Grasses

Grasses may be annual or perennial herbs.,
: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.  Grass leaves are nearly always alternate and distichous (in one plane), and have parallel veins.

Each leaf is differentiated into a lower sheath hugging the stem and a blade with entire (i.e., smooth) margins. 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.

Flowers of Poaceae are characteristically arranged in spikelets, each having one or more florets. The spikelets are further grouped into panicles or spikes . The part of the spikelet that bears the florets is called the rachilla. A spikelet consists of two (or sometimes fewer)
bracts at the base, called glumes, followed by one or more florets.  A floret consists of the flower surrounded by two bracts, one external—the lemma —and one internal—the palea. The flowers are usually hermaphroditic —maize being an important exception—and anemophilous or wind-pollinated. The perianth is reduced to two scales, called lodicules,  that expand and contract to spread the lemma and palea; these are generally interpreted to be modified sepals. This complex structure can be seen in the image on the right, portraying a wheat (Triticum aestivum ) spikelet. The fruit of grasses is a caryopsis , in which the seed coat is fused to the fruit wall. A tiller is a leafy shoot other than the first shoot produced from the seed.

Growth and development

Grass blades grow at the base of the blade and not from elongated stem tips. This low growth point evolved in response to grazing animals and allows grasses to be grazed or mown regularly without severe damage to the plant.
Three general classifications of growth habit present in grasses: bunch-type (also called caespitose), stoloniferous , and rhizomatous .[ citation needed ] The success of the grasses lies in part in their morphology and growth processes and in part in their physiological diversity. Most of the grasses divide into two physiological groups, using the C3 and
C4 photosynthetic pathways for carbon fixation. The C4 grasses have a photosynthetic pathway, linked to specialized Kranz leaf anatomy, which allows for increased water use efficiency , rendering them better adapted to hot, arid environments and those lacking in carbon dioxide .
The C3 grasses are referred to as "cool-season" grasses, while the C4 plants are considered "warm-season" grasses.

Annual cool-season - wheat , rye, annual bluegrass (annual meadowgrass, Poa annua), and oat

Perennial cool-season - orchardgrass (cocksfoot, Dactylis glomerata), fescue (Festuca spp.), Kentucky bluegrass and perennial ryegrass (Lolium perenne)

Annual warm-season - maize , sudangrass, and
pearl millet

Perennial warm-season - big bluestem,
Indiangrass , Bermudagrass and switchgrass