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contrasting their structure witli the concentrically zoned 

 growth of other Exogens, to which the collective name of 

 Cyclogens may be applied. In this manner Exogens are 

 composed of three classes, 1. Anglosperms, 2. Gi/mnosperms, 

 and 3. Homogens. 



Among Endogens I find a group of exactly the same 

 nature as the last, and differing from the mass of the order in 

 nearly the same manner. The peculiar habit of Smilax and 

 some other Endogens, which no one would suppose from their 

 general appearance to belong to that class, some time since 

 led me to propose the separation of them into a group which 

 was called the Retose. But as I had no better character for 

 it than the reticulated leaves, nobody seems to have adopted 

 it, and it has been regarded as an unnecessary separation of 

 plants essentially the same ; an opinion to which, in the ab- 

 sence of better evidence than I have before been able to offer, 

 there has been nothing to oppose beyond the conviction that 

 the Retose group is in nature well founded, although its true 

 characters may have been undiscovered. It now however 

 appears that Smilax and its allies have the wood of their 

 stem arranged upon a plan extremely similar to that of Ho- 

 mogens ; and consequently they will constitute, not a subdi- 

 vision of Endogens as I formerly supposed, but a new class or 

 primary group. If the annual branches of a Smilax are exa- 

 mined, there is nothing in their internal structure at variance 

 with that of a stem of Asparagus ; they are exactly Endoge- 

 nous ; but in the rhizomaof the whole genus (take the Sarsa- 

 parilla of the shops for instance) the wood is disposed in a 

 compact circle, below a cortical integument, and surrounding 

 a true pith ; so that the rhizoma or permanent part of the stem 

 is that of a Homogen. In Dioscorea alata the stem is formed 

 of eight fibrovascular wedges placed in pairs, with their backs 

 touching the bark, surrounding a central pith and having 

 wide medullary plates between them ; in fact, when the stems 

 of this plant are in a state of decay, the eight fibrovascular 

 wedges may be pulled asunder, like those of a Menisperma- 

 ceous plant. In Testudinaria elephantipes the structure of the 

 stem is of nearly the same kind ; several bundles of fibrovas- 

 cular tissue form a circle surrounding a pith, and pierced 

 with broad medullary processes. Lapageria and Philcsia 

 have each a zone of wood below their bark, and a central pitli 



