68 PLANT LIFE ON THE FARM. 



and consequently to raise its free end (see p. 53). It is this 

 tendency, which, as has been previously stated, permits the 

 stalks of the wheat when laid to recover their erect posi- 

 tion. Some stems or portions of stem are, however, 

 directly influenced by gravitation, as in the case of under- 

 ground stems and branches, which burrow in the ground 

 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 directions in 

 question are more especially due to varying degrees of in- 

 tensity of growth in different situations, according 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 favours 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 re- 

 ferred 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 

 contradictions may be explained by the fact that the 



