SECT. I. ORGANOGBAPHY AND GLOSSOLOGY. 49 



perhaps not so completely as in the case of the creeper 

 alluded to. That Monocotyledons increase very slowly 

 in thickness may readily be conceived, but so do the 

 trunks of dicotyledonous trees, after they have acquired 

 a great age. 



(54.) Forms of Stems. The more usual character 

 of dicotyledonous stems,, is to taper off gradually from 

 the base towards the summit, and they thus approxi- 

 mate to the form of a very lengthened cone. On the 

 other hand, the stems of woody Monocotyledons, with 

 few exceptions, approximate to the form of a cylinder. 

 Some stems, however, in the early stages of their 

 growth, and many herbaceous stems during the whole 

 period of their duration, are variously angulated, and 



channelled (fig. 40.). This is frequently owing to 

 some peculiarity in the development of the cellular 

 tissue of which the bark is composed. 



(55.) Directions of Stems. The original tendency 

 of aerial stems, is vertically upwards; but many are 

 too weak to support themselves in that position, and, 

 in consequence, either trail upon the 

 ground, or cling to the surrounding 

 herbage, by means of tendrils, hooks, 

 and various other appendages; which 

 are frequently modifications of the leaf. 

 There are certain stems, also, which, 

 by continually twisting in a spiral man- 

 ner, twine themselves round the trunks 

 and branches of neighbouring trees and 

 shrubs, and are thus supported to a great 

 height. The spiral which these stems describe, is termed 



