238 STRUCTURAL BOTANY 



the still greater differences which we shall find when 

 we come to consider the development in detail. 



Having now given a brief sketch of the chief points 

 in the external morphology of our type, we will go on 

 to examine its more minute structure. 



I. ANATOMY OF THE VEGETATIVE ORGANS 

 a. Stem 



The structure of the young stem in its main 

 features is closely similar to that of a normal Dicoty- 

 ledon (see Fig. 97). 



The central cylinder contains a single ring of 

 vascular bundles enclosing a pith. The bundles 

 themselves, just as in Dicotyledons, are collateral, 

 with their xylem directed towards the centre, and 

 their phloem towards the exterior of the cylinder. 

 These bundles pass out one by one into the leaves, each 

 of which receives a single bundle only (see Fig. 97, I). 

 The bundles in the cylinder are separated from one 

 another by the primary rays, which are only two or 

 three cells in thickness. 



The primary bundles are of simple structure. The 

 xylem consists entirely of tracheides and parenchyma ; 

 no vessels are developed at any time in the wood, 

 whether primary or secondary, of the Fir or any other 

 cone-bearing trees. The protoxylem, which lies at the 

 extreme inner side of the bundle, has the usual spiral 

 or annular tracheides ; the phloem consists of sieve- 

 tubes and parenchyma. There are no companion-cells, 

 but certain cells of the parenchyma bordering on the 



