Plant Reproduction
How Do Seeds Develop into Plants?
Fruits and Seeds
Teacher Note: Misconception
Students may think that flowers and fruit are the only kinds of plant reproductive structures. However, other plant reproductive structures include grass inflorescences, pinecones and structures enabling asexual reproduction, such as stolons and rhizomes.
In angiosperms, after fertilization has occurred, the ovule is referred to as a seedglossary term (opens in a new window). A seed consists of a tough seed coat, or testaglossary term (opens in a new window), that surrounds and protects its contents. Inside the testa is the embryo. The embryo consists of a plumuleglossary term (opens in a new window), a radicleglossary term (opens in a new window), and cotyledons. The plumule is the embryonic shoot, and the radicle is the embryonic root. There may be one or two cotyledons, depending on the species of plant. The cotyledons act as food stores and are the embryonic leaves. They are quite different from the foliage leaves that are part of the plumule. These embryonic leaves play a special role in germinationglossary term (opens in a new window). The testa may also enclose the endospermglossary term (opens in a new window), a starchy food store that provides energy needed for germination.
When attached to the parent plant, angiosperm seeds may be found together inside a fruitglossary term (opens in a new window). A fruit consists of seeds surrounded by an ovary. There are two main types of fruits, dehiscent and nondehiscent. Dehiscent fruits liberate the seeds by opening the fruit as it stays attached to the parent plant. Nondehiscent fruits and their seeds are dispersed together. Nondehiscent fruits include nuts, berries, and drupes (fleshy fruits like plums). Seed dispersal is related to fruit structure.
Seed Dispersal
Seeds that fall close to the parent plant will, when they start to grow, compete with the parent and other seeds from the same plant for light and nutrients. Plants have evolved a wide range of seed dispersal strategies to reduce the likelihood of this competition and to enable the colonization of new habitats. The mechanisms for seed dispersal include wind, animals, water, and self-dispersal.
Some plants, such as ash or maple trees, produce seeds with flat surfaces that act as wings. These wings catch the wind and carry the seeds some distance from the parent. Other plants, such as dandelions and cotton, have seeds that have hairs that trap air and help the seeds float considerable distances.
Animals disperse seeds in two main ways. In the case of fleshy fruits, such as berries and drupes, the fruit has characteristics that make it more likely to be eaten. Animals that eat the fruit may discard the seeds or pass them through their digestive systems. In the latter situation, the seeds emerge from the gut with their own ready-made fertilizer. Some seeds must pass through the gut of an animal for it to germinate. Other animal-dispersed seeds and fruits have hooks that attach to the fur or feathers of an animal. These are carried away from the parent plant and drop or are rubbed off. Many nut-producing trees rely upon the storage habits of animals to aid in the dispersal of their fruits. For example, a squirrel may bury thousands of acorns in preparation for winter, but may fail to relocate many of them.
Water is used as a seed dispersal mechanism by some plants that are closely associated with rivers, lakes, or oceans. Coconuts may be carried thousands of miles on ocean currents before being deposited on the shores of a remote island. Some water lilies have water-dispersed seeds.
There are many species of plants that use explosive mechanisms to self-disperse their seeds. Most of these mechanisms rely on the drying of the fruit to build up tension in its tissues. Many seed pods work in this way. As the pods dry out, they split open explosively, throwing their seeds several meters.
Seed Dormancy
Once dispersed, a seed may germinate. This stage may not happen immediately. Seeds have evolved to withstand long periods without water and generally only contain about ten percent of water by weight. Many seeds remain dormant for long periods of time until conditions become right for their germination. This may be until the next spring or for many years. The oldest seed to germinate was of an Arctic flower found stored in an ancient frozen squirrel burrow. It germinated after being dormant for approximately 30,000 years. Some seeds have no dormancy period. Mangrove seeds germinate while still attached to the parent. They fall pre-germinated from the parent into the mud of the mangrove swamp.
Germination
Germination is the initial growth of a plant from a seed. Germination requires the presence of water, warmth, and oxygen. Germination begins with the absorption of water by the seed. Water enters the seed through a tiny pore called the micropyle. Inside the seed, the embryo begins to hydrate. Typically, after a few days, the radicle bursts through the testa and begins to grow downward. As it pushes through the soil, the tip of the radicle is protected by a root cap. As the root elongates, root hairs appear. These absorb water and nutrients from the soil. Side roots, called lateral roots, soon develop, and the radicle begins to pass nutrients and water into the seed. Next, in most dicotyledonous plants, the cotyledons break through the testa and, protecting the plumule, emerge through the soil surface. The two cotyledons then open and spread out, forming two green, photosynthetic “seed leaves.” The plumule rapidly grows upward, developing leaves that can photosynthesize. This type of germination is called epigeal germination. As the plant grows, it uses the food stored in the endospermglossary term (opens in a new window) and cotyledons. The cotyledons eventually shrivel up and drop off. In other dicotyledonous plants, the cotyledons remain below ground and act as a food store.
In monocotyledonous plants, such as maize, the single cotyledonglossary term (opens in a new window) remains within the seed, supplying it with food for the germination process. The plumule grows straight up, protected by a sheath called a coleoptile. Once above the soil, the coleoptile breaks open, and the leaves of the plumule emerge.
How Do Plants Grow?
Like all multicellular organisms, plants grow by cell division. Most roots and stems grow by cell division near their tips. These areas of cell division are called meristems. Those found at the tips of the main root or shoot are called apical meristems. In the root, the apical meristem is situated behind a cap of protective cells. Here the meristematic cells divide and then elongate as the root grows. At this stage, the cells begin to differentiate into different cell types with different functions. Shoot meristems are found at the tip of a shoot.
As plants increase in height, they must also increase in girth. Meristems called lateral meristems make the roots and stem grow wider. This lateral growth is most easily observed in woody plants. These plants have circular layers of meristematic cells called cambium. As the cambium produces new cells, it makes the stem or root grow wider. In trees, this growth of cambium produces new xylem and phloem tissue. The xylem of these tissues can be observed as the annual rings in the cut trunks of trees. The outer layer of bark is produced by another ring of cambium cells called the cork cambium.
Plants also grow side branches and leaves. Meristems are present wherever buds are forming. As they divide, meristematic cells differentiate into a variety of organs. These organs include leaves and reproductive structures such as flowers.
