140 THE ANATOMY OF WOODY PLANTS 



the absence of any general and common anatomical characteristics 

 of the stem for vascular plants as a whole, it is necessary to define 

 it by the important criteria supplied by the mode of attachment of 

 the appendages. The axis is characterized by the fact that it is 

 divisible into nodes from which the appendages normally take their 

 origin and into segments more or less elongated separating these 

 from one another and known as internodes. By the possession of 

 nodes and internodes the stem is at once distinguished from the 

 root and the leaf. There are usually at least two kinds of append- 

 ages attached to the stem in the region of the nodes namely, the 

 leaves and secondary axes or branches. If the axis be subterranean, 

 roots may also make their appearance in the region of the nodes 

 and are clearly separated from the other appendages by their 

 internal origin. Both leaf and branch are formed superficially or 

 exogenously on the axis. The leaf and secondary axes or branches 

 are frequently related to one another in a quite definite manner, the 

 young branch appearing in the upper angle or axil of the leaf. 

 Further, the arrangement of the leaves and consequently of their 

 axial related structures, the branches, follows a somewhat definite 

 plan. Where more than one leaf occurs at a node, the foliar 

 structures are said to be opposite or whorled; and where they are 

 attached singly to the nodes, the arrangement of the leaves or 

 phyllotaxy is said to be spiral. The spirals are of different types, 

 according as the number of foliar organs intervening between two 

 vertically opposite leaves is greater or smaller; and the spiral 

 turns about the stem, drawn through the leaf bases, are more or less 

 numerous. In the grasses, for example, the spiral phyllotaxy 

 is expressed by a fraction of which one is the numerator and two the 

 denominator. In the alder the fraction is one over three, and in the 

 oak or willow two over five. The numerator indicates the number 

 of turns about the stem in passing from one leaf to the next one 

 vertically above it. The denominator is supplied by the number of 

 leaves attached to the stem in the interval between two which 

 are vertically opposite. 



The stem is not only extremely variable as regards its anatomical 

 structure in the various groups of higher plants, but its external 

 form is greatly modified in correlation to different conditions of 



