Grass survives mowing because it grows from the bottom, not the tip
Most plants grow from their shoot tips, so cutting them off does real harm. True grasses put their growing point low, at the base of each leaf blade, an adaptation to grazing animals. That trick lets lawns survive weekly mowing, and it helped grasslands spread across roughly 40 percent of Earth's land.
The true grasses, family Poaceae, include some 12,000 species in about 780 genera, the fifth-largest plant family. Bamboo, lawn grass, prairie grass and cereals all belong. Seagrasses, rushes and sedges share the name but not the family, though rushes and sedges are close relatives in the same order.
No plant family matters more economically. Maize, wheat, rice, oats, barley and millet feed people and livestock; rice alone supplies about 20 percent of human dietary energy, wheat 19 percent and maize 5 percent. Grasses also provide bamboo, thatch and straw for building, and maize is converted into ethanol for fuel.
Their design is distinctive. Stems, called culms, are usually round and hollow except at the solid nodes where leaves attach. Leaves sit in two ranks, with a sheath hugging the stem and a flat blade. Many blades are stiffened with silica particles that deter grazers, and some, such as sword grass, can slice skin. A small flap called the ligule seals the joint against water and insects. Flowers are packed into spikelets, usually wind-pollinated, and the grain is a caryopsis, a fruit whose wall is fused to the seed coat.
Part of their success is chemical diversity. Cool-season grasses such as wheat, rye and oats use the standard C3 route to fix carbon. Warm-season C4 grasses, including maize, sugar cane and sorghum, use a pathway linked to special leaf anatomy that saves water in heat and drought. Remarkably, C4 photosynthesis seems to have evolved independently twenty or more times within the family, and around 46 percent of grass species use it. In the genus Aristida, one species is C3 while roughly 300 others are C4.
Source: Poaceae
