Plants in Relation to Man 465 



Water retained in plants. These facts show that enormous 

 quantities of soil water are utilized and thrown off in tran- 

 spiration and evaporation by plants. But considerable quan- 

 tities are retained also by plants and their products. Apples, 

 for example, are about eighty-five per cent water; potatoes 

 about seventy-eight per cent, tomatoes ninety-four per cent, 

 and lettuce ninety-five per cent water. Fresh corn is seventy- 

 five per cent, string beans eighty-nine, and dried beans fourteen 

 per cent water. Oats fall to a lower point in water content, 

 having only twelve per cent ; and wheat has about ten per 

 cent. A ton of dried clover hay is about fifteen per cent water ; 

 but in its growth and maturity it has made use of more than 

 three hundred eighty-five tons of water. In general, plants 

 in the manufacture of one pound of dry substance utilize in 

 transpiration from two hundred to five hundred pounds of water. 



Plants and the soil's capacity for moisture. Knowledge 

 of the capacity of different soils for moisture is necessary in 

 order to judge the suitability of soil for different kinds of plants. 

 One kind of plant needs a comparatively moist soil, while another 

 thrives best in a dry soil. Potatoes do not require nearly as 

 much moisture as red clover. Other kinds of plants show 

 similar variation in their need of soil water. 



Crops and soil water. When the amount of soil water re- 

 quired for any given crop is known, together with the mean 

 annual rainfall of the region and the loss by drainage and 

 evaporation, it is a comparatively simple matter to select the 

 crops best adapted to the conditions. Where sufficient water 

 is not provided by rainfall, or is supplied in the wrong season 

 of the year, it may be supplemented by irrigation or conserved 

 by dry farming (page 468). Where the soil has an excessive 

 amount of water, it may be drained, as is done with marsh or 

 swamp lands. 



Exercise : Leaves. Plants show interesting adaptations of size of 

 leaves to the environmental conditions, such as soil and soil water, 

 and the heat and humidity of the atmosphere. One may compare the 

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