INTEODUCTION. ix 



grouping, has shown the plans on which all animals are con- 

 structed. He ignores the positive and negative of vertebrate 

 and non-vertebrate, and employs positive characters only in 

 defining his divisions ; these are Vertebrates, MoUusks, Arti- 

 culates, and Radiates : and a little reflection will convince 

 any botanist that there are four great divisions of plants, 

 equally capable of being distinguished by positive characters ; 

 these are Exogens, Endogens, Acrogens, and Thallogens. 



Acrogens, in common with Thallogens, are without flowers ; 

 " nothing can be found which resembles the stamens and 

 pistils of flowering plants : " they have usually distinct roots, 

 stems, and leaves, the two halves of the latter being generally 

 symmetrical ; these characters 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 Acro- 

 gens has been wholly misunderstood. Abundant evidence 

 exists that there is in these discoveries no contradiction to 

 the assertion, that Acrogens, so far as our researches have 

 extended, are perfectly 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 Lyco- 

 podiums. Quill-worts, Pill-worts, Marsilias, Equisetums, and 

 Charas : they have sometimes been called Cryptogamic Vascu- 

 lares ; but I prefer to define and divide them in the following 

 manner, which, it wiU be observed, strikingly diffei;^ 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. 



h 



