96 



BOTANY AND PHARMACOGNOSY. 



dicularly downward (Fig. 6i, 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. 88). The secondary branches are 

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

 dicular, or grow out horizontally, while still others do not appear 

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

 tion. In the case of large trees we frequently find that the lateral 



Fig. 62. Over-turned tree trunk showing spreading root-system, the main or 

 tap root having died away. 



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

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

 gravity is not very evident (Fig. 62) . But here it must be remem- 

 bered that gravity was instrumental in determining the direction 

 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 

 layers. As indicated, gravity also determines the upward perpen- 

 dicular direction of the shoot, which is therefore said to be 



