32 MORPHOLOGICAL DEVELOPMENT. 



besides simulating the stem in colour and texture. In the 

 petioles of large compound leaves, like those of the com 

 mon Heracleum, we still more distinctly see 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 Ethiopica. 

 One other fact respecting the modifications which leaves 

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

 sume 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 cha 

 racters 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 

 lamina) of the leaves being undeveloped. And the proof 

 is, that in young plants, showing their kinships by their em 

 bryonic 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 gene 

 ral aspects and functions of 

 leaves, have also usurped their 

 structures : though their ve 

 nation is not like that of the leaves they replace, yet they 

 have veins, and in some cases mid-ribs. 



Reduced to their most general expression, the truths 

 above shadowed forth are these : That group of morphologi 

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

 unit called a leaf, has, in each higher plant, a typical form; due 

 to the special arrangement of these cells around a mid-rib and 



