Chapter 32 | Plant Reproduction 971

32 | PLANT
REPRODUCTION

Figure 32.1 Plants that reproduce sexually often achieve fertilization with the help of pollinators such as (a) bees, (b)
birds, and (c) butterflies. (credit a: modification of work by John Severns; credit b: modification of work by Charles J.
Sharp; credit c: modification of work by "Galawebdesign"/Flickr)

Chapter Outline
32.1: Reproductive Development and Structure
32.2: Pollination and Fertilization
32.3: Asexual Reproduction

Introduction
Plants have evolved different reproductive strategies for the continuation of their species. Some plants reproduce
sexually, and others asexually, in contrast to animal species, which rely almost exclusively on sexual
reproduction. Plant sexual reproduction usually depends on pollinating agents, while asexual reproduction is
independent of these agents. Flowers are often the showiest or most strongly scented part of plants. With their
bright colors, fragrances, and interesting shapes and sizes, flowers attract insects, birds, and animals to serve
their pollination needs. Other plants pollinate via wind or water; still others self-pollinate.

32.1 | Reproductive Development and Structure

By the end of this section, you will be able to do the following:
• Describe the two stages of a plant’s lifecycle
• Compare and contrast male and female gametophytes and explain how they form in angiosperms
• Describe the reproductive structures of a plant
• Describe the components of a complete flower
• Describe the development of microsporangium and megasporangium in gymnosperms

Sexual reproduction takes place with slight variations in different groups of plants. Plants have two distinct
stages in their lifecycle: the gametophyte stage and the sporophyte stage. The haploid gametophyte produces
the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and females
gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid
sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte.
The new gametophyte produces gametes, and the cycle continues. This is the alternation of generations, and is
972 Chapter 32 | Plant Reproduction

typical of plant reproduction (Figure 32.2).

Figure 32.2 The alternation of generations in angiosperms is depicted in this diagram. (credit: modification of work by
Peter Coxhead)

The life cycle of higher plants is dominated by the sporophyte stage, with the gametophyte borne on the
sporophyte. In ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. In
bryophytes, such as mosses, the haploid gametophyte is more developed than the sporophyte.
During the vegetative phase of growth, plants increase in size and produce a shoot system and a root system.
As they enter the reproductive phase, some of the branches start to bear flowers. Many flowers are borne singly,
whereas some are borne in clusters. The flower is borne on a stalk known as a receptacle. Flower shape, color,
and size are unique to each species, and are often used by taxonomists to classify plants.

Sexual Reproduction in Angiosperms

The lifecycle of angiosperms follows the alternation of generations explained previously. The haploid
gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms.
Flowers contain the plant’s reproductive structures.
Flower Structure
A typical flower has four main parts—or whorls—known as the calyx, corolla, androecium, and gynoecium
(Figure 32.3). The outermost whorl of the flower has green, leafy structures known as sepals. The sepals,
collectively called the calyx, help to protect the unopened bud. The second whorl is comprised of petals—usually,
brightly colored—collectively called the corolla. The number of sepals and petals varies depending on whether
the plant is a monocot or dicot. In monocots, petals usually number three or multiples of three; in dicots, the
number of petals is four or five, or multiples of four and five. Together, the calyx and corolla are known as
the perianth. The third whorl contains the male reproductive structures and is known as the androecium. The
androecium has stamens with anthers that contain the microsporangia. The innermost group of structures in
the flower is the gynoecium, or the female reproductive component(s). The carpel is the individual unit of the
gynoecium and has a stigma, style, and ovary. A flower may have one or multiple carpels.

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 973

Figure 32.3 The four main parts of the flower are the calyx, corolla, androecium, and gynoecium. The androecium
is the sum of all the male reproductive organs, and the gynoecium is the sum of the female reproductive organs.
(credit: modification of work by Mariana Ruiz Villareal)

If the anther is missing, what type of reproductive structure will the flower be unable to produce? What term
is used to describe an incomplete flower lacking the androecium? What term describes an incomplete flower
lacking a gynoecium?

If all four whorls (the calyx, corolla, androecium, and gynoecium) are present, the flower is described as
complete. If any of the four parts is missing, the flower is known as incomplete. Flowers that contain both
an androecium and a gynoecium are called perfect, androgynous or hermaphrodites. There are two types
of incomplete flowers: staminate flowers contain only an androecium, and carpellate flowers have only a
gynoecium (Figure 32.4).
974 Chapter 32 | Plant Reproduction

Figure 32.4 The corn plant has both staminate (male) and carpellate (female) flowers. Staminate flowers, which are
clustered in the tassel at the tip of the stem, produce pollen grains. Carpellate flowers are clustered in the immature
ears. Each strand of silk is a stigma. The corn kernels are seeds that develop on the ear after fertilization. Also shown
is the lower stem and root.

If both male and female flowers are borne on the same plant, the species is called monoecious (meaning “one
home”): examples are corn and pea. Species with male and female flowers borne on separate plants are termed
dioecious, or “two homes,” examples of which are C. papaya and Cannabis. The ovary, which may contain one
or multiple ovules, may be placed above other flower parts, which is referred to as superior; or, it may be placed
below the other flower parts, referred to as inferior (Figure 32.5).

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 975

Figure 32.5 The (a) lily is a superior flower, which has the ovary above the other flower parts. (b) Fuchsia is an inferior
flower, which has the ovary beneath other flower parts. (credit a photo: modification of work by Benjamin Zwittnig;
credit b photo: modification of work by "Koshy Koshy"/Flickr)

Male Gametophyte (The Pollen Grain)

The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive
organs, development of pollen takes place in a structure known as the microsporangium (Figure 32.6). The
microsporangia, which are usually bilobed, are pollen sacs in which the microspores develop into pollen grains.
These are found in the anther, which is at the end of the stamen—the long filament that supports the anther.
976 Chapter 32 | Plant Reproduction

Figure 32.6 Shown is (a) a cross section of an anther at two developmental stages. The immature anther (top) contains
four microsporangia, or pollen sacs. Each microsporangium contains hundreds of microspore mother cells that will
each give rise to four pollen grains. The tapetum supports the development and maturation of the pollen grains. Upon
maturation of the pollen (bottom), the pollen sac walls split open and the pollen grains (male gametophytes) are
released, as shown in the (b) micrograph of an immature lily anther. In these scanning electron micrographs, pollen
sacs are ready to burst, releasing their grains. (credit a: modification of work by LibreTexts; b: modification of work by
Robert R. Wise; scale-bar data from Matt Russell)

Within the microsporangium, the microspore mother cell divides by meiosis to give rise to four microspores, each
of which will ultimately form a pollen grain (Figure 32.7). An inner layer of cells, known as the tapetum, provides
nutrition to the developing microspores and contributes key components to the pollen wall. Mature pollen grains
contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen
tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to
enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes
(sperm cells). Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 977

Figure 32.7 Pollen develops from the microspore mother cells. The mature pollen grain is composed of two cells: the
pollen tube cell and the generative cell, which is inside the tube cell. The pollen grain has two coverings: an inner layer
(intine) and an outer layer (exine). The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains.
(credit “pollen micrograph”: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine (Figure 32.7). The exine
contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows
the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without
undergoing damage.
Female Gametophyte (The Embryo Sac)
While the details may vary between species, the overall development of the female gametophyte has two distinct
phases. First, in the process of megasporogenesis, a single cell in the diploid megasporangium—an area
of tissue in the ovules—undergoes meiosis to produce four megaspores, only one of which survives. During
the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an
eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac. Two of
the nuclei—the polar nuclei—move to the equator and fuse, forming a single, diploid central cell. This central
cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the
embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus
closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into
synergid cells (Figure 32.8). The synergids help guide the pollen tube for successful fertilization, after which
they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the
978 Chapter 32 | Plant Reproduction

fertilized ovule forms the other tissues of the seed.

A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will
develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the
fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening
called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.

Figure 32.8 As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments
and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central
cell, and the egg cell.

An embryo sac is missing the synergids. What specific impact would you expect this to have on fertilization?
a. The pollen tube will be unable to form.
b. The pollen tube will form but will not be guided toward the egg.
c. Fertilization will not occur because the synergid is the egg.
d. Fertilization will occur but the embryo will not be able to grow.

Sexual Reproduction in Gymnosperms

As with angiosperms, the lifecycle of a gymnosperm is also characterized by alternation of generations. In
conifers such as pines, the green leafy part of the plant is the sporophyte, and the cones contain the male
and female gametophytes (Figure 32.9). The female cones are larger than the male cones and are positioned
towards the top of the tree; the small, male cones are located in the lower region of the tree. Because the pollen
is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 979

Figure 32.9 This image shows the lifecycle of a conifer. Pollen from male cones blows up into upper branches, where
it fertilizes female cones. Examples are shown of female and male cones. (credit “female”: modification of work by
“Geographer”/Wikimedia Commons; credit “male”: modification of work by Roger Griffith)

Male Gametophyte
A male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts are known as
microsporophylls (Figure 32.10) and are the sites where microspores will develop. The microspores develop
inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis
to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative
nucleus, and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones
and is carried by the wind to land on the female cone.

Watch this video to see a cedar releasing its pollen in the wind. (This multimedia resource will open in a
browser.) (http://cnx.org/content/m66607/1.3/#eip-id1168023726478)

Female Gametophyte
The female cone also has a central axis on which bracts known as megasporophylls (Figure 32.10) are
present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore
mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form
the multicellular female gametophyte, while the others divide to form the rest of the structure. The female
gametophyte is contained within a structure called the archegonium.
980 Chapter 32 | Plant Reproduction

Figure 32.10 This series of micrographs shows male and female gymnosperm gametophytes. (a) This male cone,
shown in cross section, has approximately 20 microsporophylls, each of which produces hundreds of male
gametophytes (pollen grains). (b) Pollen grains are visible in this single microsporophyll. (c) This micrograph shows an
individual pollen grain. (d) This cross section of a female cone shows portions of about 15 megasporophylls. (e) The
ovule can be seen in this single megasporophyll. (f) Within this single ovule are the megaspore mother cell (MMC),
micropyle, and a pollen grain. (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Reproductive Process
Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative
cell migrates towards the female gametophyte through the micropyle. It takes approximately one year for
the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the
generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After
fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of
the cones are closed during development of the seed. The seed is covered by a seed coat, which is derived
from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be
dispersed, the bracts of the female cones open to allow the dispersal of seed; no fruit formation takes place
because gymnosperm seeds have no covering.

Angiosperms versus Gymnosperms

Gymnosperm reproduction differs from that of angiosperms in several ways (Figure 32.11). In angiosperms, the
female gametophyte exists in an enclosed structure—the ovule—which is within the ovary; in gymnosperms,
the female gametophyte is present on exposed bracts of the female cone. Double fertilization is a key event
in the lifecycle of angiosperms, but is completely absent in gymnosperms. The male and female gametophyte
structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are
a part of the flower. Lastly, wind plays an important role in pollination in gymnosperms because pollen is blown by
the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination
is more common.

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 981

Figure 32.11 (a) Angiosperms are flowering plants, and include grasses, herbs, shrubs and most deciduous trees,
while (b) gymnosperms are conifers. Both produce seeds but have different reproductive strategies. (credit a:
modification of work by Wendy Cutler; credit b: modification of work by Lews Castle UHI)

View an animation of the double fertilization process of angiosperms. (This multimedia resource will open
in a browser.) (http://cnx.org/content/m66607/1.3/#eip-id4418315)

32.2 | Pollination and Fertilization

By the end of this section, you will be able to do the following:
• Describe what must occur for plant fertilization
• Explain cross-pollination and the ways in which it takes place
• Describe the process that leads to the development of a seed
• Define double fertilization

In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the
same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the
female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg.
Pollination has been well studied since the time of Gregor Mendel. Mendel successfully carried out self- as well
as cross-pollination in garden peas while studying how characteristics were passed on from one generation to
the next. Today’s crops are a result of plant breeding, which employs artificial selection to produce the present-
day cultivars. A case in point is today's corn, which is a result of years of breeding that started with its ancestor,
teosinte. The teosinte that the ancient Mayans originally began cultivating had tiny seeds—vastly different from
today’s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the
genetic difference between them is miniscule.
Pollination takes two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from
the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination
is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of
the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time, and
are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an
982 Chapter 32 | Plant Reproduction

investment from the plant to provide nectar and pollen as food for pollinators.

Explore this interactive website (http://openstaxcollege.org/l/pollination) to review self-pollination and

cross-pollination.

Living species are designed to ensure survival of their progeny; those that fail become extinct. Genetic diversity
is therefore required so that in changing environmental or stress conditions, some of the progeny can survive.
Self-pollination leads to the production of plants with less genetic diversity, since genetic material from the same
plant is used to form gametes, and eventually, the zygote. In contrast, cross-pollination—or out-crossing—leads
to greater genetic diversity because the microgametophyte and megagametophyte are derived from different
plants.
Because cross-pollination allows for more genetic diversity, plants have developed many ways to avoid self-
pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-
pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature
and can only be pollinated by pollen from another flower. Some flowers have developed physical features
that prevent self-pollination. The primrose is one such flower. Primroses have evolved two flower types with
differences in anther and stigma length: the pin-eyed flower has anthers positioned at the pollen tube’s halfway
point, and the thrum-eyed flower’s stigma is likewise located at the halfway point. Insects easily cross-pollinate
while seeking the nectar at the bottom of the pollen tube. This phenomenon is also known as heterostyly. Many
plants, such as cucumber, have male and female flowers located on different parts of the plant, thus making self-
pollination difficult. In yet other species, the male and female flowers are borne on different plants (dioecious). All
of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. The majority
of pollinators are biotic agents such as insects (like bees, flies, and butterflies), bats, birds, and other animals.
Other plant species are pollinated by abiotic agents, such as wind and water.

Incompatibility Genes in Flowers

In recent decades, incompatibility genes—which prevent pollen from germinating or growing into the stigma
of a flower—have been discovered in many angiosperm species. If plants do not have compatible genes,
the pollen tube stops growing. Self-incompatibility is controlled by the S (sterility) locus. Pollen tubes have
to grow through the tissue of the stigma and style before they can enter the ovule. The carpel is selective
in the type of pollen it allows to grow inside. The interaction is primarily between the pollen and the stigma
epidermal cells. In some plants, like cabbage, the pollen is rejected at the surface of the stigma, and the
unwanted pollen does not germinate. In other plants, pollen tube germination is arrested after growing
one-third the length of the style, leading to pollen tube death. Pollen tube death is due either to apoptosis
(programmed cell death) or to degradation of pollen tube RNA. The degradation results from the activity
of a ribonuclease encoded by the S locus. The ribonuclease is secreted from the cells of the style in the
extracellular matrix, which lies alongside the growing pollen tube.
In summary, self-incompatibility is a mechanism that prevents self-fertilization in many flowering plant
species. The working of this self-incompatibility mechanism has important consequences for plant breeders
because it inhibits the production of inbred and hybrid plants.

Pollination by Insects
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees (Figure
32.12). The most common species of bees are bumblebees and honeybees. Since bees cannot see the color
red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees collect energy-rich pollen
or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored,

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 983

have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A
nectar guide includes regions on the flower petals that are visible only to bees, and not to humans; it helps
to guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks
to the bees’ fuzzy hair, and when the bee visits another flower, some of the pollen is transferred to the second
flower. Recently, there have been many reports about the declining population of honeybees. Many flowers will
remain unpollinated and not bear seed if honeybees disappear. The impact on commercial fruit growers could
be devastating.

Figure 32.12 Insects, such as bees, are important agents of pollination. (credit: modification of work by Jon Sullivan)

Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which
produce nectar, usually have dull colors, such as brown or purple. They are found on the corpse flower or voodoo
lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapleia, Rafflesia). The nectar provides
energy, whereas the pollen provides protein. Wasps are also important insect pollinators, and pollinate many
species of figs.
Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters.
These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to
make access to nectar easier. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other
hand, pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white
and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant,
which is pollinated by the yucca moth. The shape of the flower and moth have adapted in such a way as to allow
successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female
moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and
developing seeds. Thus, both the insect and flower benefit from each other in this symbiotic relationship. The
corn earworm moth and Gaura plant have a similar relationship (Figure 32.13).

Figure 32.13 A corn earworm sips nectar from a night-blooming Gaura plant. (credit: Juan Lopez, USDA ARS)
984 Chapter 32 | Plant Reproduction

Pollination by Bats
In the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning
glory. The flowers are usually large and white or pale-colored; thus, they can be distinguished from the dark
surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar.
They are naturally large and wide-mouthed to accommodate the head of the bat. As the bats seek the nectar,
their faces and heads become covered with pollen, which is then transferred to the next flower.

Pollination by Birds
Many species of small birds, such as the hummingbird (Figure 32.14) and sun birds, are pollinators for plants
such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in such a way
as to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower
typically has a curved, tubular shape, which allows access for the bird’s beak. Brightly colored, odorless flowers
that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited
on the bird’s head and neck and is then transferred to the next flower it visits. Botanists have been known to
determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from
the same site.

Figure 32.14 Hummingbirds have adaptations that allow them to reach the nectar of certain tubular flowers. (credit:
Lori Branham)

Pollination by Wind
Most species of conifers, and many angiosperms, such as grasses, maples and oaks, are pollinated by wind.
Pine cones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green,
small, may have small or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated
flowers, flowers adapted to pollination by wind do not produce nectar or scent. In wind-pollinated species, the
microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it (Figure
32.15). The flowers usually emerge early in the spring, before the leaves, so that the leaves do not block the
movement of the wind. The pollen is deposited on the exposed feathery stigma of the flower (Figure 32.16).

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 985

Figure 32.15 A person knocks pollen from a pine tree.

Figure 32.16 These male (a) and female (b) catkins are from the goat willow tree (Salix caprea). Note how both
structures are light and feathery to better disperse and catch the wind-blown pollen.

Pollination by Water
Some weeds, such as Australian sea grass and pond weeds, are pollinated by water. The pollen floats on water,
and when it comes into contact with the flower, it is deposited inside the flower.
986 Chapter 32 | Plant Reproduction

Pollination by Deception
Orchids are highly valued flowers, with many rare varieties (Figure 32.17). They grow in a range of specific
habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of
orchids have been identified.

Figure 32.17 Certain orchids use food deception or sexual deception to attract pollinators. Shown here is a bee
orchid (Ophrys apifera). (credit: David Evans)

Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid
are an exception to this standard: they have evolved different ways to attract the desired pollinators.
They use a method known as food deception, in which bright colors and perfumes are offered, but no
food. Anacamptis morio, commonly known as the green-winged orchid, bears bright purple flowers and
emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong
scent—which usually indicates food for a bee—and in the process, picks up the pollen to be transported to
another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as
the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent,
lands on the orchid flower, and in the process, transfers pollen. Some orchids, like the Australian hammer
orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps. The
flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries
to mate with what appears to be a female wasp, and in the process, picks up pollen, which it then transfers
to the next counterfeit mate.

Double Fertilization
After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The
microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell
grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires
water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen
tube’s growth is supported by the tissues of the style. In the meantime, if the generative cell has not already split
into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by
the synergids present in the embryo sac, and it enters the ovule sac through the micropyle. Of the two sperm
cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei,
forming a triploid cell that develops into the endosperm. Together, these two fertilization events in angiosperms

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 987

are known as double fertilization (Figure 32.18). After fertilization is complete, no other sperm can enter. The
fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.

Figure 32.18 In angiosperms, one sperm fertilizes the egg to form the 2n zygote, and the other sperm fertilizes the
central cell to form the 3n endosperm. This is called a double fertilization.

After fertilization, the zygote divides to form two cells: the upper cell, or terminal cell, and the lower, or basal, cell.
The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal
tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing
embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo (Figure 32.19a). In dicots
(eudicots), the developing embryo has a heart shape, due to the presence of the two rudimentary cotyledons
(Figure 32.19b). In non-endospermic dicots, such as Capsella bursa, the endosperm develops initially, but is
then digested, and the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge,
they run out of room inside the developing seed, and are forced to bend (Figure 32.19c). Ultimately, the embryo
and cotyledons fill the seed (Figure 32.19d), and the seed is ready for dispersal. Embryonic development is
suspended after some time, and growth is resumed only when the seed germinates. The developing seedling
will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
988 Chapter 32 | Plant Reproduction

Figure 32.19 Shown are the stages of embryo development in the ovule of a shepherd’s purse (Capsella bursa).
After fertilization, the zygote divides to form an upper terminal cell and a lower basal cell. (a) In the first stage of
development, the terminal cell divides, forming a globular pro-embryo. The basal cell also divides, giving rise to the
suspensor. (b) In the second stage, the developing embryo has a heart shape due to the presence of cotyledons. (c)
In the third stage, the growing embryo runs out of room and starts to bend. (d) Eventually, it completely fills the seed.
(credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Development of the Seed

The mature ovule develops into the seed. A typical seed contains a seed coat, cotyledons, endosperm, and a
single embryo (Figure 32.20).

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 989

Figure 32.20 The structures of dicot and monocot seeds are shown. Dicots (left) have two cotyledons. Monocots,
such as corn (right), have one cotyledon, called the scutellum; it channels nutrition to the growing embryo. Both
monocot and dicot embryos have a plumule that forms the leaves, a hypocotyl that forms the stem, and a radicle
that forms the root. The embryonic axis comprises everything between the plumule and the radicle, not including
the cotyledon(s).

What of the following statements is true?

a. Both monocots and dicots have an endosperm.
b. The radicle develops into the root.
c. The plumule is part of the epicotyl.
d. The endosperm is part of the embryo.

The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, such as
corn and wheat, the single cotyledon is called a scutellum; the scutellum is connected directly to the embryo
via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Upon germination,
enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the
endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins and lipids, the products
of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo.
Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.
The two cotyledons in the dicot seed also have vascular connections to the embryo. In endospermic dicots, the
food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive
organs to take up the enzymatically released food reserves, much like in monocots (monocots, by definition,
also have endospermic seeds). Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper
(Capsicum annuum) are examples of endospermic dicots. In non-endospermic dicots, the triploid endosperm
develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and
moved into the developing cotyledon for storage. The two halves of a peanut seed (Arachis hypogaea) and the
split peas (Pisum sativum) of split pea soup are individual cotyledons loaded with food reserves.
The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac.
In dicots, the seed coat is further divided into an outer coat known as the testa and inner coat known as the
tegmen.
The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo
between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the
cotyledons”). The embryonic axis terminates in a radicle (the embryonic root), which is the region from which the
root will develop. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant. In monocots,
the hypocotyl does not show above ground because monocots do not exhibit stem elongation. The part of the
embryonic axis that projects above the cotyledons is known as the epicotyl. The plumule is composed of the
epicotyl, young leaves, and the shoot apical meristem.
Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards. This
shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the
990 Chapter 32 | Plant Reproduction

epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to
light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl
continues to elongate. During this time, the radicle is also growing and producing the primary root. As it grows
downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system.
In monocot seeds (Figure 32.21), the testa and tegmen of the seed coat are fused. As the seed germinates,
the primary root emerges, protected by the root-tip covering: the coleorhiza. Next, the primary shoot emerges,
protected by the coleoptile: the covering of the shoot tip. Upon exposure to light (i.e., when the plumule has
exited the soil and the protective coleoptile is no longer needed), elongation of the coleoptile ceases and the
leaves expand and unfold. At the other end of the embryonic axis, the primary root soon dies, while other,
adventitious roots (roots that do not arise from the usual place – i.e., the root) emerge from the base of the stem.
This gives the monocot a fibrous root system.

Figure 32.21 As this monocot grass seed germinates, the primary root, or radicle, emerges first, followed by the
primary shoot, or coleoptile, and the adventitious roots.

Seed Germination
Many mature seeds enter a period of inactivity, or extremely low metabolic activity: a process known as
dormancy, which may last for months, years, or even centuries. Dormancy helps keep seeds viable during
unfavorable conditions. Upon a return to favorable conditions, seed germination takes place. Favorable
conditions could be as diverse as moisture, light, cold, fire, or chemical treatments. After heavy rains, many new
seedlings emerge. Forest fires also lead to the emergence of new seedlings. Some seeds require vernalization
(cold treatment) before they can germinate. This guarantees that seeds produced by plants in temperate
climates will not germinate until the spring. Plants growing in hot climates may have seeds that need a heat
treatment in order to germinate, to avoid germination in the hot, dry summers. In many seeds, the presence of a
thick seed coat retards the ability to germinate. Scarification, which includes mechanical or chemical processes
to soften the seed coat, is often employed before germination. Presoaking in hot water, or passing through an
acid environment, such as an animal’s digestive tract, may also be employed.
Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have
enough food reserves to germinate deep below ground, and still extend their epicotyl all the way to the soil
surface. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only
germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath
the surface, the developing seedling would not have enough food reserves to reach the sunlight.

Development of Fruit and Fruit Types

After fertilization, the ovary of the flower usually develops into the fruit. Fruits are usually associated with having
a sweet taste; however, not all fruits are sweet. Botanically, the term “fruit” is used for a ripened ovary. In most
cases, flowers in which fertilization has taken place will develop into fruits, and flowers in which fertilization has
not taken place will not. Some fruits develop from the ovary and are known as true fruits, whereas others develop
from other parts of the female gametophyte and are known as accessory fruits. The fruit encloses the seeds and
the developing embryo, thereby providing it with protection. Fruits are of many types, depending on their origin

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 991

and texture. The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull
of corn (the tough, thin part that gets stuck in your teeth when you eat popcorn) are all fruits. As the fruit matures,
the seeds also mature.
Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin (Figure 32.22).
If the fruit develops from a single carpel or fused carpels of a single ovary, it is known as a simple fruit, as seen
in nuts and beans. An aggregate fruit is one that develops from more than one carpel, but all are in the same
flower: the mature carpels fuse together to form the entire fruit, as seen in the raspberry. Multiple fruit develops
from an inflorescence or a cluster of flowers. An example is the pineapple, where the flowers fuse together to
form the fruit. Accessory fruits (sometimes called false fruits) are not derived from the ovary, but from another
part of the flower, such as the receptacle (strawberry) or the hypanthium (apples and pears).

Figure 32.22 There are four main types of fruits. Simple fruits, such as these nuts, are derived from a single ovary.
Aggregate fruits, like raspberries, form from many carpels that fuse together. Multiple fruits, such as pineapple, form
from a cluster of flowers called an inflorescence. Accessory fruit, like the apple, are formed from a part of the plant
other than the ovary. (credit "nuts": modification of work by Petr Kratochvil; credit "raspberries": modification of work by
Cory Zanker; credit "pineapple": modification of work by Howie Le; credit "apple": modification of work by Paolo Neo)

Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of
the fruit), and the endocarp (the inner part of the fruit). Together, all three are known as the pericarp. The
mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the endocarp
is the edible part. In many fruits, two or all three of the layers are fused, and are indistinguishable at maturity.
Fruits can be dry or fleshy. Furthermore, fruits can be divided into dehiscent or indehiscent types. Dehiscent
fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release
their seeds.

Fruit and Seed Dispersal

The fruit has a single purpose: seed dispersal. Seeds contained within fruits need to be dispersed far from the
992 Chapter 32 | Plant Reproduction

mother plant, so they may find favorable and less competitive conditions in which to germinate and grow.
Some fruit have built-in mechanisms so they can disperse by themselves, whereas others require the help of
agents like wind, water, and animals (Figure 32.23). Modifications in seed structure, composition, and size help
in dispersal. Wind-dispersed fruit are lightweight and may have wing-like appendages that allow them to be
carried by the wind. Some have a parachute-like structure to keep them afloat. Some fruits—for example, the
dandelion—have hairy, weightless structures that are suited to dispersal by wind.
Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are
well known for their ability to float on water to reach land where they can germinate. Similarly, willow and silver
birches produce lightweight fruit that can float on water.
Animals and birds eat fruits, and the seeds that are not digested are excreted in their droppings some distance
away. Some animals, like squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash
of fruit, and if conditions are favorable, the seeds germinate. Some fruits, like the cocklebur, have hooks or sticky
structures that stick to an animal's coat and are then transported to another place. Humans also play a big role in
dispersing seeds when they carry fruits to new places and throw away the inedible part that contains the seeds.
All of the above mechanisms allow for seeds to be dispersed through space, much like an animal’s offspring can
move to a new location. Seed dormancy, which was described earlier, allows plants to disperse their progeny
through time: something animals cannot do. Dormant seeds can wait months, years, or even decades for the
proper conditions for germination and propagation of the species.

(a) (b) (c)

Figure 32.23 Fruits and seeds are dispersed by various means. (a) Dandelion seeds are dispersed by wind, the (b)
coconut seed is dispersed by water, and the (c) acorn is dispersed by animals that cache and then forget it. (credit a:
modification of work by "Rosendahl"/Flickr; credit b: modification of work by Shine Oa; credit c: modification of work by
Paolo Neo)

32.3 | Asexual Reproduction

By the end of this section, you will be able to do the following:
• Compare the mechanisms and methods of natural and artificial asexual reproduction
• Describe the advantages and disadvantages of natural and artificial asexual reproduction
• Discuss plant life spans

Many plants are able to propagate themselves using asexual reproduction. This method does not require
the investment required to produce a flower, attract pollinators, or find a means of seed dispersal. Asexual
reproduction produces plants that are genetically identical to the parent plant because no mixing of male and
female gametes takes place. Traditionally, these plants survive well under stable environmental conditions when
compared with plants produced from sexual reproduction because they carry genes identical to those of their
parents.
Many different types of roots exhibit asexual reproduction (Figure 32.24). The corm is used by gladiolus and
garlic. Bulbs, such as a scaly bulb in lilies and a tunicate bulb in daffodils, are other common examples. A potato
is a stem tuber, while parsnip propagates from a taproot. Ginger and iris produce rhizomes, while ivy uses an
adventitious root (a root arising from a plant part other than the main or primary root), and the strawberry plant
has a stolon, which is also called a runner.

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 993

Figure 32.24 Different types of stems allow for asexual reproduction. (a) The corm of a garlic plant looks similar to (b) a
tulip bulb, but the corm is solid tissue, while the bulb consists of layers of modified leaves that surround an underground
stem. Both corms and bulbs can self-propagate, giving rise to new plants. (c) Ginger forms masses of stems called
rhizomes that can give rise to multiple plants. (d) Potato plants form fleshy stem tubers. Each eye in the stem tuber can
give rise to a new plant. (e) Strawberry plants form stolons: stems that grow at the soil surface or just below ground
and can give rise to new plants. (credit a: modification of work by Dwight Sipler; credit c: modification of work by Albert
Cahalan, USDA ARS; credit d: modification of work by Richard North; credit e: modification of work by Julie Magro)

Some plants can produce seeds without fertilization. Either the ovule or part of the ovary, which is diploid in
nature, gives rise to a new seed. This method of reproduction is known as apomixis.
An advantage of asexual reproduction is that the resulting plant will reach maturity faster. Since the new plant is
994 Chapter 32 | Plant Reproduction

arising from an adult plant or plant parts, it will also be sturdier than a seedling. Asexual reproduction can take
place by natural or artificial (assisted by humans) means.

Natural Methods of Asexual Reproduction

Natural methods of asexual reproduction include strategies that plants have developed to self-propagate. Many
plants—like ginger, onion, gladioli, and dahlia—continue to grow from buds that are present on the surface of
the stem. In some plants, such as the sweet potato, adventitious roots or runners can give rise to new plants
(Figure 32.25). In Bryophyllum and kalanchoe, the leaves have small buds on their margins. When these are
detached from the plant, they grow into independent plants; or, they may start growing into independent plants if
the leaf touches the soil. Some plants can be propagated through cuttings alone.

Figure 32.25 A stolon, or runner, is a stem that runs along the ground. At the nodes, it forms adventitious roots and
buds that grow into a new plant.

Artificial Methods of Asexual Reproduction

These methods are frequently employed to give rise to new, and sometimes novel, plants. They include grafting,
cutting, layering, and micropropagation.
Grafting
Grafting has long been used to produce novel varieties of roses, citrus species, and other plants. In grafting,
two plant species are used; part of the stem of the desirable plant is grafted onto a rooted plant called the stock.
The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than
a right angle), placed in close contact with each other, and are then held together (Figure 32.26). Matching up
these two surfaces as closely as possible is extremely important because these will be holding the plant together.
The vascular systems of the two plants grow and fuse, forming a graft. After a period of time, the scion starts
producing shoots, and eventually starts bearing flowers and fruits. Grafting is widely used in viticulture (grape
growing) and the citrus industry. Scions capable of producing a particular fruit variety are grafted onto root stock
with specific resistance to disease.

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 995

Figure 32.26 Grafting is an artificial method of asexual reproduction used to produce plants combining favorable stem
characteristics with favorable root characteristics. The stem of the plant to be grafted is known as the scion, and the
root is called the stock.

Cutting
Plants such as coleus and money plant are propagated through stem cuttings, where a portion of the stem
containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start
producing a root even when placed only in water. For example, leaves of the African violet will root if kept in
water undisturbed for several weeks.
Layering
Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can
be bent easily without any injury are preferred. Jasmine and bougainvillea (paper flower) can be propagated this
way (Figure 32.27). In some plants, a modified form of layering known as air layering is employed. A portion
of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some
gardeners also apply rooting hormone. After some time, roots will appear, and this portion of the plant can be
removed and transplanted into a separate pot.
996 Chapter 32 | Plant Reproduction

Figure 32.27 In layering, a part of the stem is buried so that it forms a new plant. (credit: modification of work by
Pearson Scott Foresman, donated to the Wikimedia Foundation)

Micropropagation
Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a
single plant in a short time under laboratory conditions (Figure 32.28). This method allows propagation of rare,
endangered species that may be difficult to grow under natural conditions, are economically important, or are in
demand as disease-free plants.

Figure 32.28 Micropropagation is used to propagate plants in sterile conditions. (credit: Nikhilesh Sanyal)

To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used.
The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that
species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all
the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8

Chapter 32 | Plant Reproduction 997

mass known as callus, from which individual plantlets begin to grow after a period of time. These can be
separated and are first grown under greenhouse conditions before they are moved to field conditions.

Plant Life Spans

The length of time from the beginning of development to the death of a plant is called its life span. The life cycle,
on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of
the mature plant. Some plants, such as annuals, only need a few weeks to grow, produce seeds and die. Other
plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented
age of 4,500 years (Figure 32.29). Even as some parts of a plant, such as regions containing meristematic
tissue—the area of active plant growth consisting of undifferentiated cells capable of cell division—continue
to grow, some parts undergo programmed cell death (apoptosis). The cork found on stems, and the water-
conducting tissue of the xylem, for example, are composed of dead cells.

Figure 32.29 The bristlecone pine, shown here in the Ancient Bristlecone Pine Forest in the White Mountains of
eastern California, has been known to live for 4,500 years. (credit: Rick Goldwaser)

Plant species that complete their lifecycle in one season are known as annuals, an example of which is
Arabidopsis, or mouse-ear cress. Biennials such as carrots complete their lifecycle in two seasons. In a
biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its
reproductive phase. Commercial growers harvest the carrot roots after the first year of growth, and do not allow
the plants to flower. Perennials, such as the magnolia, complete their lifecycle in two years or more.
In another classification based on flowering frequency, monocarpic plants flower only once in their lifetime;
examples include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as
120 years in some bamboo species), these plants may reproduce asexually and accumulate a great deal of food
material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon
after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such
as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials,
flower several times during their life span, but not each year. By this means, the plant does not require all its
nutrients to be channelled towards flowering each year.
As is the case with all living organisms, genetics and environmental conditions have a role to play in determining
how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and
competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow,
despite the presence of dead tissue such as cork. Individual parts of plants, such as flowers and leaves, have
different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is
triggered by factors such as a decrease in photosynthetic efficiency, due to shading by upper leaves, or oxidative
damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled
by the plant for use in other processes, such as development of seed and storage. This process is known as
nutrient recycling.
The aging of a plant and all the associated processes is known as senescence, which is marked by several
complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which
is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery
such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are
998 Chapter 32 | Plant Reproduction

broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of
other plant tissues.
The complex pathways of nutrient recycling within a plant are not well understood. Hormones are known to play
a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic
acid causes premature onset of senescence.

This OpenStax book is available for free at http://cnx.org/content/col24361/1.8