412 RELATION TO ENVIRONMENT. 



taining moisture, the roots grow mainly into the moist substratum 

 (hydrotropism). If the soil is too wet, the roots grow away from 

 it to soil with less water, or in some cases will grow to and upon 

 the surface of the soil. 



The roots need aeration, and where the supply of water is too 

 great, the air is shut out, and we know that corn, wheat, and 

 many other plants become "sickly" in low and undrained soil 

 in wet seasons. This can only be said in the case of our ordinary 

 dry land plants, i.e., those that occupy an intermediate position 

 between water-loving plants and dry-conditioned plants. This 

 phase of the subject must be reserved for special treatment. 

 (See Chapter XLVI.) 



788. (4) Distribution of roots for the purpose of reaching 

 food-laden soil. This is one of the essential relations of the root 

 in the case of the land plant, and probably accounts for the very 

 extensive ramification of the roots. To some extent it also 

 explains the different root systems in some plants. The pines, 

 spruces, etc., usually grow in regions where the soil is very shal- 

 low. The root system does not extend deeply into the soil. It 

 spreads laterally and extends widely through the shallow surface 

 soil and presents a very different aspect from the stem system in 

 the air. The root-system of the broad-leaved trees usually extends 

 more deeply into the soil, while of course, extending laterally 

 to great distances. The hickory, walnut, etc., especially have 

 strong tap roots which extend deeply into the soil, and the root 

 system of such a tree is more comparable in aspect, if it were 

 entirely uncovered, to the stem system in the air. The tap-root 

 is more pronounced in some trees than in others. It may be that 

 in the hickory and walnut the deep tap-root is important in 

 supplying the tree with water in dry seasons, especially when 

 growing on dry, gravelly soil which does not retain moisture on 

 the surface nor hold it within two or three feet of the surface. 

 Experiment has demonstrated, by pot culture of plants, that- 

 where soil rich in plant food lies adjacent to poor soil, no matter 

 in what part of the pot the rich soil is, the greatest growth and 

 branching of roots is in the rich soil. 



