Grasses may be annual or perennial herbs,[8]: 10 generally with the following characteristics (the image gallery can be used for reference): The stems of grasses, called culms, are usually cylindrical (more rarely flattened, but not 3-angled) and are hollow, plugged at the nodes, where the leaves are attached.[8][9] Grass leaves are nearly always alternate and distichous (in one plane), and have parallel veins.[8]: 11 Each leaf is differentiated into a lower sheath hugging the stem and a blade with entire (i.e., smooth) margins.[8]: 11 The leaf blades of many grasses are hardened with silica phytoliths, which discourage grazing animals; some, such as sword grass, are sharp enough to cut human skin. A membranous appendage or fringe of hairs called the ligule lies at the junction between sheath and blade, preventing water or insects from penetrating into the sheath.
