252 STRUCTURAL BOTANY 



We will describe the latter first. It is generally 

 a double bundle in the lower part of the leaf, but 

 nearer the apex only one bundle can be distin- 

 guished, as in Fig. 101 The wood consists of 

 tracheides, with plates of parenchyma, resembling 

 medullary rays, between them. The first - formed 

 tracheides, on the upper side of the xylem, are spiral, 

 the rest pitted; the phloem has much the same 

 structure as in the young stem. Between wood and 

 bast is a layer of cambium. As compared with the 

 leaves of most Dicotyledons or Monocotyledons, we see 

 that the Fir leaf is very scantily supplied with vascular 

 bundles. It has, however, an arrangement of its own, 

 which to some extent makes good the deficiency. 

 Among the cells of the conjunctive parenchyma there 

 are a number of short tracheides with bordered pits 

 (see Fig. 103, t). These tracheides are in communica- 

 tion both with one another and with the xylem of the 

 bundle. They no doubt take up water from the 

 bundle, and pass it on through the pitted endodermis 

 to the assimilating tissue, where it is wanted. These 

 tracheides outside the bundle form what is called the 

 transfusion tissue, a very characteristic feature in the 

 anatomy of coniferous leaves. 



The phloem as well as the xylem enjoys the 

 advantage of an extension into the conjunctive tissue. 

 On each side of the bundle we find a large group of 

 cells with dense protoplasmic contents (see Fig. 103, 

 al). They join on to the phloem of the bundle. 

 These albuminous cells, as they are called, appear to 

 answer the same purpose as the enlarged companion- 



