48 MORPHOLOGICAL DEVELOPMENT. 



of the ordinary colour, and still bearing leaves. But in 

 kindred plants, as Euphorbia neriifolia, this swelling of the 

 lateral axes is carried to a far greater extent; and, at the 

 same time, a green colour and a fleshy consistence have been 

 acquired: the typical relations nevertheless being still shown 

 by the few leaves that grow out of these soft and swollen 

 axes. In the Cactacece, which are thus resembled by plants 

 not otherwise allied to them, we have indications of a 

 parallel transformation. Some kinds, not commonly brought 

 to England, bear leaves; but in the species most familiar to 

 us, the leaves are undeveloped and the axes assume their 

 functions. Passing over the many varieties of form and 

 combination which these green succulent growths display, we 

 have to note that in some genera, as in Phyllocactus, they 

 become flattened out into foliaceous shapes, having mid-ribs 

 and something approaching to veins. So that here, and in 

 the genus Epiphyllum, which has this character still more 

 marked, the plant appears to be composed of fleshy leaves 

 growing one upon another. And then, in Rhipsalis, the 

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

 would regard them as leaves. These which are axial organs 

 in their homologies, have become foliar organs in their 

 analogies. When, instead of comparing these 



strangely-modified axes in different genera of Cactuses, we 

 compare them in the same individual, we meet with transfor- 

 mations 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 devi- 

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

 Rhipsalis rhombea 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 resulted 



