Grass lawns are endemic in America and are the subject of many studies. In addition to lawns, grasses are important components of FORBS, which are forage crops for domestic livestock. Understanding how grasses grow is critical to lawn maintenance and use by both domestic and wild herbivores.
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Grass seed, or caryopsis, is dried grass fruit. Caryopsis has three main structural elements: the pericarp, which is the outer hull surrounding the seed, the endosperm, which is the stored food source for the developing seed, and the embryo. The embryo is the new plant in miniature and includes the first leaf or cotyledon, the radical or root, and the coleoptile. Because grass is a monocot, it has only one cotyledon or seed leaf. The endosperm primarily consists of starches and some sugars. The coleoptile is a protective layer that surrounds the growing point of the grass embryo.
When conditions for germination aren't right, a seed becomes dormant to await the next growing season when the chances for survival are better. Being dormant allows the seeds hard coat and low water content to protect the seed from both freezing and the dry season. The main signals for the seed to break its dormancy are water availability, warmer temperatures and sufficient oxygen for the grass embryo to breathe. The increase in water will soften the seed coat of the grass, allowing the embryo to push through, if other conditions are suitable. The requirement for oxygen, aside from its necessity for a plant's chemical processes, acts as a counterbalance in case the grass seed is too far underground or under water to be able to survive if it germinates. In these situations, oxygen availability is low, preventing germination.
Once the grass seed senses the right conditions for germination, it begins to take up water through the softened seed coat, causing seed enlargement. Once the embryo starts to take up the water, and in the presence of the oxygen, the embryo manufactures enzymes that change the starches stored in the endosperm into simple sugars. The embryo uses these sugars, or carbohydrates, to power the mechanisms of growth.
Once the hull, or seed coat, is soft enough, and the embryo has absorbed enough water, the germinating embryo uses hydraulic pressure to push the root radicle and the first leaf through the seed coat. With monocots, the seedling has only one seed leaf. This first leaf, along with the coleoptile, grows away from gravity while the root grows towards gravity, an ability called geotropism. As mentioned previously, the grass coleoptile protects the growing point of the grass. When the coleoptile senses sunlight, it stops growing but the leaf continues to grow. Other leaves arise from the growth point that hugs the soil surface. All growth of the leaves originates from the growth point, and not the tip of the grass leaves. This arrangement is what makes grasses good food crops. The growing point at the soil surface allows animals to eat the top portion of the grass leaves without harming the growing point. If the leaf grew from the tip, once an animal or mower removed the tip, the grass would stop growing. Of course, this would require less mowing but would mean that any grazing by insects or wildlife would eventually kill the plant.
While the leaf was reaching for the surface, the root radical was reaching into the soil. At this point, the true roots, or primary roots, begin to grow, but unlike the leaves, the growing point, or meristem, is at the ends of the roots. Grasses have a third kind of root, adventitious, which mainly aids in support. These roots grow from the crown, where the growing point of the leaves is located, and in creeping grasses, from nodes along the grass stem.
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