144 Science of Plant Life 



proving the chemical composition of the soil and to a less 

 extent its physical condition. Irrigating the land where the 

 water supply is ina4equate, and under draining it where the 

 soil moisture is excessive, are further means of improving soil 

 conditions for the growth of crops. The soil requires these 

 special attentions for the growth of domesticated plants be- 

 cause they must not only live, as do wild plants, but they 

 must, in one form or another, yield a profitable return. 



Atmospheric water. The water in the air affects plants 

 directly in several ways. The moistness or dry ness of the 

 air determines whether less water or more is required for 

 transpiration, and the amount of water precipitated from the 

 air in the form of rain determines to a large extent the amount 

 of water available in the soil. Atmospheric water conderised 

 in the form of fog and cloud reduces transpiration and also 

 lessens the amount of light that reaches the plant. 



The distribution of rainfall through the year is of the great- 

 est importance to vegetation. When the period of heaviest 

 rainfall coincides with the hottest part of the year, the con- 

 ditions are best for the rapid growth of plants. If the rain- 



FIG. 82. 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 

 04); when regularly irrigated, the same wheat produces soft, starchy grains (C). An 

 intermediate condition is shown by B. This exemplifies the effect of the water balance 

 on the composition of a grain. 



