90 THE MECHANICAL FUNCTIONS. 



process, as that of the trunk. The buds from 

 which they originate, spring from the angle 

 formed by the stalk which supports a leaf, and 

 which is termed by botanists the axilla of that 

 leaf. A law of symmetry is established by 

 nature in the developement of all the parts 

 of plants. The leaves, in particular, are fre- 

 quently observed to arise in a circle, or symme- 

 trically round the parent stem ; forming what 

 is termed a tvhorl, or, in botanical language, a 

 verlicillated arrangement. In other cases they 

 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 intervals 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 observable 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 pri- 

 mitive direction 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 generally twisted throughout 

 their whole length ; a disposition evidently con- 

 ducive to the purpose of their formation, namely, 



