DEVELOPMENT OF VEGETABLES. 77 



the light reaches it only on one side, it will always, by de- 

 grees, turn itself to that side, as if eagerly pressing forward 

 to obtain the beneficial action of that agent. The leaves, 

 whose functions in a more especial manner require its ope- 

 ration, will always 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 superior at- 

 traction of the light, when it can reach them only laterally. 

 Thus, while those on the upper part spread out in full luxu- 

 riance in all directions, those below them are obliged to ex- 

 pand more in a lateral direction: and this is still more the 

 case with the lowest branches, which shoot out horizontal- 

 ly 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 be- 

 ing the result of the sun's motion, that were it so, the direc- 

 tion of the spiral would always be the same, that is, ascend- 

 ing 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 be- 

 ing the same in all. Dr. Wollaston ingeniously suggested 

 that a verification of the theory would be obtained, were it 

 found that plants transported from tlie southern to the nor- 

 thern hemispheres, would have this direction reversed,- for 

 it is evident that the motion of the sun's lisht in the two 

 hemispheres is in opposite directions; being, in the southern 

 hemisphere, from right to left, to a spectator facing the me 

 ridian 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 we may consider the hypothesis as 

 untenable. 



The roots dificr considerably from the stems both in their 

 structure, and in their mode of growth. They exhibit, in- 

 deed, the appearance of medullary rays and of concentric lay- 

 ers, but they are destitute of any central pith, and they have 

 no tracheae; neither does their surface present any appear- 



