DEVELOPEMENT OF VEGETABLES. 91 



that of laying hold of the objects with which 

 they come in contact, and of twining round them 

 in search both of nourishment and of support. 

 The twisted stems of the hop and of ivy show 

 this structure in a remarkable degree, and the 

 purpose for which this tendency was given can- 

 not be mistaken. 



A conjecture has been offered that this ten- 

 dency to a spiral growth might be the effect 

 of the influence of the sun's light, acting succes- 

 sively on different sides of the plant, in the 

 course of its diurnal motion. In these northern 

 latitudes the direction of that motion is from east 

 to west ; or, to an observer facing the south, from 

 left to right. That light has a powerful influence 

 in determining the direction of the growth of all 

 the parts of the plant which are above ground, is 

 manifest to every one who has observed the 

 habits of vegetables. If a growing plant be 

 placed in a situation where the light reaches it 

 only on one side, it will always, by degrees, 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 operation, 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 attraction of the light, when it can 

 reach them only laterally. Thus while those on 

 the upper part spread out in full luxuriance in 



