116 THE NATURAL SYSTEM. 



Class VI. Thallogens, consisting merely of cellular tissue, with a tendency to 

 grow into a flat expansion called thallus, but having no distinction of 

 root, stem, leaves, or flowers. Ex. Lichens, seaweeds, liverworts, 

 fungi. 



352. Affinities of the Six Classes. These may be represented 

 to the sight by the following arrangement • 



Angiosperms. 



Gymnosperms. Agkimaceae. 



Acrogens. Glumaceae. 



Thallogens. 



Angiosperms stand in the highest rank, as they justly merit, by their superior 

 organization. These are nearly alHed to Gymnosperms by their mode of growth ; 

 and, on the other hand, to Aglumacese by their mode of flowering. Gymnos- 

 perms are intimately connected with Acrogens through EquisetaceJB of the latter, 

 which stands intermediate ; and the Aglumaceffi approach the Glumacese, almost 

 indefinitely, through the Junceas (rushes). Between the Acrogens and Thallo- 

 gens a close relationship is established thi-ough the Musci (mosses), while the 

 sporogens form the connecting link between the Endogens and the lowest tribes 

 of vegetation, as the Fungi. Thus, from the highest rank we descend to the low- 

 est, through Gymnosperms and Acrogens on the one hand, and through Aglu- 

 macese and Glumace£e on the other, forming a circle of afiinities. 



353. The mutual relations of the six classes -with the higher divisions, are pre- 

 sented in the following synopsis : 



fExOGENS: \^]^'' I Angiosperms. 

 f Ph^nogamia ; ] ' ^kss II. Gymnosperms. 

 Vegetable I ' [ Endogens ; \ ^\^'' l^/ ^^^^'^^^-'^^^o^s. 

 Kingdom A ^ ' f ^]^'' ^Z' Glitmaceods. 

 n^^^rr^r^^ . ,,, . i Class V. Acrogens. 



I Cryptogamia 5 I ^^^^^ ^ Thallogens. 



354. Sub-classes. The classes are next to be broken up 

 into smaller divisions. In eifecting tliis object most writers 

 have employed artificial methods, since no natural one, founded 

 upon clear and comprehensive distinctions, has yet been de- 

 vised. Thus Angiosperms, which class is by far the largest of 

 the six, is divided into three sub-classes. Polypetal^, or poly- 

 petalous Exogens, flowers with distinct petals; Monopetal^, 

 or monopetalous Exogens, flowers with united petals ; Apet- 

 al^, or apetalous Exogens, flower with no floral envelopes, or 

 with a calyx only. 



355. Orders, or Families, are the most important of all the 

 natural associations. On the accuracy and distinctness of the 



