330 LYCOPODIALES 
as to the nature and origin of the more complicated steles of Lycopodium, 
Comparison of these with polystelic stems of Se/aginel/a 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.! But the polystelic condition 
seen in some Sé/aginel/as 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 Se/a- 
ginella, while it is only in the more complicated Se/aginellas 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 Se/ago 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 ZL. clavatum. The origin of the xylem-islands as seen in JZ. 
sguarrosum or L. 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- 
1This is specifically stated in Strasburger’s Lectangsbahnen, p..458, and the view has 
been retained in his Text-book, in the German edition of 1906. 
