MORPHOLOGY OF HIGHER PLANTS. 



303 



It is well known that, whatever the position of the seed at the 

 time of germination, the young radicle begins to grow perpen- 

 dicularly downward ( Fig. 163, A ) . The branches, however, which 

 arise on the primary root are less positively geotropic and, instead 

 of growing downward parallel with the primary or tap root, di- 

 verge at an angle from it (Fig. 161 ) . The secondary branches are 

 still less affected by gravity and diverge still more from the per- 

 pendicular, or grow out horizontally, while still others do not 



FIG. 164. Over-turned tree trunk showing spreading root-system, the main or 

 tap root having died away 



appear to be in the least affected by gravity and grow freely in any 

 direction. In the case of large trees we frequently find that the 

 lateral roots spread out in a more or less horizontal plane near the 

 surface of the earth, and if the main root has died the influence of 

 gravity is not very evident (Fig. 164). But here it must be re- 

 membered that gravity was instrumental in determining the direc- 

 tion of growth at an earlier stage. This spreading of the roots 

 near the surface of the earth is of decided advantage to plants, for 

 it enables them to avail themselves of the better soil of the surface 



