66 STEUCTUKAL BOTANY 



structure with those of a young stem, but the two 

 parts of the bundle are so placed that the xylem is 

 directed upwards, and the phloem downwards. This 

 is a very constant rule for vascular bundles in flat 

 leaves, and holds good both for the main bundles and 

 their branches. We know that in the stem the 

 xylem faces inwards and the phloem outwards, and 

 as the bundle passes into the leaf without any twisting, 

 the xylem necessarily comes to lie towards the upper, 

 and the phloem towards the lower surface of the 

 leaf. 



The loosely coiled spiral vessels of the protoxylem 

 are found at the extreme upper edge of the wood, 

 while the protophloem lies at the extreme lower edge 

 of the bast. The development of the parts of the 

 bundle follows precisely the same order as in the 

 stem. In the leaf, however, the bundles are dosed. The 

 leaf being of limited growth, there is no need for a 

 permanently active cambium. 



In the finer bundles of the leaf the structure is 

 greatly simplified, the xylem consisting exclusively 

 of spirally thickened elements. In the ultimate 

 branches these elements are of the kind called 

 tracheides] they resemble vessels in all respects 

 except that the cells of which they are composed do 

 not fuse with one another, so that they do not form 

 continuous tubes. If we trace the finest bundles to 

 their termination, we find that the phloem comes to 

 an end before the xylem, so that the extreme end of 

 the bundle consists of spiral tracheides only. All the 

 bundles, including their finest branches, are enclosed in 



