Plant reproduction is the process by which plants generate new
individuals, or offspring. Reproduction is either sexual or asexual.
Sexual reproduction is the formation of offspring by the fusion of
gametes
. Asexual reproduction is the formation of offspring without the fusion of
gametes. Sexual reproduction results in offspring
genetically different
from the parents. Asexual offspring are genetically identical except for
mutation. In higher plants, offspring are packaged in a protective seed,
which can be long lived and can disperse the offspring some distance from
the parents. In flowering plants (angiosperms), the seed itself is
contained inside a fruit, which may protect the developing seeds and aid
in their dispersal.
Sexual Reproduction in Angiosperms: Ovule Formation
All plants have a life cycle that consists of two distinct forms that
differ in size and the number of
chromosomes
per cell. In flowering plants, the
A hibiscus flower, showing anthers, five stigmas, and pollen.
large, familiar form that consists of roots, shoots, leaves, and
reproductive structures (flowers and fruit) is
diploid
and is called the sporophyte. The sporophyte produces
haploid
microscopic
gametophytes
that are dependent on tissues produced by the flower. The reproductive
cycle of a flowering plant is the regular, usually seasonal, cycling back
and forth from sporophyte to
gametophyte.
The flower produces two kinds of gametophytes, male and female. The female
gametophyte arises from a cell within the
ovule
, a small structure within the ovary of the flower. The ovary is a larger
structure within the flower that contains and protects usually many
ovules. Flowering plants are unique in that their ovules are entirely
enclosed in the ovary. The ovary itself is part of a larger structure
called the carpel, which consists of the stigma, style, and ovary. Each
ovule is attached to ovary tissue by a stalk called the funicle. The point
of attachment of the funicle to the ovary is called the
placenta.
As the flower develops from a bud, a cell within an ovule called the
archespore enlarges to form an embryo-sac mother cell (EMC). The EMC
divides by
meiosis
to produce four megaspores. In this process the number of chromosomes is
reduced from two sets in the EMC to one set in the megaspores, making the
megaspores haploid. Three of the four megaspores degenerate and disappear,
while the fourth divides
mitotically three times to produce eight haploid
cells. These cells together constitute the female gametophyte, called the
embryo sac.
The eight embryo sac cells differentiate into two synergids, three
antipodal cells, two fused
endosperm
nuclei, and an egg cell. The mature embryo sac is situated at the outer
opening (micropyle) of the ovule, ready to receive the sperm cells
delivered by the male gametophyte.
Pollen
The male gametophyte is the mature pollen grain. Pollen is produced in the
anthers, which are attached at the
distal
end of filaments. The filament and
anther together constitute the stamen, the male sex organ. Flowers
usually produce many stamens just inside of the petals. As the flower
matures, cells in the anther divide mitotically to produce pollen mother
cells (PMC). The PMCs divide by meiosis to produce haploid microspores in
groups of four called tetrads. The microspores are housed within a single
layer of cells called the tapetum, which provides nutrition to the
developing pollen grains.
Each microspore develops a hard, opaque outer layer called the exine,
which is constructed from a
lipoprotein
called sporopollenin. The exine has characteristic pores, ridges, or
projections that can often be used to identify a species, even in fossil
pollen. The microspore divides mitotically once or twice to produce two or
three haploid nuclei inside the mature pollen grain. Two of the nuclei
function as sperm nuclei that can eventually fuse with the egg and
endosperm nuclei of the embryo sac, producing an embryo and endosperm,
respectively.
For sexual fusion to take place, however, the pollen grain must be
transported to the stigma, which is a receptive platform on the top of the
style, an elongated extension on top of the carpel(s). Here the moist
surface or chemicals cause the pollen grain to germinate. Germination is
the growth of a tube from the surface of a pollen grain. The tube is a
sheath of
pectin
, inside of which is a solution of water,
solutes
, and the two or three nuclei, which lack any cell walls. Proper growth of
the pollen tube requires an
aqueous
solution of appropriate solute concentration, as well as nutrients such
as boron, which may aid in its synthesis of pectin.
At the apex of the tube are active ribosomes and
endoplasmic reticulum
(types of cell
organelles
) involved in
protein
synthesis. Pectinase and a glucanase (both
enzymes
that break down
carbohydrates
) probably maintain flexibility of the growing tube and aid in
penetration. The pollen tube apex also releases
ribonucleic acid (RNA) and
ribosomes into the tissues of the style. The tube grows to eventually
reach the ovary, where it may travel along intercellular spaces until it
reaches a placenta. Through chemical recognition, the pollen tube changes
its direction of growth and penetrates through the placenta to the ovule.
Here the tube reaches the embryo sac lying close to the micropyle, and
sexual fertilization takes place.
Double Fertilization
Fertilization in flowering plants is unique among all known organisms, in
that not one but two cells are fertilized, in a process called double
fertilization. One sperm
nucleus
in the pollen tube fuses with the egg cell in the embryo sac, and the
other sperm nucleus fuses with the diploid endosperm nucleus. The
fertilized egg cell is a
zygote
that develops into the diploid embryo of the sporophyte. The fertilized
endosperm nucleus develops into the
triploid
endosperm, a nutritive tissue that sustains the embryo and seedling. The
only other known plant group exhibiting double fertilization is the
Gnetales in the genus
Ephedra,
a nonflowering seed plant. However, in this case the second fertilization
product degenerates and does not develop into endosperm.
Double fertilization begins when the pollen tube grows into one of the two
synergid cells in the embryo sac, possibly as a result of chemical
attraction to calcium. After penetrating the synergid, the apex of the
pollen tube breaks open, releasing the two sperm nuclei and other contents
into
the synergid. As the synergid degenerates, it envelops the egg and
endosperm cells, holding the two sperm nuclei close and the other expelled
contents of the pollen tube. The egg cell then opens and engulfs the sperm
cell, whose membrane breaks apart and allows the nucleus to move near the
egg nucleus. The
nuclear envelopes
then disintegrate, and the two nuclei combine to form the single diploid
nucleus of the zygote. The other sperm cell fuses with the two endosperm
nuclei, forming a single triploid cell, the primary endosperm cell, which
divides mitotically into the endosperm tissue.
Double fertilization and the production of endosperm may have contributed
to the great ecological success of flowering plants by accelerating the
growth of seedlings and improving survival at this vulnerable stage.
Faster seedling development may have given flowering plants the upper hand
in competition with gymnosperm seedlings in some habitats, leading to the
abundance of flowering plants in most temperate and tropical regions.
Gymnosperms
nevertheless are still dominant at higher elevations and latitudes, and
at low elevations in the Pacific Northwest coniferous forests, such as the
coastal redwoods. The reasons for these patterns are still controversial.
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