THE MORPHOLOGICAL COMPOSITION OF PLANTS. 41 



shapes, and approaching in their sizes, to those on the stem; 

 besides simulating the stem in colour and texture. In the 

 petioles of large compound leaves, like those of the com- 

 mon Heracleum, we see still more distinctly both internal 

 and external approximations in character to axes. Nor are 

 there wanting plants whose large, though simple, leaves, are 

 held out far from the stems by foot-stalks that are, near the 

 ends, sometimes so like axes that the transverse sections of 

 the two are indistinguishable ; as instance the Calla palustris. 



One other fact respecting the modifications which leaves 

 undergo, should be set down. Not only may leaf-stalks 

 assume to a great degree the characters of stems, when they 

 have to discharge the functions of stems, by supporting many 

 leaves or very large leaves; but they may assume the charac- 

 ters of leaves, when they have to undertake the functions 

 of leaves. The Australian Acacias furnish a remarkable 

 illustration of this. Acacias elsewhere found bear pinnate 

 leaves; but the majority of those found in Australia bear 

 what appear to be simple leaves. It turns out, however, that 

 these are merely leaf-stalks flattened out into foliar shapes: 

 the laminae of the leaves being undeveloped. And the proof 

 is that in young plants, showing their kinships by their 

 embryonic characters, these leaf-like petioles bear true leaflets 

 at their ends. A metamorphosis of like kind occurs in Oxalis 

 bupleurifolia, Fig. 66. The fact most deserving of notice, 

 however, is that these leaf- 

 stalks, in usurping the gen- 

 eral aspects and functions 

 of leaf-blades, have, to some 

 extent, also usurped their 

 structures : though their venation is not like that of the leaf- 

 blades they replace, yet they have veins, and in some cases 

 mid-ribs. 



Eeduced to their most general expression, the truths above 

 shadowed forth are these: — That group of morphological 

 units, or cells, which we see integrated into the compound 



