414 RELATION TO ENVIRONMENT. 



of the chemical compounds of potash, phosphoric acid, etc.) which 

 are insoluble in water. Certain acids excreted by the roots aid 

 in making these substances soluble (see Chapter III). In a num- 

 ber of plants the roots have become associated with fungus or 

 bacterial organisms which assist in the manufacture of nitro- 

 genous food substances, or even in the absorption of ordinary 

 food solution from the soil, or in making use of the decaying 

 humus of the forest (.see Chapter IX). 



792. (8) The maintenance of the required balance between 

 the environment and the increasing or changing requirements 

 of the plant. — In this matter the entire plant participates. Men- 

 tion is made liere only of the general relation which the root 

 sustains to its own environment and the increased burden placed 

 upon it by the shoot. The increase in the root system keeps 

 pace with the increasing size of the stem system. The roots 

 become stronger, their ramifications wider, and tlie number of 

 absorbing rootlets more numerous. The obser^-ation is some- 

 times offered that the correlation between the root system of a 

 plant, and the form of the stem s}stem and position of the leaves, 

 is of such a nature that plants with a tap-root svstem have their 

 leaves so arranged as to shed the water to the center of the sys- 

 tem, while plants with a fibrous root svstem have their leaves so 

 arranged as to shed the water outward. In support of this 

 attention is called to the radiate t}'pe of the leaf svstem of the 

 dandelion, beet, etc. In the second place the imbricate type as 

 manifested in broad leaved trees, and in the overlapping branch 

 systems of many pines, etc. One should note, howe\-er, that in 

 the former class the lea\'cs are often arranged to shed as much 

 water outward as inward. As to the latter class, there is need 

 of experiment to determine whether these empirical obser\'ations 

 are correct, for the fullowing reasons: ist. Root and leaf distri- 

 bution are g()\-erncd ])y other and more important laws, the root 

 lieing inlUieiKcd l'\' (he loialion of I'ood in the soil which usually 

 forms a ww thin straUim wliik' llic shoot and leaf i> mainh' in- 

 lluenced b\' light, and root distribution is much wider in a lateral 

 direction tli.in that of the branches. 2d, In light rains the leaf 



