22 DISTBIUDTION AND AFFINITIES. 
As to their affinities, the ferns approach flowering plants 
through the Cycads, a group of Gymnogens, which may 
be considered as closely allied to them, on account of the 
imperfect degree in which the vascular system of that or- 
der is developed, of their pinnate leaves with a gyrate 
vernation, and of their naked ovules borne on the margins 
of contracted leaves, as the spore-cases are upon the leaves 
of Osmunda and Ophioglossum. To Conifers, another 
group of Gymnogens, they also advance very closely 
through Salisburia, whose leaves might be mistaken for 
those of a fern. Among Acrogens, their affinity with 
Horsetails (Equisetum) is not very obvious, consisting 
chiefly in the presence of annular vessels, and in the ab- 
sence of flowers. The Club-mosses (Lycopodium) which 
have some relationship, are distinguished readily by their 
axillary spore-cases, which dehisce by regular valves. 
The Pepperworts (Marsileacea) are, according to Lindley, 
much more remote. These latter, however, the Horsetails. 
Club-mosses, and Pepperworts. are popularly considered 
as being specially " fern- allies." 
