92 THE MECHANICAL FUNCTIONS. 



all directions-, those below them are obliged to 

 expand more in a lateral direction : and this is 

 still more the case with the lowest branches, 

 which shoot out horizontally to a considerable 

 distance before they turn upwards, and present 

 their leaves to the light. Often, however, from 

 the deficiency of this necessary agent, their 

 growth is much stinted, or entirely prevented. 

 The operation of this cause is extensively seen in 

 the interior of a dense forest. 



It may be objected to the theory of the spiral 

 growth being the result of the sun's motion, that 

 were it so, the direction of the spiral would 

 always be the same, that is, ascending from left 

 to right with reference to the axis. But this is 

 not found to be the case, for the direction of the 

 turns, though generally constant in the same 

 plant, is far from being the same in all. Dr. 

 Wollaston ingeniously suggested that a verifica- 

 tion of the theory would be obtained were it 

 found that plants transported from the southern 

 to the northern hemispheres, would have this 

 direction reversed ; for it is evident that the 

 motion of the sun's light in the two hemispheres 

 is in opposite directions ; being, in the southern 

 hemisphere, from right to left, to a spectator 

 facing the meridian position of the sun, which 

 in those regions is to the north. But, the facts 

 are not in accordance with this view of the sub- 

 ject ; so that we may consider the hypothesis 

 as untenable. 



