STRUCTURE, MORPHOLOGY, AND PHYSIOLOGY. 29 



specific gravity, and consequently the seed is better 

 adapted for transportation by the wind (very small fruits, 

 like those of Agrostis and Erogrostis, which are light 

 enough in themselves, usually fall entirely out of the 

 bracts), especially if the empty glumes accompany it 

 and have a considerable surface (Holcus), or generally 

 the fruit-bracts are large in comparison with the fruit 

 (Briza), the whole having the effect of a winged seed. 

 A still greater effect is produced when a large number 

 of sterile glumes, or whole groups of sterile spikelets 

 (Phalaris ccerulescens Desf.), fall off with the fruit. The 

 glumes or parts of the axis, if they bear long hairs, form 

 a peculiar winged apparatus (Phragmites, Arundo, Cala- 

 magrostis, very many Andropogonew, especially those re- 

 lated to Saccharum ; Melica ciliata L., and Stipa pennata 

 L. (by its featheiy awns), as well as many Aristida spe- 

 cies). Many grasses are also adapted to distribution 

 by the fur of animals, for either the bracts themselves 

 (Tragus) or the awns (Hordeum, Echinarid) are very 

 rough or have hooked bristles. In Streptochcvta the 

 ripe fruit hangs from delicate spiral threads (the awns), 

 which are fastened together at the end of the spike ; they 

 are free below, and their stiff* pointed bracts, bent out- 

 ward, act like fish-hooks by catching into the fur of any 

 animal that touches them in passing. The fleshy berries 

 of many Bambusece are evidently fitted for distribution 

 by animals, which neither destroy nor digest the seeds. 

 Not a few grass-fruits possess a powerful instrument in 

 the surrounding bracts for fastening themselves into the 

 ground. First we find the base of such bracts prolonged 

 into a pointed often prickly callus, which makes its pene- 

 tration easy, and above the point are stiff hairs pointing 

 upwards which make it difficult to draw out a bract that 

 has once penetrated the soil (Fig. 44, caV). A very 

 strong bent awn, twisted like a rope below the bend, 

 often acts as a motor. This awn is very hygroscopic ; 

 the coils unroll or unwind when damp, and when dry 

 they return to their former condition ; and every time 

 this happens the point of the bract is rotated with lateral 

 variations, especially when the upper half of the awn 



