248 STRUCTURAL BOTANY 



The phloem ray is simply the outer part of the 

 same medullary ray which passes through the xylem, 

 and the cambium serves for the growth of both (see 



rig. 98). 



We see, then, that the portions of the medullary 

 rays which pass through the phloem show a similar 

 differentiation to that of the same rays in the wood. 

 In both alike there are two kinds of elements, one 

 kind serving for the storage of reserve materials, 

 while the other forms part of the conducting system. 



b. Leaf 



The leaves of the Fir and those of most Coniferae 

 differ greatly from the typical leaves of Dicotyledons. 

 On the whole they are much simpler in structure, 

 and the bundle - system especially is rudimentary 

 in character. As we have seen, a single vascular 

 bundle enters each leaf from the stem. Some- 

 times this bundle divides into two, which run side 

 by side through the leaf. In other cases, as in 

 that figured, the bundle is a single one (see Fig. 

 101). The bundle forms part of a central cylinder 

 which traverses the leaf from end to end, and is con- 

 tinuous with the tissues of the central cylinder of the 

 stem. The cylinder of the leaf is surrounded by the 

 mesophyll, which in this species shows no distinction 

 between palisade and spongy parenchyma, though in 

 some other Conifers this differentiation is present. 



We will now consider the structure of a leaf more 

 in detail. The form of the leaf as seen in transverse 

 section is quadrangular. Its natural position is such 



