DEVELOFEMENT i\l- VEGETA15LES. 7f) 



wood of tlie pine, however straight may be the 

 direction of the whole trunk. This tendency is 

 shown even in the epidermis of the cherry tree ; for 

 it may be stripped oflf with more facility in a spiral 

 direction than in any other. The primitive direc- 

 tion of the leaves of endogenous plants is a spiral 

 one. It is particularly marked, also, in the stems 

 of creepers and of parasitic plants, which are gene- 

 rally twisted throughout their whole length ; a dis- 

 position evidently conducive to the purpose of their 

 formation, namely, 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 tendency 

 to a spiral growth might be owing to the influence 

 of the sun's light, acting successively on the 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 fac- 

 ing 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 press- 

 ing forward to obtain the beneficial action of that 

 agent. The leaves, whose functions in a more 

 especial manner require its operation, will always 



