44 MORPHOLOGICAL DEVELOPMENT. 



modifications of form and structure. Ity contemplating the 

 changes here displayed within the limits of a single order, 

 we shall greatly widen our conception of the possibilities of 

 metamorphosis in the vegetal kingdom, taken as a whole. 

 Two different, but similarly-significant, truths are illustrated. 

 First, we are shown how, of these two components of a 

 flowering plant, commonly regarded as primordially distin- 

 guished, one may assume, throughout numerous species, the 

 functions, and to a great degree the appearance, of the other. 

 Second, we are shown how, in the same individual, there 

 may occur a re-metamorphosis — the usurped function and 

 appearance being maintained in one part of the plant, while 

 in another part, there is a return to the ordinary appearance 

 and function. We will consider these two truths separ- 

 ately. Some of the Euphorbiacece, which simulate 

 Cactuses, show us the stages through which such abnormal 

 structures are arrived at. In. Eupliorbia sple?idens, the lateral 

 axes are considerably swollen at their distal ends, so as often 

 to be club-shaped : still, however, being covered with bark 

 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 Ertiphyllum, which has this character still more 



