126 



THE VEGETATIVE PTJNCTIONS OF PLANTS 



responds by growing either more or less rapidly, as the 



case may be. 



127. Relation of Roots and 

 Stems to Gravity. — It is common 

 knowledge that, in general, roots 

 grow downward, while stems 

 grow upward. If a. germinating 

 seed of lupine is so placed that 

 the axis of the emerging embryo- 

 plant is horizontal, the position 

 of the elongating root and shoot 

 after a given interval of time 

 (e.g. 48 hours) will be as shown 

 in Fig. 84. The result is the 

 same whether the plant is placed 

 in diffused light or in the dark. 

 The difference in the direction of 

 growth may not, therefore, be 

 attributed to the action of light. 

 If the seedling is so placed that 

 the only external influence is 

 gravity — the attraction of the 

 earth — the result, is as shown in 

 the figure. We must, therefore, 

 conclude that the result is due 

 to the earth's gravitational at- 

 traction. 



The correctness of this conclu- 

 sion may be assured by subject- 

 ing the seedling to another influ- 

 ence than gravity, but, like it, 

 acting on both root and shoot, at 



Fig. 84. — Seedling of white 

 lupine {Lupimts alhus), after 

 having been placed horizon- 

 tally in the dark for 48 hours. 

 The hypocotyl {h) has re- 

 sponded negatively, and the 

 root (r) positively to the stim- 

 ulus of gravity. The portion 

 (0) remains in the original 

 horizontal position. 



