68 SOILS AND FERTILIZERS 



the greatest importance in the growth of plants. There are 

 certain more or less definite relations that obtain, so that 

 for any given condition of the water supply certain results 

 in crop growth may be expected. As we shall see later, 

 these conditions of water supply are, within certain limits, 

 subject to the control of man and consequently the growth 

 of crops may be regulated to some extent by these means. 



74. Ways in which water is useful to plants. — In many 

 indirect ways water contributes to plant growth, as for 

 instance in aiding in the disintegration of rocks, in the pro- 

 motion of decay of organic matter and in numerous other 

 ways, but it is with the use of water as it occurs in soils and 

 as taken up by plants that we are now concerned. The 

 functions that water thus serves may be listed as follows : 



1. Water is a direct source of food material, for it either 

 becomes a part of the plant substances without change (about 

 90 percent of most plants is water), or it is decomposed and 

 its elements are used in building plant tissue. 



2. Water acts as a solvent and carrier of plant-food ma- 

 terials, taking up these substances in the soil and transferring 

 them to the plant, where they are utilized in the formation 

 of plant tissue. 



3. Water in the plant serves to keep the cells expanded, 

 to regulate the temperature and to carry in solution sub- 

 stances from those portions of the plant in which they are 

 formed, to the places where they are needed, as for instance, 

 to transport soluble matter from the leaves of the potato, 

 where the starch is formed, to the tuber, where it is stored. 



75. Water requirements of plants. — Most of the water 

 that enters the roots passes on through the plant and evapo- 

 rates from openings in the leaves. A large crop will, other 

 things being equal, require more water for its production 

 than a small crop. The ratio of the quantity of water used, 

 to the quantity of dry matter that the plants contain, is 



