CHAPTER XAV. 
COMPARATIVE ANATOMY OF THE LYCOPODIALES. 
Ir has been already noted that the Lycopods are marked off from other 
Vascular Plants by the simple and regular arrangement of their sporangia 
in relation to the other parts of the shoot: also that the characters of 
the shoot themselves suggest in their simple form and arrangement a 
primitive state. The Lycopods are no less notable for their anatomical 
characters, and especially those of the Vascular System. They stand apart 
from almost all other Vascular Plants in the presence in their mature axes 
of a stele having peripheral protoxylem, and often showing the solid 
xylem-core characteristic of the protostele. The leaf-traces insert them- 
selves with the minimum of local disturbance upon the periphery of the 
columnar stele, which is further shown by its development to be cauline 
(compare Fig. 67, p. 125). Exceptions from this simple vascular construc- 
tion occur within the phylum: but a comparative examination of the various 
forms will show that the®non-medullated monostele may be accepted as 
a central type of construction for them all, upon which certain modi- 
fications and variants have arisen: some of these are exemplified in the 
fossils, some in plants now living. The comparisons will be primarily 
based upon the structure of the mature shoot. The same order will be 
maintained as in the description of the external morphology, and it will 
be found that the anatomical complexity follows, with some degree of 
exactness, that of the external form. A 
Taking, therefore, first the less differentiated Se/ago section of the genus 
Lycopodium, as seen in L. Selago, serratum, or lucidulum, the cylindrical 
stele is there found to consist of a connected central mass of xylem of 
irregularly star-like form: the rays of the star vary in number in different 
species, as well as in different regions of the same plant, and are specially 
characterised by the form of the periphery of the rays: these expand 
outwards into a wide-spread, almost fan-like outline, as seen in the trans- 
verse section (Fig. 171 Cc). Small tracheides forming the protoxylem lie at 
the extreme periphery, while the centrally-disposed metaxylem is composed 
