90 



PLANT RELATIONS. 





by numerous thread-like processes spreading in every direc- 

 tion than by fiat, expanded processes. 



It should also be noted that as soil roots are subterra- 

 nean they are used often for the storage of food, as in the 

 case of many subterranean stems. Certain prominent root 

 types may be noted as follows : 



(i4. Soil roots. — These roots push into the ground with 



great energy, 

 and their ab- 

 sorbing sur- 

 faces are en- 

 tirely covered. 

 Only the young- 

 est parts of a 

 root system 

 absorb actively, 

 the older parts 

 transporting 

 the absorbed 

 material to the 

 stem, and help- 

 ing to grip the 

 soil. The soil 

 root is the most 

 common root 

 typ(>. being 

 used by the great majority of seed plants and fern plants, 

 and among the moss plants the very simple root-like pro- 

 cesses are mostly soil-related. To such roots the water of 

 the soil presents itself either ah free wafer — that is, water 

 that can be drained away — or as films of water adhering to 

 each soil particle, often called water of adhesion. To come 

 in contact with this water, not only does the root system 

 usually branch profusely in every direction, but the youngest 

 branches develop abundant absorbing hairs, or root hairs 

 (see Fig. 84), which crowd in among the soil particles and 



Fig . 84. Koot tips of corn , showiiig root hairs, tlieir position 

 in reference to the growing tip, and the effect of the 

 surrounding medium upon their development : 1, in soil ; 

 2, in air ; 3, in water. 



