12 



THE ANATOMY OF WOODY PLANTS 



than in living pines. The fibrovascular strand is sharply bounded 

 in the median region by the endodermis, a layer circular in con- 

 figuration and well developed in Pinus, though often absent in the 

 higher conifers. The organization of the fibrovascular strand need 

 not particularly occupy our attention at this stage, as it will be 

 considered in detail more appropriately in the sequel. It is enough 

 to note that its upper or woody part is composed of empty cells 

 often showing bordered pits in other words, of tracheids. These 



are continuous on the 

 flanks of the strand with 

 short-pitted elements, the 

 transfusion cells, which are 

 of great interest from the 

 evolutionary standpoint. 

 The cellular complex lying 

 outside the endodermis 

 and within the epidermis 

 is the mesophyll, the repre- 

 sentative of the funda- 

 mental tissues in the leaf. 

 The cells of the mesophyll 

 are infolded in a manner 

 characteristic of most 



FIG. 9. Transverse section of the root of 

 the sarsaparilla (Smilax), showing the three 

 systems of tissues in root organs. 



living pines. In the meso- 

 phyll lie also, on the lower 

 side of the leaf, conspicuous secretory spaces, the resin canals. 

 In the root of the pine, as in the leaf, the same sharp distinction 

 between the fibrovascular structures and the fundamental tissues 

 is present. In the stem of the conifers generally, however, the 

 limit between the tissues belonging to the central conducting 

 cylinder (the fibrovascular system) has become obsolete and can be 

 judged to have been formerly present only on theoretical grounds. 

 In the case of the root of vascular plants in general, from the 

 lowest to the highest, the limit between conducting or fibrovascular 

 tissues and the fundamental system is usually very distinct and is 

 one of the features which so clearly and universally mark the root 

 as the most conservative of the organs of plants. Fig. 9 illustrates 



