Plants reproduce either sexually or asexually. In sexual reproduction, two parents produce a genetically different individual. In asexual reproduction, a plant propagates (reproduces) itself and produces a genetically identical individual. Some plants reproduce both ways.
Sexual reproduction in plants requires separate male and female parts whose separate sex cells come together and fuse or unite. This unification produces a seed that if cultivated under the proper conditions, will grow into a unique offspring. In flowering seed plants or angiosperms, the reproductive parts are in the flowers. For humans, flowers may be simply a source of pleasure since they add color and fragrance to the world, but for plants they are complex and hardworking organs. Flowers vary in size, shape, and color, yet all have the same common structures. A flower's male reproductive parts, called stamens, consist of a filament or stalk and a pollen head called an anther. Its female parts, called carpels or pistils, consist of an ovary (where the seed eventually will develop), and a stalk or style at the end of which is a sticky flat top called a stigma, which will receive the pollen. The entire male part looks like an antennae with pads on their ends. The female parts resemble a long-necked, round-bottom bottle with a flat top.
Before a flower can produce seeds, pollination must occur. This means that pollen (which contains the plant's male sex cells) has to somehow travel from its anthers to its stigma. While many plants have both male and female organs and can therefore easily pollinate themselves (self-pollination), this does not result in much genetic variety. Therefore, flowering plants have evolved several different methods and strategies to make sure that pollen is transferred from one plant to another (cross-pollination). Since plants cannot move on their own and accomplish this task, they require agents to do their work for them. The most common method is animal pollination in which a colorful, perfumed flower attracts an insect, bird, or bat by producing a sugary liquid called nectar. In extracting the nectar from the flower, the animal picks up sticky pollen grains, which it then accidentally rubs onto another flower's stigma as it gets more nectar. This transfer of pollen is called pollination. Besides animals, the wind is another useful way of transferring pollen from one plant to another.
After a grain of pollen lands on the stigma of a suitable flower, the pollen begins to grow a thin tube that penetrates down the long neck or style of the female part and enters the embryo sac containing the ovary (female sex cells). The male cells travel down the tube and fertilize the egg or ovum. Soon a seed is formed, containing an embryo (the beginning of a new individual plant that has its own unique collection of genes), some stored food (the endosperm), and a protective coat (the testa). After a seed has developed, a plant's next job is to distribute the seed some distance from the parent plant, mainly to prevent its own habitat from becoming overcrowded. Plants that use animals to disperse their seeds often produce fruit that contain seeds. Fruit can be soft and juicy like a peach or hard and dry like a burr. Some fruit are eaten by animals that discard the seed somewhere else or have it pass through their system undigested. Other fruit have hooks or barbs that easily catch on to the fur of passing animals. Some seeds are mechanically dispersed by fruit that explode (like impatiens), while other seeds can float and use water to move about. When a seed falls into an environment where conditions are right for it, the seed takes in water and begins to grow. This beginning growth is called germination.
Plants can also produce more of their kind through asexual reproduction. Many perennial plants (plants that grow for several years) reproduce asexually by producing new parts that become entirely new plants. This form of asexual reproduction is called vegetative propagation. This method is favored by plants that live in especially harsh or severe conditions, such as those near a mountaintop, where the opportunity to attract animal pollinators is minimal or unreliable. The grass that grows on our lawns reproduces asexually by sending up new above-ground plants from its underground root system. The strawberry plant sometimes found as a weed in our lawns sends out runners along the ground that take root and produce a new plant while still attached to the parent plant. Strawberry plants can also reproduce by making seeds. Other plants can regenerate (regrow) parts from any plant parts that remain. Thus if we pull up a dandelion from our lawn but do not get the entire taproot and leave a piece underground, the plant will regrow new roots, stems, and leaves. Many plants with belowground tubers (stems), rhizomes (stems like roots), or bulbs (like tulips) can do the same and produce an entirely new plant. However, all of the plants produced by vegetative propagation are genetically identical to the parent plant. Farmers and gardeners use this technique to produce large numbers of desirable, identical plantlets. Nature, however, favors sexual over asexual reproduction since it prefers to have plants that vary genetically. Plants that are genetically identical can be susceptible to disease or a sudden climate change, and they can all be destroyed. If a species has genetic variety, though, some may be more resistant to disease or others may tolerate climate change and survive.
[See alsoBotany; Plant Anatomy; Plant Hormones; Plants; Reproduction, Asexual; Reproduction, Sexual ]