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The true grasses, family Poaceae (formerly Gramineae), is one of the most speciose plant families, comprising over 10,000 species with a Gondwanan origin approximated at about 80-100 million years ago (although there are fossil specimens that potentially push the origin earlier; Prasad et al. 2011; Vicentini et al. 2008; Stevens 2013).
Distributed world-wide, the true grasses are absent only in parts of Greenland and Antarctica, and are the most economically important group of monocots, as this family includes the true grains, pasture grasses, sugar cane, and bamboo. Species in this family have been domesticated for staple food crops (grains and sugar, for example), fodder for domesticated animals, biofuel, building materials, paper and ornamental landscaping, among other things. Grasslands cover at least 20% of the earth’s surface, although grasses also grow in biomes other than grasslands.
Grasses are primarily wind pollinated, most have dangling anthers. They have hollow stems and grow from the plant base, rather than the tip, as an evolutionary response to predation. Many also protect themselves from predation by secreting silica crystals in their leaves. There are two main kinds of grasses, cool-season (C3) and warm-season (C4) grasses, which are distinct in their means for fixing Carbon. The evolution of C4 fixation has arisen independently in 4 of the 12 currently recognized grass subfamilies; a combination of changes in paleoclimate including temperature, aridness, seasonality are thought to select for new origins of C4 lineages (Vicentini et al. 2008).
Genomic duplications are common in the true grasses, and thought to play important role in the evolution of the group as well as innovations leading to diversification of branches within Poaceae (for example, the evolution of flowers arranged as spikelets).
(The Plant List 2010; Prasad et al. 2011; Stevens 2013; Vicentini et al. 2008; Wikipedia 2013)