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 gametophyte produces male or female gametes or both , by a process of cell division called mitosis. 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 mega gametophyte is contained within the ovule that may develop into a seed and the micro gametophyte is contained within a pollen grain.
History of sexual reproduction of plants[ edit ] Main article: Evolution of sexual reproduction Unlike animals, plants are immobile, and cannot seek out sexual partners for reproduction. In the evolution of early plants, abiotic means, including water and 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 like 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 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 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.
The male gametophyte consists of the pollen grains. The sperm of seed plants are non-motile, except for two older groups of plants, the Cycadophyta and the Ginkgophyta , which have flagellated sperm. Flowering plants[ edit ] Flowering plants are the dominant plant form on land and they reproduce by sexual and asexual means.
Often their most distinguishing feature is their reproductive organs, commonly called flowers. Sexual reproduction in flowering plants involves the production of male and female gametes , the transfer of the male gametes to the female ovules in a process called pollination.
After pollination occurs, fertilization happens and the ovules grow into seeds within a fruit. After the seeds are ready for dispersal , the fruit ripens and by various means the seeds are freed from the fruit and after varying amounts of time and under specific conditions the seeds germinate and grow into the next generation.
The anther produces male gametophytes which are pollen grains , which attach to the stigma on top of a carpel , in which the female gametophytes inside ovules are located.
After the pollen tube grows through the carpel's style, the sperm from the pollen grain migrate into 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 ovary, which produced the female gametophyte s , then grows into a fruit , which surrounds the seed s. Plants may either self-pollinate or cross-pollinate. Pollination[ edit ] An orchid flower In plants that use insects or other animals to move pollen from one flower to the next, plants 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 back 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, as such they have no need for large fancy flowers.
Plants have a number of different means to attract pollinators including colour, 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 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.
Flower heads showing disk and ray florets. The largest family of flowering plants is the orchids Orchidaceae , estimated by some specialists to include up to 35, 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 '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, species,  which also have highly modified inflorescences that are flowers collected together in heads composed of a composite of 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, though some radiate heads may be heterogamous too. Ferns[ edit ] Ferns typically produce large diploid sporophytes with rhizomes , roots and leaves; and on fertile leaves called sporangium , spores are produced. The spores are released and germinate to produce short, thin gametophytes that are typically heart shaped, small and green in color.
The gametophytes or thallus , 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 are released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of 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 call alternation of generations. Other plants with similar reproductive means include the Psilotum , Lycopodium , Selaginella and Equisetum.
Alternation of generations and Bryophyte life cycle The bryophytes , which include liverworts , hornworts and mosses , reproduce both sexually and vegetatively.
The gametophyte is the most commonly known phase of the plant. All are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominate form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antherida 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 by mitotic division and grows into a sporophyte that is diploid. The multicellular diploid sporophyte produces structures called spore capsules. The spore capsules produce spores by meiosis, and when ripe, the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. In some species each gametophyte is one sex while other species produce both antheridia and archegonia on the same gametophyte which is thus hermaphrodite.
Plant reproductive morphology Many plants have evolved complex sexual reproductive systems, which is expressed in different combinations of their reproductive organs. Some species have separate male and female plants, and some have separate male and female flowers on the same plant, but the majority of plants have both male and female parts in the same flower.
Some plants change their morphological expression depending on a number of factors like age, time of day, or because of environmental conditions.
Plant sexual morphology also varies within different populations of some species.