Factors Affecting Growth and Reproduction 211 



Methods of conserving soil water. — In dry regions there are two 

 methods by which the soil water is conserved. By cultivating 



Fig. 124. Cross-sections of kernels of hard or macaroni wheat. This wheat is grown in 

 dry regions and is valued because of its large content of protein. In the figures the flinty 

 or high-protein parts are shaded and the soft or starchy parts are white. When the wheat is 

 grown under the conditions of dry farming, the protein content is highest (.4) ; when regu- 

 larly irrigated, the same wheat produces soft, starchy grains (C). An intermediate con- 

 dition is shown by B. This exemplifies the effect of the water balance on the composition 

 of a grain. 



the soil several times during the growing season, the soil struc- 

 ture is broken a few inches below the surface, where evapora- 

 tion takes place. Consequently the capillary water columns are 

 broken and water rises only to the top of the undisturbed soil 

 layer. The cultivated layer soon dries out and forms a blanket 

 that reduces water loss by evaporation. The rough, loose sur- 

 face is of further advantage when it rains, in that the water settles 

 into the soil quickly and there is little run-off. This method of 

 conserving water is called the "dust mulch.." 



A second method of making land suitable for crop production 

 in regions of sKght rainfall is to plant crops only in alternate 

 years. By plowing the land so that it will take up water as fast 

 as it falls, and especially by destroying weeds which would other- 

 wise remove large amounts of water, each crop has a large part of 

 two years' rainfall available. These two methods of conserving 

 water form the basis of the so-called '' dry farming." 



Effect of temperature. As one goes north or south from the 

 equator, the temperatures of the soil and the air decrease. In- 

 creasing altitude in mountains brings about the same effects. 

 Temperature directly influences the rate of all plant processes, and 



