CHAPTER XXV. 



COMPARATIVE ANATOMY OF THE LYCOPODIALES. 



IT 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. 



Taking, therefore, first the less differentiated Selago 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 c). Small tracheides forming the protoxylem lie at 

 the extreme periphery, while the centrally-disposed metaxylem is composed 



