6&ASSES — HITCaCOCK 309 



Grasses of high altitudes and high latitudes have leaf structures 

 similar to those of desert grasses. The water supply may be ample, 

 but the low temperature of the water and often also of the air inter- 

 feres with water absorption. The leaves thus have a structure that 

 reduces evaporation to correspond to water supply. 



Even in humid regions the water supply may be reduced on rocks 

 or sand, and grasses growing on rocky or sandy soil may have the 

 structure of dry land grasses. The grasses of salt meadows or 

 marshes also have structures that reduce evaporation to meet re- 

 duced absorption by the roots. The absorption of soluble salts is 

 hindered more and more as the concentration of the soil water 

 increases. 



SEED DISPERSAL 



Grasses owe their dominance to their ability to make a living 

 under all conditions where the higher plants can live at all, and 

 also to their ability to reproduce themselves and spread their seeds 

 far and wide. The seeds are often provided with special structures 

 that aid in their dissemination. 



The means of dispersal are chiefly wind and animals, though water 

 may play a minor part. Small seeds may be carried by wind great 

 distances without especial adaptive attachments. Some seeds in- 

 crease their chances of dispersal by wind by having outgrowths of 

 hairs or fuzz. Such seeds are able to remain longer in the air and 

 can be carried further in proportion to their weight than can naked 

 seeds. Examples of this are broomsedge {Andropogon) , plumegrass 

 {Erianthus) , and reed {Phragmites) . The long spreading or reflexed 

 awns of certain species of Aristida aid in wind dispersal. On the 

 Great Plains the seed^ of A. longifolia and allied species may be seen 

 drifting across the surface of the soil in countless numbers. The 

 three spreading awns give a surface upon which the wind acts. 



TUMBLEWEEDS 



This adaptation to wind dispersal is further developed in tumble- 

 weeds. In a typical tumbleweed the whole plant at maturity is a 

 more or less globular mass of hard gtiff branches. The stem breaks 

 off near the ground and the whole plant goes tumbling and rolling 

 before the wind, scattering seed as it goes. Tumbleweeds are charac- 

 teristic of prairies and plains, as they can function only in open 

 ground. 



In grasses it is only the inflorescence that acts as a tumbleweed. 

 Witchgrass (Panicum capillar e) and ticklegrass {Agrostis hiemalis) 

 are examples. At maturity the panicles with their stiff, slender, 

 spreading branches become light, open skeleton balls which break 

 away and roll before the wind. 



