An important feature that grasses have developed for survival in open, sunny environments is a well-developed root system. In fact, only 30% of a grass plant may be above ground. Some fully-grown grasses have enormous, fibrous roots that can extend twice as deep as the above-ground shoots stand high. For example, North American prairie cordgrass penetrates to 3.9 m (13 ft.) deep. This adaptation allows grass roots to absorb underground moisture for as long as it is available. Dense root systems stabilize the plants as well as aerate soils. They sustain the plants through extreme seasonal temperature and moisture changes, fire, unrelenting sunshine, and constantly blowing, drying winds. And, if above ground vegetation is damaged by weather, fire, or grazing animals, the tubers and roots store ample energy to grow new tissue.
An oversized, underground storage system is just one means of defense that tasty, nutritious grasses have against grazers. A grass leaf's growing center is at the base where it joins the stem and is frequently at or below ground level so it is out of harm's way. Nibbled grass leaves can continue growing without permanent damage. Outer grass leaves protect new shoots that replace eaten foliage. One other protective trait detected even in early grass fossils is the presence of silica, a hard, crystalline compound, found in sand and glass. Silica provides structural strength to resist hungry, rearing teeth and trampling hooves.