79 
contrasting their structure with 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. Angiosperms, 2. Gymnosperms, 
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. Ifthe annual branches of a Smilax are exa- 
mined, there 1s nothing in their internal structure at variance 
with that of a stem of Asparagus; they are exactly Endoge- 
nous ; but in the rhizoma of 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 Philesia 
have each a zone of wood below their bark, and a central pith 
