COMPARATIVE ANATOMY 



329 



of larger elements without any parenchyma interspersed between them. 

 The spaces between the xylem-rays are occupied by the sieve- tubes, with 

 the protophloem lying at the periphery, while conjunctive parenchyma 

 forms a complete sheath intervening between the phloem and the xylem. 

 The whole is invested by a parenchymatous sheath resembling a pericycle, 

 but derived, according to Strasburger, 1 from the cortex : outside this is 

 the endodermis, recognisable while young as a single layer, but later 

 obscured by extension of the corky development. A very similar structure 

 to the above is seen also in the thinner branches of L. inundatiim, a 

 species, which as we have seen above, stands in its external morphology in 

 near relation to the section Selago. These species may be taken as repre- 

 senting the structure usually found in the simpler upright, ground-growing 

 members of the genus. 



FIG. 171. 



Diagrammatic transverse sections of the stele of various species of Lycopodiitm ; the 

 phloem is dotted, the xylem drawn as tr.icheides. C = Lyc. scrratiun, Thb.n.. with 

 stellate arrangement. Z> = upright stem of L. cinnotimtin, L., with somewhat stellate 

 arrangement. E=L. cetnitnni, L., with uniform distribution of the small groups. /'= 

 L. volttbile, Forst, with strongly bilateral structure. x8o. (From Engler and Prantl.) 



But a more elaborate construction of the stele is found to accompany 

 the greater differentiation of external form. In creeping and climbing stems 

 there is apt to be an increase in the number of the protoxylems, accom- 

 panied by a development of alternating bands of xylem and phloem : the 

 xylem becomes isolated into distinct masses as seen in the transverse section, 

 and these are roughly disposed parallel to the surface of the substratum 

 (Fig. 171 F). In other cases, and especially in the epiphytes, the xylem 

 and phloem are more uniformly distributed, the former as patches em- 

 bedded in the latter, as seen in tranverse section (Fig. 171 E). Both these 

 conditions may be connected by intermediate steps with the simpler type 

 seen in L. Selago, and as they occur in plants with more specialised form 

 and habit, it may be concluded with some degree of certainty that the type 

 with a connected xylem-tract shows the more primitive state. 



It would seem hardly necessary to insist on this rather obvious outcome 

 of comparison within the genus Lycopodiiun, were it not that a certain 

 misconception, which dates back to the Text-book of Sachs, still survives 



1 Leititngsbahnen, p. 460. 



