20. Poaceae Tribe ARISTIDEAE
三芒草族 san mang cao zu
Authors: Shou-liang Chen, Sheng-lian Lu & Sylvia M. Phillips
Perennials, or occasionally annuals. Leaf blades narrow, often convolute; ligule a line of hairs. Inflorescence a contracted or open panicle. Spikelets all alike, with 1 floret, bisexual without rachilla extension, laterally compressed or terete, disarticulating above glumes; glumes usually longer than floret, persistent, membranous to scarious, 1–5-veined, apex acute to acuminate or mucronate; floret callus pungent to obtuse, bearded; lemma usually terete, cartilaginous becoming indurated at maturity, margins tightly convolute and enclosing the palea, 1–3-veined, veins converging at apex, extending directly into 3 awns, or more often combining into a single 3-branched awn raised upon a twisted column, laterals sometimes reduced or rarely suppressed, awn branches scabrid or all or only the central branch plumose; palea less than 1/2 length of lemma, often obscure, hyaline or membranous. Stamens 3, rarely 1. Caryopsis terete or fusiform, tightly enclosed within toughened lemma, embryo 1/3 its length; hilum linear. Leaf anatomy: either Kranz PS (Stipagrostis) or a form of Kranz unique to Aristida; microhairs absent or 1-celled, rarely 2-celled. x = 11, 12.
Three genera and ca. 350 species: tropical and subtropical regions of the world; two genera and 12 species (six endemic) in China.
This tribe is usually instantly recognizable on account of the 3-branched awn. Stipagrostis has sometimes been included in Aristida, but besides the obvious awn difference, separation of the two genera is supported by differences in leaf anatomy. In Stipagrostis the cells of the outer bundle sheath are larger than the inner, and only the inner contain chloroplasts, whereas in Aristida the cells of the outer bundle sheath are smaller, and both sheaths contain chloroplasts. There is also usually a difference in chromosome number: 2n = 22 in Aristida and 2n = 44 in Stipagrostis.
Species of Aristideae are mainly found on the poor, stony soils of dry plains and deserts.