134 A TEXT-BOOK OF GRASSES 



hairs are found on the lemmas in Arundo, on the rachilla 

 joints in Phragmites, on the whole spikelet in Saccharum 

 (Fig. 14), on the awns in Stipa pennata L. of Europe and 

 >S. spedosa Trin. & Rupr. of Cahfornia, on the long pedi- 

 cels of S. elegantissima Labill. of Australia. Awns and 

 bristles often aid dispersal by increasing the surface. Clus- 

 ters of spikelets, with their surrounding involucre of bris- 

 tles, fall away from the rachis, the bristles catching air 

 currents. Long-awned species of Hordese, with disarticu- 

 lating rachis, are adapted to wind dispersal. Sitanion and 

 Hordeum are good examples of this. The joints of Sitan- 

 ion, with their numerous long awns spreading in all direc- 

 tions, are sent whirling across the open grassland in the 

 western states. In many species of Aristida (Fig. 35) the 

 3 awns spread horizontally or are somewhat reflexed. On 

 the Great Plains it is common to see, at the proper season, 

 the fruits of these grasses being hurled along by the high 

 winds, the sharp-pointed callus to the front ready to catch 

 in the wool or hair of animals. From such fruits it is an 

 easy transition to wing-fruits, in which the increased sur- 

 face is furnished by wings, appendages or sterile parts. 

 The inflated lemma of Briza, the winged crests on the 

 lemmas of Phalaris, the group of sterile spikelets of 

 Phalaris paradoxa, all aid in dispersal. In some grasses 

 the whole inflorescence breaks away and becomes a 

 ''tumble-weed." The panicles of Panicum capillare L., 

 Agrostis hiemalis (Walt.) B. S. P., Chloris vertidllata Nutt. 

 and Er agrostis pectinacea (Michx.) Nees, are familiar exam- 

 ples. At maturity the panicles separate from the plant 

 and roll over the surface of the ground before the wind, 

 the widely spreading branches making the whole very 

 light. The small fruits are dropped here and there as the 

 panicle travels. The inflorescence of Schedonnardiis panic- 



