68 PLANT LIFE 02ST THE FAKM. 



to produce tubers ; and it is clear, from the position and 

 direction of the branches of a tree, that the influence of 

 gravity, direct or negative, varies greatly in different 

 cases, so that on the whole it is probable that the direc- 

 tions in question are more especially due to varying de- 

 grees of intensity of growth in different situations, accord- 

 ing to local necessities and the action of light, than to 

 gravitation pure and simple. 



Influence of Light on Stems. The remarks made 

 under the corresponding heading in regard to leaves, 

 apply with the necessary modifications to stems. The 

 stems have often a marked tendency to move or grow 

 towards the light, but the opposite tendency is shown in 

 other instances, as in the ivy, the runners of the straw- 

 berry, and other cases, where this peculiarity favors the 

 application of the stem to the surface of the ground, of 

 a wall, or of any means of support, as in many climbing 

 plants. 



The action of light in retarding growth, already referred 

 to, seems opposed to many of the phenomena just recorded 

 such as the bending of the stems towards the light, 

 the fact that stems grow by day as well as by night, the 

 circumstance that the tissues of plants grown in the dark 

 are feeble and ill-developed. These apparent contradic- 

 tions may be explained by the fact that the retarding 

 influence of growth, which is so manifest when the plant 

 is grown under artificial conditions, when the influence 

 of other agencies is prevented or excluded, is compensated 

 for or overcome by other agencies temperature, moisture, 

 etc. when the plant is grown under natural conditions. 

 Again, what is called the "after effect "has to be con- 

 sidered the facilities for growth afforded by the absence 

 of light, by the agency of heat, or other forces, may con- 

 tinue after those influences have ceased to act, and so a 

 plant may grow for a time under adverse influences by 



