258 PHYSIOLOGICAL DEVELOPMENT. 



branch continues to show, by the formation of chlorophyll, 

 that it shares in the duties of the leaves; but in proportion 

 as a bark which the light cannot penetrate is produced by 

 the adherent flakes of dead skin, or by the actual deposit of 

 a protective substance, the differentiation of duties becomes 

 more decided. Cactuses and Euphorbias supply 



us with converse facts having the same implication. The 

 succulent axes so strangely combined in these plants, main- 

 tain for a long time the translucency of their outer layers 

 and their greenness ; and they so efficiently perform the offices 

 of leaves that leaves are not produced. In some cases, axes 

 that are not succulent participate largely in the leaf-function, 

 or entirely usurp it still, however, by fulfilling the same 

 essential conditions. Occasionally, as in Statice brassiccefolla, 

 stems become fringed; and the fringes they bear assume, 

 along with the thinness of leaves, their darker green and 

 general aspect. In the genus Ruscus, the flattened axis 

 simulates so closely the leaf-structure, that were it not for the 

 flower borne on its midrib, or edge, its axial nature would 

 hardly be suspected. And let us not omit to note that where 

 axes usurp the characters of leaves, in their attitudes as well 

 as in their shapes and thicknesses, there are contrasts between 

 their under and upper surfaces, answering to the contrasts 

 between the relations of these surfaces to the light. Of this 

 Ruscus androgynus furnishes a striking example. In it the 

 difference which the unaided eye perceives is much less con- 

 spicuous than that disclosed by the microscope; for I find 

 that while the face of the pseudo-leaf has no stomata, the back 

 is abundantly supplied with them. One more illustration 

 must be added. Equally for the morphological and physio- 

 logical truths which it enforces, the MiihlenbecJcia platyclada 

 is one of the most instructive of plants. In it the simulation 

 of forms and usurpation of functions, are carried out in a 

 much more marvellous way than among the Cactacece. 

 Imagine a growth resembling in outline a very long willow- 

 leaf, but without a midrib, and having its two surfaces alike. 



