LINNEAN SOCIETY OF LONDON. 73 



into anything more complex *. On the whole, Prof. Lignier's 

 idea that the Lycopods stand apart from the rest of the Vasculares 

 appears quite teuahle, though by no means proved. It is con- 

 firmed by the simple relation between sporangium and sporophyll 

 which prevails throughout the group, and by the fact that the 

 Lycopods are the only vascular plants in which there is a want of 

 sharp differentiation between root and shoot. The former 

 character may not be a primitive one (Prof. Lignier himself 

 regards tlie terminal position of the sporangium on a branch as 

 the original ari-angement ; other botanists suggest the presence of 

 a reduced sporangiophore) ; but the existence of so many transi- 

 tional forms between root or rootlet and stem or leaf is a strong 

 indication of a relatively primitive and isolated ])osition. 



I may here recall that Mr. Tansley has touched on the position 

 of the Lycopods in a very illuminating way in the first and the 

 last of his lectures on the Evolution of the Filicinean Vascular 

 System t. He recognizes the peculiar cliaracter of their leaves, 

 contrasting so sharply with the megaphylly of other Pteridophytes, 

 but the explanation he suggests is different from Prof. Lignier's. 

 He says that the Lycopods " may be independently derived from 

 the primitive Propteridophytes by foliar specialisation of short 

 undivided hranchlets of the thalliis, instead of ivJiole branch systems 

 as in the Filicinean type " (p. 9). This, as he points out, would 

 bring the Lycopods into line with the other Pteridophytes without 

 assuming any extensive reduction, or abandoning, in this case, the 

 thallus-branch theory of the leaf, which he regards as by far 

 the most rational and convincing which has yet been suggested. 



I should like to dwell on the wonderfully instructive comparison 

 which Mr. Tansley draws between the morphological construction 

 of Selaginella and that of a Fern with its fronds, but must content 

 myself with a couple of short quotations. " In Selaginella we 

 have a very old if not a primitively microphyllous stock which 

 modifies whole branch-s3'stems for assimilatimg purposes. The 

 leaf itself is so small as to exercise no influence on the general 

 conformation of the vascular system, and corresponds physio- 

 logically with the ultimate pinnule or segment of the lamina in a 

 fern-frond. But the branch-system as a whole retains its plas- 

 ticity and becomes moulded on lines parallel with those of the 

 fern-frond as a whole" (p. 135), "In the frond-like dorsiventral 

 type of branch-system seen in some species of Selaginella we have 

 in fact a kind of working model of the hypothetical thallus of the 

 ' pro-Lycopod,' the leaves representiug the ultimate assimilating 

 branchlets, and the whole showing a convergence with a fern- 

 frond hypothetically derived by integration of a whole thalloid 

 branch-system'*' (p. 136). 



To return to Prof. Lignier. The Phylloideae are after all a 

 limited group now, though so prominent in the Palaeozoic Floras. 



* The doubling of the vascular bundle in Siyil/oriopais is the only case in 

 point, but does not seem to iiave led to anything iurther. 

 t ' New rhytologist," Reprint, No. 2. Cambridge, 1908. 



