106 THE STEM 



stud}'. According to the work of P]sau (1938, p. 396) on tobacco, 

 the primary xylem differentiates both upwardly into the leaf and 

 downwardly into the axis. But "the phloem of a leaf trace fol- 

 lows a different course of development from that of xylem. It 

 differentiates, at least in the species considered in these studies, 

 acropetally from the stem into the leaf." 



3. The secondary structure of the stem. In gymnosperms and 

 many angiosperms, the so-called primary tissue regions are rela- 

 tively short-lived and eventually are destroyed or embedded by 

 the development of secondary tissues. As pointed out in Exer- 

 cise X, the distinction between primary and secondary vascular 

 tissues is rather arbitrary and depends fundamentally upon the 

 way in which "procambium" is distinguished from the "vascu- 

 lar cambium." The process of secondary growth in stems is 

 extremely complex and the following outline is intended merely 

 as an introductory guide. (For a detailed treatment cf. Eames 

 and MacDaniels, Ch. VI.) 



The vascular cambium, theoretically regarded, is a uniseriate 

 meristem composed of ray initials, which produce the vascular 

 rays, and fusiform initials which give rise to the "vertical" cell 

 types in the secondary phloem and secondary xylem. In examin- 

 ing developing siphonosteles in trans-section, iiowever, it proves 

 difficult or impossible to distinguish the cambial initials from 

 their most recent derivatives. For this reason, the term "cambial 

 zone" may be used collectively to designate the cambial initials 

 and their adjacent undifferentiated phloem and xylem mother 

 cells. The vascular cambium may be thouglit of as the direct 

 continuation of the undifferentiated procambial tissue situated 

 between the metaxylem and the metaphloeni. If one is concerned 

 with a dictyostele, a distinction is made between the caml)ium 

 witliin each bundle (the fascicular camhium) and the cambium 

 Avhich arises fi-om i)arenchyma — like tissue l)etween the bundles 

 (interfascicular camhium). The latter may pi-oduce additional 

 phloem and xylem or as in certain vines, broad rays of "sec- 

 ondary" pai-enchyma. The effects of sustained cainl)!;!! activity 

 upon the ])riinai-y tissue i-egioiis is jirofound. All cxt i-a-cambial 

 tissues, i.e., ]n-imary plilocin, pcricycle, coi-tex and epidermis, are 

 affected and eventually, through the added activity of the 



