32 FIELD CROPS 



in the supply of soil moisture is soon evident from the wilting 

 which takes place when water is transpired more rapidly 

 than it can be taken in by the roots. Plants can not draw 

 all the water from the soil; clay soils will retain more than 

 sandy soils. It is easy to see that a reduction of the root sur- 

 face lessens the supply of water which the plant can obtain, 

 hence cultivating corn so deep that some of the roots are cut 

 causes the plants to wilt and checks their growth. The 

 gardener removes part of the leaves from his plants and 

 prunes his trees in transplanting them so as to reduce trans- 

 piration and lessen the danger from wilting, for he knows 

 that part of the roots have been broken off and those that 

 remain can not supply enough water for the full leaf surface. 

 30. Roots Require Air. As with all other living parts of 

 the plant, the roots are constantly taking in oxygen and giv- 

 ing off carbon dioxid; that is, the process of respiration is in 

 progress. Consequently, roots require air. Most plants 

 can not grow in a soil that is so filled with moisture that air 

 is largely excluded, though a few plants have become adapted 

 to this condition. Plants do not root deeply when there is an 

 over-supply of moisture, for it is not necessary for them to 

 extend their roots to obtain water, nor is there sufficient air in 

 the soil for the healthy growth of roots. Consequently, 

 plants in wet locations often suffer first when dry weather 

 comes, for their root systems are so small and so shallow 

 that they are unable to obtain enough water. For the same 

 reason, plants are more likely to be damaged by a drouth 

 which follows a wet spring than by one succeeding a moder- 

 ately dry one. Enough water, but not too much, is essential 

 for the best growth of plants. Good drainage helps by tak- 

 ing off the surplus water and allowing air to penetrate, thus 

 inducing deeper rooting. 



