364 PLANT PHYSIOLOGY 



water absorption and expenditure, and the plant will begin to 

 wilt. Atmospheric drought as such may frequently be observed 

 in spring, when the soil is saturated with water after the melting 

 of the snow, and likewise under conditions of artificial irrigation, 

 especially in arid climates. If the plants have sufficiently devel- 

 oped roots and an ample conductive system and the temperature 

 does not exceed the limits that can be endured by plants, atmos- 

 pheric drought as such is not harmful. If during the noon hours 

 with a high evaporating power of the atmosphere the plants wilt 

 slightly, this is of no consequence, for such wilting is only tem- 

 porary. During the night, the plants recover their turgor. 



Soil drought is a phenomenon much more harmful to plants. 

 It usually takes place toward the end of the summer, when all 

 the moisture reserves are exhausted and summer precipitation 

 has been insufficient to replenish them. The dry soil ceases to 

 yield water to the plant, and it goes into a state of permanent 

 wilting, when the tissues become considerably dehydrated and 

 growth is retarded or stops entirely. Soil drought therefore 

 always leads to a considerable decrease of the yield. If it begins 

 very early as a consequence of insufficient water reserves in the 

 soil from the absence of either winter or summer precipitation, 

 it may lead to a complete failure of the crop. 



A permanent disturbance of the water balance is accompanied 

 by changes in the physiological processes in the plant. The 

 closing of stomata, as a consequence of deficiency of water, 

 checks photosynthesis by impeding the penetration of carbon 

 dioxide into the leaf. The dehydration of the chloroplasts has 

 the same result, for their normal function is possible only when 

 they are sufficiently supplied with water. Simultaneously, other 

 synthetic processes are retarded, such as the formation of starch 

 and synthesis of protein substances, while catabolic processes 

 are accelerated. Consequently, the leaves and other organs of 

 the plant are enriched in monosaccharides and amino acids. 

 Increased dishitegration of protein substances is caused by poor 

 aeration, a consequence of the closing of stomata and of the 

 checking of assimilation, which represents a source of oxygen. 

 Oxygen deficiency contributes to the reduction of sulphydryl 

 groups, which, as has been seen, are important respiratory 

 activators. This leads to an increased disintegration of the 

 proteins. Accumulation of the products of catabolism stimu- 



