THE OUTER TISSUES OF PLANTS. 241 



us with converse facts having the same implication. The 

 swollen succulent axes so strangely combined in these plants, 

 maintain for a long time the transparency of their outer 

 layers ; and doing this, 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-func- 

 tion, or entirely usurp it — still, however, by fulfilling the 

 same essential conditions. Occasionally, as in Sfatice bras- 

 sicoeplia, stems become fringed ; and the fringes they bear 

 assume, along with the thinness of leaves, their darker green 

 and general aspect. In the genus JRuscas, 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 thickness, there exist 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 physiological 

 truths which it enforces, the Coccoloba platycladon 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. Imagine that 

 across this thin, green, semi-transparent structure, there are 

 from ten to thirty divisions, which prove to be the successive 

 nodes of an axis. Imagine that along the edges of this 

 leaf-shaped aggregate of internodes, there arise axillary 

 buds, some of which unfold into flowers, and others of which 

 shoot up vertically into growths like the one which bears 



