650 SECONDARY GROWTH IN THICKNESS 



radial rows characteristic of a typical cambium. To begin with, these 

 procambial strips give rise to small additional vascular strands. Later, 

 the two or three innermost layers become transformed into mechanical 

 elements ; these, together with the stereides previously differentiated in 

 the hadrome-strands of the vascular bundles, constitute a complete intra- 

 cambial fibrous cylinder (Fig. 268 b). The outermost layer of the 

 interfascicular procambium is ultimately converted into the inter- 

 fascicular portion of the actual cambium, its cells becoming elongated 

 radially and undergoing division principally by tangential walls (Fig. 

 268 c). The cambial cylinder is completed, by these interfascicular 

 strips of serial cambium becoming continuous with the corresponding 

 fascicular tissue. 



A somewhat more complicated arrangement is found in the young 

 internodes of Pelargonium gibbosum. Here, as in the preceding case, 

 the young vascular strands become joined together by strips of primary 

 procambial tissue, within which additional bundles are differentiated. 

 In this instance, however, the procambial strips comprise as many as 

 five to seven layers, and give rise to four distinct tissues, in place 

 of the two produced by them in Salvia. The two outermost layers 

 become transformed into fibres. The two or three next in order give 

 rise to narrow conducting-parenchyma cells. The elements of the fifth 

 or sixth layer, by elongating radially and undergoing tangential division, 

 produce a serial cambium which, in conjunction with the fascicular 

 cambium, constitutes the cambial cylinder. The innermost procambial 

 layer, finally, becomes converted into narrow but elongated conducting- 

 parenchyma cells. According to Hartig, Sanio and De Bary, the 

 cambial cylinder originates in an essentially similar fashion in Ephedra 

 monostachya, Cheiranthus Cheiri, Bumex, Lunaria, Cobaca, Galium, 

 Pyrcthmm, Hieracium, in the Cakyophyllaceae, Ckassulaceae, Plan- 



TAGINACEAE, etc. 



Where the cambial cylinder arises independently of the primary 

 procambium, or where a primary procambial cylinder is never formed, 

 the development of the actual cambium is comparatively straightfor- 

 ward. In this case, tangential divisions appear in the medullary 

 rays, close to the margins of the strips of fascicular cambium ; the 

 rays thus become gradually bridged by strips of secondary meristem. 

 Every cambial cylinder produced in this way is therefore composite in 

 character, being made up of alternate strips of fascicular and interfas- 

 cicular tissue, the former representing persistent portions of primary 

 meristem, while the latter are secondarily derived from the fundamental 

 parenchyma of the medullary rays. We have here another illustration 

 of the fact already mentioned on more than one occasion that a tissue 

 which is wholly concerned with a single physiological function, may 



