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,! 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 ZL. zxundatum, a 
species, which as we have seen above, stands in its external morphology in 
near relation to the section Se/ago. These species may be taken as repre- 
senting the structure usually found in the simpler upright, ground-growing 
members of the genus. : 
Fic. 171. 
Diagrammatic transverse sections of the stele of various species of Lycopodium ; the 
phloem is dotted, the xylem drawn as tracheides. C=Lyc. serratum, Thbg., with 
stellate arrangement. D-=upright stem of L. annotinum, L., with somewhat stellate 
arrangement. E£=L. cernuum, L., with uniform distribution of the small groups. F= 
L. volubile, Forst, with strongly bilateral structure. X80. (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 £). Both these 
conditions may be connected by intermediate steps with the simpler type 
seen in LZ. Sedago, 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 Lycopodium, were it not that a certain 
misconception, which dates back to the Text-book of Sachs, still survives 
1 Lectungsbahnen, p. 460. 
