161 
* 
plants, equally capable of being distinguished by positive characters ; 
these are Exogens, Endogens, Acrogens, and Thallogens. 
*“‘ Acrogens, in common with Thallogens, are without flowers ; ‘no- 
thing can be found which resembles the stamens and pistils of flow- 
ering plants:’ they have usually distinct roots, stems, and leaves, 
the two halves of the latter being generally symmetrical; these cha- 
racters serving at once to distinguish Acrogens from Sea-weeds, 
Lichens, or Fungi. Interesting as are the discoveries which Nageli 
and his followers have made on the pro-embryo of ferns, and which I 
had the pleasure of introducing to the notice of British botanists 
(Phytol. iii. 613 and 925), their ,bearing on the diagnostic characters 
of Acrogens has been wholly misunderstood. Abundant evidence 
exists that there is in these discoveries no contradiction to the asser- 
tion, that Acrogens, so far as our researches have extended, are per- 
fectly asexual. 
“ Acrogens are either vascular and Pteridoid, or cellular and 
Mnioid: the first including all ferns and their allies, and the last, all 
mosses and their allies. The allies of ferns are Lycopodiums, Quill- 
worts, Pill-worts, Marsilias, Equisetums, and Charas: they have 
sometimes been called Cryptogamic Vasculares ; but I prefer to de- 
fine and divide them in the following manner, which, it will be 
observed, strikingly differs from the most popular and most recent 
arrangements. The division of the Filicales splits the universally 
received genera of Pteris, Polypodium, Asplenium, Davallia, and 
many others. 
“ACROGEN PTERIDOID.4, Pteridoid Acrogens, or Ferns and 
their allies, are plants of vascular structure, but which pro- 
duce fruit without preliminary flowers: they may be divided 
thus :— 
FILICALES [or annulate Ferns] have distinct leaves bearing 
one-celled capsules which are encircled by an elastic 
ring: they comprise :— 
RHIZOPHYLLACE2, in which the leaves are attached to a rhi- 
zome or root. 
CoRMOPHYLLACE, in which the leaves are attached to a 
cormus or trunk. 
OSMUNDALES [or exannulate Ferns] have distinct leaves 
and one-celled capsules detached from the leaves, and 
not encircled by an elastic ring: they comprise :— 
VOL. V. Ne 
