102 FREDERICK ORPEN BOWER 



In the first place Lycopodium belongs to the Eligulate Lycopodi- 

 ales, which are, and appear always to have been homosporous. 

 Selaginella belongs to the Ligulate Lycopodiales, which are, and 

 from Palseozoic times have certainly been heterosporous. There is 

 good reason to believe that the two phyla have been distinct from 

 one another from a very early period, and a comparison of their 

 living representatives, together with the evidence of their fossil 

 history, leads to a different conclusion from that above stated. 

 Taking the Ligulate series, comparison of those now living indi- 

 cates that the radial upright types which centre round S. spinulosa 

 are relatively primitive, while those with dorsiventrally flattened 

 shoots are derivative. It is in some of the latter only that the 

 polystelic state, which was the basis for comparison with Lyco- 

 podium, exists. It is thus seen to be probably a late and deriva- 

 tive state. The species of the type of S. spinulosa have a single 

 protostele corresponding in its main features to that of the early 

 Ligulate fossils, and this supports the comparative conclusion 

 arrived at on the evidence of the fossils. From both sources the 

 conclusion is thus attained that the primitive state was proto- 

 stelic, and the polystelic state seen in the dorsiventral species is 

 relatively late and derivative. Turning to the Eligulate series, 

 the evidence is less complete though still sufficient. A compari- 

 son of living species of Lycopodium indicates that those which are 

 in general features most primitive, such as the upright types 

 associated with Lycopodium Selago, have a relatively simple stele, 

 with a continuous xylem-tract, which may, however, be variously 

 lobed as seen in transverse section. The types where the xylem 

 appears in distinct plates are those which on general lines of com- 

 parison are held to be more differentiated and advanced, as is the 

 case in L. clavatum. Thus in both Ligulate and Eligulate Lyco- 

 podiales the vascular structure is referable on a basis of broad com- 

 parison to the protostelic state, with a simple and solid xylem - 

 core. Both of these apparently distinct phyletic lines have pro- 

 ceeded to more elaborate structures, in the former case with dis- 

 tinct steles, in the latter with more or less distinct xylem-plates 

 within the stele. There appears to have been a parallel response 

 to a need for elaboration, which has led to the states exemplified 



