Sexual reproduction involves two fundamental processes:
meiosis, which rearranges the
genes and reduces the number of
chromosomes, and
fertilization, which restores the chromosome to a complete
diploid number. In between these two processes, different types of
plants and
algae vary, but many of them, including all
land plants, undergo
alternation of generations, with two different multicellular structures (phases), a
gametophyte and a
sporophyte. The evolutionary origin and adaptive significance of sexual reproduction are discussed in the pages
Evolution of sexual reproduction and
Origin and function of meiosis. The gametophyte is the multicellular structure (plant) that is
haploid, containing a single set of chromosomes in each cell. The gametophyte produces male or female
gametes (or both), by a process of cell division, called
mitosis. In vascular plants with separate gametophytes, female gametophytes are known as mega gametophytes (mega=large, they produce the large egg cells) and the male gametophytes are called micro gametophytes (micro=small, they produce the small sperm cells). The fusion of male and female gametes (fertilization) produces a diploid
zygote, which develops by mitotic cell divisions into a multicellular sporophyte. The mature sporophyte produces
spores by meiosis, sometimes referred to as reduction division because the chromosome pairs are separated once again to form single sets. In mosses and liverworts, the gametophyte is relatively large, and the sporophyte is a much smaller structure that is never separated from the gametophyte. In
ferns,
gymnosperms, and
flowering plants (angiosperms), the gametophytes are relatively small and the sporophyte is much larger. In gymnosperms and flowering plants the megagametophyte is contained within the
ovule (that may develop into a seed) and the microgametophyte is contained within a
pollen grain.
History of sexual reproduction of plants Unlike animals, plants are immobile, and cannot seek out sexual partners for reproduction. In the evolution of early plants, abiotic means, including water and much later, wind, transported
sperm for reproduction. The first plants were
aquatic, as described in the page
Evolutionary history of plants, and released sperm freely into the water to be carried with the currents. Primitive land plants such as liverworts and mosses had motile sperm that swam in a thin film of water or were splashed in water droplets from the male reproduction organs onto the female organs. As taller and more complex plants evolved, modifications in the alternation of generations evolved. In the
Paleozoic era
progymnosperms reproduced by using spores dispersed on the wind. The seed plants including
seed ferns,
conifers and
cordaites, which were all
gymnosperms, evolved about 350 million years ago. They had pollen grains that contained the male
gametes for protection of the sperm during the process of transfer from the male to female parts. It is believed that insects fed on the pollen, and plants thus evolved to use
insects to actively carry pollen from one plant to the next. Seed producing plants, which include the angiosperms and the gymnosperms, have a heteromorphic alternation of generations with large sporophytes containing much-reduced gametophytes. Angiosperms have distinctive reproductive organs called flowers, with
carpels, and the female gametophyte is greatly reduced to a female embryo sac, with as few as eight cells. Each pollen grains contains a greatly reduced male gametophyte consisting of three or four cells. The sperm of seed plants are non-motile, except for two older groups of plants, the
Cycadophyta and the
Ginkgophyta, which have flagella. Sexual reproduction in plants was not universally agreed upon during the establishment of modern
botany, occasioning a bitter dispute in the 1730s and 1740s between
Carolus Linnaeus and his correspondent
Johann Georg Siegesbeck, who averred that "
God would never have allowed such abominable un
chastity among his innocent plants". The botanists who took Linnaeus's side in the dispute were distinguished during the 18th century as the Sexualists ().
Flowering plants Flowering plants, the dominant plant group, reproduce both by sexual and asexual means. Their distinguishing feature is that their reproductive organs are contained in
flowers. Sexual reproduction in flowering plants involves the production of separate male and female gametophytes that produce
gametes. The
anther produces
pollen grains that contain male
gametophytes. The pollen grains attach to the stigma on top of a
carpel, in which the female gametophytes (inside ovules) are located. Plants may either
self-pollinate or
cross-pollinate. The transfer of pollen (the male gametophytes) to the female
stigmas occurs is called
pollination. After pollination occurs, the pollen grain germinates to form a pollen tube that grows through the carpel's style and transports male nuclei to the ovule to fertilize the egg cell and central cell within the female gametophyte in a process termed
double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus a binucleate female cell) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The fertilized ovules develop into seeds within a
fruit formed from the ovary. When the seeds are ripe they may be
dispersed together with the fruit or freed from it by various means to
germinate and grow into the next generation.
Pollination Plants that use insects or other animals to move pollen from one flower to the next have developed greatly modified flower parts to attract pollinators and to facilitate the movement of pollen from one flower to the insect and from the insect to the next flower. Flowers of wind-pollinated plants tend to lack petals and or sepals; typically large amounts of pollen are produced and pollination often occurs early in the growing season before leaves can interfere with the dispersal of the pollen. Many trees and all grasses and sedges are wind-pollinated. Plants have a number of different means to attract pollinators including color, scent, heat, nectar glands, edible pollen and flower shape. Along with modifications involving the above structures two other conditions play a very important role in the sexual reproduction of flowering plants, the first is the timing of flowering and the other is the size or number of flowers produced. Often plant species have a few large, very showy flowers while others produce many small flowers, often flowers are collected together into large inflorescences to maximize their visual effect, becoming more noticeable to passing pollinators. Flowers are attraction strategies and sexual expressions are functional strategies used to produce the next generation of plants, with pollinators and plants having co-evolved, often to some extraordinary degrees, very often rendering mutual benefit. The largest family of flowering plants is the orchids (
Orchidaceae), estimated by some specialists to include up to 35,000 species, which often have highly specialized flowers that attract particular insects for pollination. The stamens are modified to produce pollen in clusters called
pollinia, which become attached to insects that crawl into the flower. The flower shapes may force insects to pass by the pollen, which is "glued" to the insect. Some orchids are even more highly specialized, with flower shapes that mimic the shape of insects to attract them to attempt to 'mate' with the flowers, a few even have scents that mimic insect
pheromones. Another large group of flowering plants is the
Asteraceae or sunflower family with close to 22,000 species, which also have highly modified inflorescences composed of many individual flowers called florets. Heads with florets of one sex, when the flowers are pistillate or functionally staminate or made up of all bisexual florets, are called homogamous and can include discoid and liguliflorous type heads. Some radiate heads may be homogamous too. Plants with heads that have florets of two or more sexual forms are called heterogamous and include radiate and disciform head forms.
Ferns Ferns typically produce large diploids with stem, roots, and leaves. On fertile leaves
sporangia are produced, grouped together in
sori and often protected by an
indusium. If the spores are deposited onto a suitable moist substrate they germinate to produce short, thin, free-living gametophytes called
prothalli that are typically heart-shaped, small and green in color. The gametophytes produce both motile sperm in the
antheridia and egg cells in separate
archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the antheridia where they fertilize the egg. To promote out crossing or cross-fertilization the sperm is released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of the different thallus. A
zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called
alternation of generations. Other plants with similar reproductive strategies include
Psilotum,
Lycopodium,
Selaginella and
Equisetum.
Bryophytes The
bryophytes, which include
liverworts,
hornworts and
mosses, can reproduce both sexually and
vegetatively. The
life cycles of these plants start with haploid spores that grow into the dominant form, which is a multicellular haploid gametophyte, with thalloid or leaf-like structures that
photosynthesize. The gametophyte is the most commonly known phase of the plant. Bryophytes are typically small plants that grow in moist locations and like ferns, have motile sperm which swim to the ovule using
flagella and therefore need water to facilitate sexual reproduction. Bryophytes show considerable variation in their reproductive structures, and a basic outline is as follows: Haploid gametes are produced in antheridia and archegonia by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells, thus producing zygotes that are diploid. The zygote divides repeatedly by
mitotic division and grows into a diploid sporophyte. The resulting multicellular diploid sporophyte produces spore capsules called
sporangia. The spores are produced by
meiosis, and when ripe, the capsules burst open to release the spores. In some species each gametophyte is
one sex while other species may be
monoicous, producing both
antheridia and
archegonia on the same gametophyte which is thus
hermaphrodite.
Algae Sexual reproduction in the multicellular facultatively sexual
green alga Volvox carteri is induced by
oxidative stress. A two-fold increase in cellular
reactive oxygen species (associated with oxidative stress) activates the
V. carteri genes needed for
sexual reproduction. Exposure to
antioxidants inhibits the induction of sex in
V. carteri. It was proposed on the basis of these observations that sexual reproduction emerged in
V. carteri evolution as an adaptive response to oxidative stress and the
DNA damage induced by reactive oxygen species. Oxidative stress induced DNA damage may be repaired during the
meiotic event associated with germination of the
zygospore and the start of a new generation. ==Dispersal and offspring care==