THE MOKI'HOT.OGICAL COMPOSITION OF PLANTS. 



45 



rnailced, the plant appears to be composed of fleshy 

 leaves growing one upon another. And then, in Rhipsalls^ 

 the same parts are so leaf-like that an uncritical observer 

 would regard them as leaves. These which are axial organs 

 in theii' homologies, have become foliar organs, in their 

 ajialogies. When, instead of comparing these 



strangely-modified axes in different genera of Cactuses, we 

 compare them in the same indi\idual, we meet with transform- 

 ations no less striking. Where a tree-like form is pro- 

 duced by the growth of these foliaceous shoots, one on another ; 

 and where, as a consequence, the first- formed of them become 

 the main stem that acts as support to secondary and tertiary 

 stems ; they lose their green, succulent character, acquire 

 bark, and become woody — ^in resuming the functions of axes 

 they resume the structures of axes, from which they had de- 

 viated. In Fig. 71 are shown some of the leaf-like axes of 

 Rliijjsalis rJiomhea in their young state ; while Fig. 72 repre- 

 sents the oldest portion of the 

 same plant, in which the foli- 

 aceous characters are quite 

 obliterated, and there has re- 

 sulted an ordinary stem-struc- 

 ture. One further 

 fact is to be noted. At the' 

 same time that their leaf-like appearances are lost, the 

 Axes also lose their separate individualities. As they become 

 stem-like, they also become integrated ; and they do this so 

 efiectually, that their original points of junction, at first so 

 strongly marked, are effaced, and a consolidated trunk is 

 produced. 



Joined with the facts previously specified, these facts 

 help us to conceive how, in the evolution of flowering plants 

 in general, the morphological components that were once 

 distinct, may become extremely disguised. We may ration- 

 ally expect that during so long a course of modification, 

 much greater changes of form, and much more decided fiisioue 



