70 THE MECHANICAL FUNCTIONS. 



formed by the stalk which supports a leaf, and which is 

 termed by botanists the axilla of that leaf A law of sym- 

 metry is established by nature in the development of all the 

 parts of plants. The leaves, in particular, are frequently 

 observed to arise in a circle, or symmetrically round the pa- 

 rent stem; forming what is termed a lohorl, or, in botanical 

 lano-uase, a verticillated arrangement. In other cases the}^ 

 are found to have their origins at equal intervals of a spiral 

 line, which may be conceived to be drawn along the stem, 

 or the branch from which they grow. When these inter- 

 vals correspond to the semi-circumference of the stem, the 

 leaves alternate with one another on its opposite sides. 



The stems of most plants, even those that are perfectly 

 erect, exhibit a tendency to a spiral growth. This is obser- 

 vable in the fibres of the wood of the 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 off with more facility in a spiral direction than 

 in any other. The primitive direction of the leaves of en- 

 dogenous plants is a spiral one. It is particularly marked 

 also in the stems of creepers and of parasitic plants, which 

 are generally twisted throughout their whole length; a dis- 

 position evidently conducive to the purpose of their forma- 

 tion, 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 

 cannot be mistaken. 



A conjecture has been offered that this tendency to a spi- 

 ral growth might be the effect of the influence of the sun's 

 light acting successively 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 ve- 

 getables. If a growing plant be placed in a situation where 



