xiii LYCOPODINEM 495 



broad (Fig. 287) and only slightly convex. Its centre is occu- 

 pied by a group of similar initial cells, which in L. selago, 

 according to Strasburger ((10), p. 240), usually show two 

 initials in longitudinal section (Fig. 287, i). From these in- 

 itials are cut off lateral segments which, by further periclinal 

 and anticlinal walls, produce the epidermis and cortex, and sec- 

 ondarily the leaves. Periclinal walls also are formed from 

 time to time in the initial cells, by which basal segments are 

 cut off, which produce the large central plerome cylinder. 



The leaves arise as conical outgrowths near the stem apex, 

 and owe their origin to the three or four outer cell layers of the 

 growing point. The separation of the epidermis does not oc- 

 cur until the leaf has formed a conspicuous conical protuber- 

 ance. The differentiation of the procambium in the young 

 leaf begins early, and the strand joins the central procambial 

 cylinder of the stem, which, however, is quite independent 

 of the leaf-traces. Each young leaf-trace joins an older one 

 at the point of junction with the stem cylinder, and thus the 

 complete stem possesses two systems of vascular bundles, the 

 strictly cauline central cylinder, and the system of common 

 bundles formed by the united leaf-traces. 



The first elements of the vascular bundles to become recog- 

 nisable are spiral tracheids, both in the stem and leaves, and 

 these are followed in the former by the much wider scalari- 

 form tracheids that occupy the central part of the tracheary 

 plates in the fully-developed bundles. 



The fully-developed central cylinder of the stem (Russow 

 (i),p. 128; De Bary (3), p. 281; Strasburger (n), vol. iii., 

 p. 458; Strasburger, /. c., p. 460; Van Tieghem (5), p. 553) 

 is undoubtedly to be considered as a group of confluent vascu- 

 lar bundles or as gamostelic. The oval or nearly circular cross- 

 section (Fig. 288, A) is sharply separated from the surround- 

 ing ground tissue by a clearly-marked endodermis, within 

 which is a pericycle which may be only one cell thick, but is 

 usually several-layered. According to Strasburger this peri- 

 cycle does not properly belong to the central cylinder, but is 

 of cortical origin. The cutinised band ("radial folding") of 

 the endodermal cells is only observable in the younger stages, 

 as later the whole wall of the endodermal cells become cutin- 

 ised. This cutinisation extends also through a number of the 

 succeeding cortical layers. The rest of the cortical region is" 



