All grasses are flowering plants, formally named angiosperms, even though the flowers are often extremely inconspicuous. Graminoids, the grasses, include turf grass and wild grasses, but there are many other types. Bamboo is a grass, as are cereal plants. Rushes and sedges are also grass. About 3,500 species of grasses exist, and all of them reproduce by seed. The genetic changes developed through this sexual method of reproduction have created this diversity.
All grass seeds have three essential elements. The embryo will grow into a new plant. The endosperm stores nutrition for the developing embryo. The seed coat encloses the embryo and endosperm, protecting them from harm. Most types of grass seeds have fruits as well.
The embryo, which has genes from both parents, is essentially an extremely immature version of the plant. The roots of the plant develop from the radicle, which is sensitive to gravity and grows downward. The shoot develops from a plumule. A seed leaf, or cotyledon, is attached to the plumule. When the shoot grows, the cotyledon appears. The area above the cotyledon is the epicotyl. It will develop true leaves, capable of photosynthesis, when it is exposed to sunlight.
Grass seeds are monocotyledons, which means that they have only one cotyledon. Other angiosperms have more. Dicotyledon seeds have two cotyledons, and gymnosperms have at least three.
Nourishment for the embryo is held in the endosperm. For some grasses, the endosperm is quite small, while in others it is large enough to be of great importance to animals and humans. Proteins in barley, wheat and rye endosperm combine to form gluten.
The testa, or seed coat, is usually fused to the fruit in grass plants. The proper term for this structure is pericarp, but often it is casually called a seed or a kernel. Turf grass usually has a small pericarp, while cereal grasses produce larger ones. Larger fruit is very attractive to animals, so the seed is dispersed widely. Small turf grass seed often has slightly hairy structures on the surface, which cling to passing animals.
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