CHAPTER TEN 



THE WATER RELATIONS OF LEAVES 



During a prolonged drought in Illinois, in 1914, oats in some 

 places failed to attain a height of more than 4 inches and pro- 

 duced practically no grain. Corn which should have averaged 10 

 feet in height reached only 5 feet in many fields, and yielded only 

 half the normal amount of grain. In the four great corn-growing 

 states there must be 3 inches of rainfall in July for the best yield 

 of corn ; and if the rainfall during July is 2 J inches instead of 3, 

 it is estimated that at normal prices there is an average loss of 

 $5 an acre, or a total loss of $150,000,000. Those who cultivate 

 plants know from experience the importance of a sufficient water 

 supply in the production of crops. The reason why the water 

 supply is important will be apparent when we understand the 

 uses made of water by the plant. 



Why water is necessary to a plant. The active protoplasm of 

 all plant cells is in a semiliquid condition. More than 90 per 

 cent of its weight is made up of water, and in consistency it closely 

 resembles white of egg. The several parts of the protoplasm — 

 the cytoplasm, the nucleus, and the plastids — differ somewhat 

 in their water content, but all of them must be nearly saturated 

 with water to carry on the hfe processes. When the amount of 

 water in the cell falls much below this point, the protoplasm 

 becomes rigid and all its activities are retarded. The curled-up 

 leaves of corn during a summer drought illustrate this effect. 

 The corn manufactures little food, and consequently growth is 

 retarded. In many plants the protoplasm may even die if the 

 water content is greatly reduced. For example, the seeds of the 

 soft or silver maple which are shed in late spring and germinate 

 soon afterward die if the water content is reduced below 30 per 

 cent. Water is necessary for the life of the protoplasm of plant 



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