The Filicinean Vascular System. 33 
reduced form like a Vittaria is phyllosiphonic, though if 
its immediate ancestors had been protostelic no leaf-gaps could 
have been formed. If the reduction of leaf-traces relatively to the 
stele is great enough, a siphonostelic form like Platyzoina may have 
no leaf-gaps. And in a type like Tmesipteris, which is no doubt 
reduced, but in which the leaf-trace has a considerable importance 
in relation to the size of the stele, leaf-gaps may or may not exist. 
The Morphological Construction of Selaginella compared 
WITH THAT OF THE FERNS. 
Enough has now been said as to the relations between the 
Ferns and the modern microphyllous forms in general, but the 
striking vascular system of the genus Selaginella presents a very 
interesting case of analogy with that of the Ferns, and to this 
analogy it will be useful to devote a little consideration. 
The “ gross anatomy ” of the vascular system of the shoot 
axes of Selaginella is strikingly variable within the genus, 1 which 
indeed presents a range of vascular structure not only unequalled 
among other Pteridophytic genera, but almost comparable with the 
range exhibited by the whole phylum of Leptosporangiate Ferns. 
The points of resemblance between Selaginella and the Ferns 
are numerous and striking. With the exception of the Ferns, 
Selaginella is the most successful Pteridophytic stock in the 
modern world. The five hundred known species inhabit for the 
most part the forests of the damp tropical region. Here they are 
very abundant not only holding their own in the struggle with 
angiospermous types but frequently dominating the ground 
vegetation over considerable areas. Some species are positively 
weeds in many of the wayside ditches and on damp banks bordering 
the roads in the Eastern Tropics. The species are also very 
polymorphic and often difficult to separate, a common sign of 
actively progressive evolution. 
In habit the dorsiventrally organised aerial branch-systems of 
many species strongly resemble the typical repeatedly pinnate fern- 
frond. Each axis of this branch-system bears four rows of leaves, 
the leaves of the two ventral rows standing out on each side of the 
stem in the plane of the branch system, while the smaller leaves of 
the two dorsal rows lie flat upon the upper surface of the stem. The 
whole branch-system with its leaves, like the pinnre of a fern-frond, 
1 R. J. Harvey Gibson. Contributions to the Anatomy of 
Selaginella. I.—The Stem. Ann. of Bot., Vol. VIII., 1894. 
