80 THE MECHANICAL FUNCTIONS. 



be found turned towards the light. The branches 

 of a tree, which have naturally a tendency to rise 

 vertically, have this tendency modified by the su- 

 perior attraction of the light, when it can reach 

 them only laterally. Thus while those on the upper 

 part spread out in full luxuriance in 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 tlieir leaves to the light. Often, how- 

 ever, 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 verification of the theory would be 

 obtained were it found that plants transported from 

 the southern to the northern hemispheres had this 

 spiral 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 hemis- 

 phere, 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 subject ; so that the hypothesis 

 is untenable. 



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