EXOGENS. 237 



in the form of medullary processes ; and the woody matter of their bark, 

 so to call their cortical integument, is not parallel with that of the wood and 

 spontaneously separable from it. The only way in which the growth of the 

 stem of Exogcns corresponds with that of Endogens is that in both classes the 

 woody matter is connected with the leaves ; and in both a cellular substance 

 is the foundation of the whole structure. Nevertheless, attempts have been 

 made by some modern physiologists to identify the two, and to show that 

 the one is very little different from the other. 



It is not, however, to be supposed that themanner of growth in Exogens, 

 is in all cases exactly what has been thus described as its normal condition. 

 On the contrary, a great variety of modifications has been found to exist, 

 dependent in part upon an excessive development of cellular matter, and in 

 part upon the formation of angles, lobes, or sinuosities, upon the loss of con- 

 centric rings of wood for which a great homogeneity of structure is substi- 

 tuted, and upon the production of irregular zones of cellular matter resem- 

 bling bark, between the zones of wood. Cases of this kind have attracted 

 the attention of most modern botanists. Several have been noticed in my 

 Introduction to Botany, in the Penny CydopcBdia, art. Exogens, and by 

 Decaisne, Adrien de Jussieu, Schultz, Gaudichaud and Schleiden ; but they 

 have not been applied successfully to systematical purposes. In a sketch of 

 a possible plan of extending the classes of plants at the expense of Exogens 

 [Bot. Reg. 1839. Misc. p. 76), I have suggested the formation of a group to 

 be called Homogens, to which it has been proposed to unite Birthworts, 

 Nepenths, Lardizabalads, Menisperms, Peppers, and several other Orders. 

 The character upon which reliance was placed was the remarkable nature 

 of the wood of these plants, which never have more than one zone of woody 

 matter, to whatever age they may have arrived. M. Decaisne has liowever 

 shown {Memoire sur les Lardizabalees), that although this peculiarity is 

 extremely striking in some cases, as for example, in Aristolochia labiosa, yet 

 that it is not constant in even the same Order, A. Clematitis having annual 

 zones ; and that in Menispermads, while there is a great departure from 

 the ordinary structure of Exogens, except Aristolochia so far as regards 

 the liber, the wood is regularly zoned in many instances, although the 

 dotted vessels are wanting. 



Nevertheless, although from the very imperfect state of infomiation 

 concerning the true structm-e of the stems of plants, I am unable to offer, for 

 retaining this division, such reasons as would be satisfactory, yet I think it will 

 be recognised hereafter, either wholly or in part ; at least I am persuaded that 

 the time will come when the internal structure of the stem will be far more 

 extensively consulted than it now is, and be made the basis of good and 

 important systematic divisions. Schultz preceded me in this attempt, in pre- 

 paring his Synorgana dichorganoidea {Naturliches System des Pflanzenreichs, 

 p. 319, 1832), to which he referred Piperaceje, Saururaceae, Chloranthacese, 

 Nyctaginacese, Callitrichacese, Hippuridacese, Myriophyllaceae, Amarantha- 

 cese, Cycadeacese, Nymphseacese, Nelumbiacese, and Diphylleiaceae, and his 

 proposition, like mine, has fallen to the ground. But although the genera 

 he collects under Diphylleiacese, namely, Diphylleia, Podophyllum, and 

 others, are in no wise different from the ordinary state of herbaceous Exo- 

 gens, yet it must be admitted that Hippurids and several of the others offer 

 less resemblance to that plan of organisation. It is difficult to say whether 

 Schleiden contemplates the possibility of any similar division ; but it is 

 worthy of notice that he, in his paper On the Anatomko-'physiohgical Differ- 

 ences in the Structure of Stems, translated in the Annals of Botany (iv. 240) 



