eroding hillsides. Plants forms of vegetative reproduction. They have also developed

(a)

(b)

Storage leaves grow
from the stem of this
onion.
The plantlets forming on the margin of this
Kalanchoe leaf will fall to the ground and
start independent lives.
The short stem is
visible at the bottom
of the bulb. 39.16 Vegetative Organs Modified for Reproduction (a) Bulbs are
short stems with large buds that store nutrients and can give rise to new
Allium sp. plants. (b) In Kalanchoe, new plantlets can form on leaves.


Recombinant DNA techniques applied to tissue cultures
can provide plants with increased resistance to pests or in-
Scion creased nutritive value to humans. There is also interest in
making certain valuable, sexually reproducing plants capable
In grafting, the scion is aligned so that
its vascular cambium is adjacent to the
of apomixis. By causing cells of different types to fuse, one can
vascular cambium in the stock. obtain plants with exciting new combinations of properties.

Stock Chapter Summary
Many Ways to Reproduce
Almost all flowering plants reproduce sexually, and many
39.17 Grafting Grafting—attaching a piece of a plant to the root or also reproduce asexually. Both sexual and asexual reproduction
root-bearing stem of another plant—is a common horticultural tech- are important in agriculture.
nique.The “host” root or stem is the stock; the upper grafted piece is
the scion.
Sexual Reproduction in Plants
Sexual reproduction promotes genetic diversity in a popula-
tion, which may give the population an advantage under chang-
new types of asexual reproduction by manipulating plants. ing environmental conditions or in exploiting new territory.
One of the oldest methods of vegetative reproduction used The flower is an angiosperm’s device for sexual reproduction.
in agriculture consists of simply making cuttings of stems, Flowering plants have microscopic gametophytes that devel-
inserting them in soil, and waiting for them to form roots and op within the flowers of the sporophytes. The megagameto-
phyte is the embryo sac, which typically contains eight nuclei in
thus become autonomous plants. The cuttings are usually en- a total of seven cells. The microgametophyte is the pollen grain,
couraged to root by treatment with a plant hormone, auxin, which usually contains two cells. Review Figure 39.1. See
as described in Chapter 38. Web/CD Tutorial 39.1
Horticulturists reproduce many woody plants by graft- Pollination enables fertilization in the absence of external water.
ing—attaching a bud or a piece of stem from one plant to the In self-incompatible species, the stigma or style rejects pollen
from the same plant. Review Figure 39.3
root or root-bearing stem of another plant. The part of the re-
The pollen grain delivers sperm cells to the embryo sac by
sulting plant that comes from the root-bearing “host” is means of a pollen tube.
called the stock; the part grafted on is the scion (Figure 39.17). Most angiosperms perform double fertilization: One sperm
In order for a graft to succeed, the vascular cambium of the nucleus fertilizes the egg, forming a zygote, and the other
scion must become associated with that of the stock. By cell sperm nucleus unites with the two polar nuclei to form a
triploid endosperm. Review Figure 39.5
division, both cambia form masses of wound tissue. If the two The zygote develops into an embryo (with an attached sus-
masses meet and fuse, the resulting continuous cambium can pensor), which remains quiescent in the seed until conditions
produce xylem and phloem, allowing transport of water and are right for germination. The endosperm supplies the nutritive
minerals to the scion and of photosynthate to the stock. Grafts reserve upon which the embryo depends at germination.
Review Figures 39.6, 39.7. See Web/CD Activity 39.1
are most often successful when the stock and scion belong to Flowers develop into seed-bearing fruits, which often play
the same or closely related species. Most fruit grown for mar- important roles in the dispersal of the species.
ket in the United States is produced on grafted trees.
Scientists in universities and industrial laboratories have
The Transition to the Flowering State
For a vegetatively growing plant to flower, an apical meri-
been developing new ways to produce useful plants via tis- stem in the shoot system must become an inflorescence meri-
sue culture. Because many plant cells are totipotent (see Fig- stem, which gives rise to bracts as well as more meristems. The
ure 19.3), cultures of undifferentiated tissue can give rise to meristems it produces may become floral meristems or addi-
tional inflorescence meristems. Review Figure 39.9
entire plants, as can small pieces of tissue cut directly from a
Flowering results from a cascade of gene expression. Floral
parent plant. Tissue cultures are used commercially to pro- organ identity genes are expressed in floral meristems that give
duce numerous new plants rapidly without resorting to seeds. rise to sepals, petals, stamens, and carpels.
Culturing tiny bits of apical meristem can produce plants
Photoperiodic Control of Flowering
free of viruses. Because apical meristems lack developed vas-
Photoperiodic plants regulate their flowering by measuring
cular tissues, viruses tend not to enter them. Treatment with the length of light and dark periods.
hormones causes a single apical meristem to give rise to 20 Short-day plants flower when the days are shorter than a
or more shoots; thus, a single plant can give rise to millions species-specific critical day length; long-day plants flower when
of genetically identical plants within a year by repeated the days are longer than a critical day length. Review Figure 39.10
Some angiosperms have more complex photoperiodic
meristem culturing. Using this approach, strawberry and po- requirements than short-day or long-day plants have, but most
tato producers are able to start each year’s crop from virus- are day-neutral.
free plants.

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