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clothed with leaves ; in others they are irregular cellular ex- 

 pansions, destitute of true leaves ; in the former we find a 

 trace of something equivalent to the sexes of Exogens and 

 Endogens, in the latter all indications of the kind disappear. 

 Thus are formed the two groups now called 5. Cormophytes, 

 where there is a stem and leaves, &c., and 6. Thallophytes, 

 where there is no separation of those parts. 



To what extent dismemberments of the three classes of 

 Jussieu may be further carried, there is no evidence to shew ; 

 it is not however probable that they are capable of much 

 further increase. For with a few exceptions, the affinities of 

 the six primary groups now indicated are too continuous and 

 complete to allow us to suppose that any great physiological 

 or fundamental difterences of organization exist among them. 

 Upon the few exceptions that do exist I propose to offer some 

 observations. 



Among Angiospermous Exogens the Natural orders Aris- 

 tolochiacecB, Nepenthacece, Lardizahalacecs, Menispermacecs, 

 Piperacece, and some others allied to the latter, stand isolated, 

 as it were, in whatever part of the group they are stationed, 

 having no obvious affinity with any other orders ; for we can 

 only regard the approximation of Mcnispermacece to Anonacece, 

 &c. as the result of altogether artificial considerations. Now 

 all these orders agree in one remarkable circumstance. In- 

 stead of their wood being formed by zone deposited over zone, 

 season after season, as is the case in the great mass of Exo- 

 gens, they never have more than one zone of woody matter, 

 to whatever age they ma}^ have arrived. Whether their 

 wood itself is formed exactly in the same way as that of other 

 Exogens, namely, by a gradual external addition of stratum 

 upon stratum, is doubtful ; it is probable that they have a 

 mode of growth of their own, analogous to that of Aristolo- 

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

 successive 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 Dico- 

 tyledonous embryo, and in general appearance, but their 

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

 be presently noticed, and I therefore think 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 



