228 NEW DISTRIBUTION OF 



chia, in which the wood when young is augmented by the suc- 

 cessive introduction of wedge upon wedge of wood between 

 wedges originally placed concentrically around a medullary 

 axis. Such plants as these agree with Exogens in their Di- 

 cotyledonous embryo, and in general appearance, but their 

 mode of growth is an approach to that of some Endogens to 

 be presently noticed, and it therefore appears they ought to be 

 regarded as a fundamental group, which from the homoge- 

 neity of the wood may be called Homogens, for the sake of 

 contrasting their structure with the concentrically zoned growth 

 of other Exogens, to which the collective name of Cyclogens 

 might be applied. In this manner Exogens are composed of 

 three classes, 1. Exogens proper, 2. Gymnogens, and 3. Homo- 



7. Among Endogens we 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, led me formerly to 

 propose the separation of them into a group which was called 

 the Retose. But as no better character could be found for it 

 than the reticulated leaves, nobody adopted it, and it has been 

 regarded as an unnecessary separation of plants essentially the 

 same ; an opinion to which, in the absence of evidence, there 

 has been nothing to oppose beyond the conviction that the 

 Retose group is in nature well founded, although its true cha- 

 racters may have been undiscovered. It now, however, ap- 

 pears that Smilax and its allies have the wood of their axis 

 arranged upon a plan wholly or in part similar to that of Homo- 

 gens ; and consequently they will constitute, not a subdivision of 

 Endogens as was formerly supposed, but a new class or pri- 

 mary 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 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 



