38 LYCOPODIALES 
Ww 
differentiation of the primary wood is nearly simultaneous over its whole 
area, but with indications of centripetal succession. The cambial activity 
starts early, being continuous from that of the primary meristem. As a 
rule the same cambium is active throughout, producing secondary ground 
tissue, wood, and phloem on its inner side, and cortical parenchyma 
only towards its exterior; but other arrangements are found, while in 
some cases a second cambial activity may arise inside or without the 
first. The adjoining diagram, quoted from Scott and Hill (Fig. 177), 
shows the relation of the primary and secondary tissues usual in Z. Aystrix, 
and it will be noted that the secondary phloem is internal to the secondary 
xylem; the cambium les outside the latter in direct contiguity with the 
secondary cortex, which arises externally 
from it. The stele which is cylindrical 
above becomes in JZ. Ayst77x triquetrous 
below, in Z /acus¢r7s it is usually flattened 
bilaterally: the change of form is a 
secondary consequence of the abutment 
of the numerous, successively formed 
root-bases upon it, and does not affect 
the general comparisons. Scott and Hill 
conclude that the anatomy of the stem 
with its solid stele, from which the 
densely crowded small and simple leaf- 
traces pass off, is just what might be 
expected in a stunted Lycopod, while the 
anomalous character of the secondary 
Diagrammatic transverse section of the upper thickening in /soe/es agrees in some 
cylindrical part of the stele of /soetes, Hystr?x. measure with that in certain fossil Lyco- 
-v=primary wood ; £/2=region in Ww hich secon- 
dary phloem has dev eloped ; ®, that in which C 1 a ‘a 
sscnnes ary xylem has developed ; "¢b=cambium pods. Scott? has remarked on the stem 
ondary cee ?¢=leaf-traces. x 42. 2 7, > y ‘oy, ing 
(after Scott and Hill) of Lepidodendron fuliginosum as having 
Fic. 177. 
an anomalous cambium producing a 
good deal of secondary parenchyma, among which there are scattered 
groups of wood; and he regards this species as exhibiting a primitive 
and rudimentary form of secondary growth. It seems to offer a distinct 
analogy with Jsoe¢es. On the other hand, the slight cambial increase 
discovered in Se/aginella spinulosa by Bruchmann affords some link as 
regards secondary thickening, though a feeble one, with a living Lycopod. 
The general result of this anatomical examination and comparison of 
Ssoetes is accordingly to strengthen its position among the Lycopodiales, 
and to show that its primary vascular arrangement corresponds in 
essentials to the type as exemplified in living species of Lycopodium, 
but much abbreviated, and with the xylem reduced in accordance 
with the aquatic or amphibious habit prevalent in the genus. It also 
appears that the secondary development, though showing fluctuating 
1 Studies, p. 143. 
