330 LYCOPODIALES 



as to the nature and origin of the more complicated steles of Lycopodium. 

 Comparison of these with polystelic stems of Selaginella long ago suggested 

 that the former structure was derived phylogenetically from the latter, by 

 the lateral fusion of several distinct steles; and thus that the stele of 

 Lycopodium is in reality a compound one. 1 But the polystelic condition 

 seen in some Selaginellas is not uniform for that genus, as we shall see 

 below : moreover it seems improbable that the simpler, homosporous 

 Lycopodium should show structural derivation from the heterosporous Sela- 

 ginella, while it is only in the more complicated Selaginellas that the 

 polystelic condition appears : again, the species of Lycopodium which show 

 distinct xylem-plates are in our view morphologically more advanced than 

 those with the xylem more closely connected. Such considerations go 

 far to negative any idea of the more complex steles of Lycopodium, being 

 compound in their origin. Comparison within the genus is usually a safer 

 guide in such questions than more far-fetched references; and in the 

 present case it suggests a different explanation, which is as follows : that, 

 in a primitively protostelic Lycopod-stock with cylindrical solid xylem, the 

 phloem became progressively intrusive as the morphological differentiation 

 of the plant increased : at first it appeared in the transverse section as 

 occupying a few narrow involutions of the margin of the still connected 

 xylem, this then showing the stellate outline, with fan-like peripheral arms, 

 as seen in the Selago type. But in others the number of the involutions 

 and their depth became greater, till the coherence of the xylem-tract as 

 seen in the single transverse section became interrupted, and the appearance 

 of more or less isolated plates with narrow peripheral edges was attained, 

 as in Z. clavatum. The origin of the xylem-islands as seen in L. 

 squarrosum or Z. cernuum was substantially the same, the difference being 

 that they are not merely intrusive from the margin, but the xylem-tracts 

 are actually for some distance occluded in the phloem. In point of fact 

 these two types of more complicated derivative structure are not strongly 

 differentiated from one another. Thus, from comparison within the genus, 

 it may be figured how from the condition of a primitive protostele with 

 phloem about its periphery the Lycopod-stele became in the more advanced 

 cases a sort of xylem-sponge, with phloem and conjunctive parenchyma 

 occupying the interstices. It will be seen later that the simple protostelic 

 state without intrusive phloem is represented among the fossil Lycopods. 



The relation of the leaves to the central stele in Lycopodium is interest- 

 ing, both in respect to the young and to the mature condition. If the 

 apex of the shoot be investigated, the plerome-cylinder is seen to extend 

 beyond the youngest leaves, to a point immediately below the apical group 

 of cells ; and thus the central region of the stele is cauline in its origin 

 (Fig. 172). The leaves originate from the three or four outer layers of 

 cells of the growing point, quite apart from the plerome, while procambium- 



1 This is specifically stated in Strasburger's Leitungsbahnen, p. 458, and the view has 

 been retained in his Text-book, in the German edition of 1906. 



