308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



When the alkali is too strong for vegetation of any kind, the glaring 

 white, usually rather smooth area is colloquially known as a " slick 

 desert." 



Salt lakes, salt basins, and alkali depressions are produced by 

 the evaporation from undrained areas. The more soluble salts are 

 dissolved in the soil water during wet periods and left on the 

 surface by evaporation during dry periods. 



Certain faulty agricultural practices in irrigated regions illus- 

 trate the artificial production of alkali-meadows. In the early days 

 where water for irrigation was plentiful, the ranchmen often allowed 

 an excess of water to flow over meadows to encourage the growth of 

 grass or alfalfa. Unless there was good drainage the soaking 

 brought soluble minerals from below and deposited them at or near 

 the surface as the water evaporated. As the meadows became more 

 and more alkaline the desirable plants gradually disappeared, their 

 place being taken by species more resistant to alkali but less valuable 

 agriculturally. This alkaline condition can be prevented by proper 

 drainage and by supplying water in only such quantity as is nec- 

 essary. 



VARIATION IN LEAF STRUCTURE 



The structure of grasses varies with the necessity for resisting 

 evaporation. In regions of abundant rainfall throughout the grow- 

 ing season the grasses have abundant foliage and mostly flat leaf- 

 blades. Of course, the plant must obtain the necessary minerals 

 from the soil by an upward current from the roots. Evaporation 

 from the leaves draws the soil water upward. If the supply of 

 water from the soil does not balance the evaporation from tlie leaves, 

 the plant wilts. Even in humid regions there is variation in the 

 proportion between absorption and evaporation. Normally, the 

 leaves meet this variation through control of the stomata or breath- 

 ing pores, which are mostly on the under surface of the blades. 

 These close when the air is dry and open when it is moist. 



If the conditions are such that the grasses have difficulty in ob- 

 taining from the soil the necessary water, the leaves are structurally 

 different from those grasses that grow in a humid climate. The 

 epidermis is thick and impervious to moisture, so that evaporation 

 takes place only or chiefly through the breathing pores. The blades 

 are often tightly rolled with the breathing pores on the inside. The 

 leaves may be short and clustered close to the ground. In extremely 

 dry regions the plants retreat underground for protection during 

 the dry season, as do herbaceous perennials in winter in humid re- 

 gions. Life persists in the perennial base beneath the surface ready 

 to send forth shoots after a rain. 



