32 MORPHOLOGICAL DEVELOPMENT. 



besides simulating tlie stem in colour and texture. In the 

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

 mon Ileroicleum, we still more distinctly see both internal 

 and external aj)proximations in character to axes, l^or 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 

 laminae of the leaves being undeveloped. And the proof 

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

 bryonic characters, these leaf-Kke petioles bear true leaflets at 

 their ends. A metamorphosis of like kind occurs in Oxalis 

 buplcurifolia, 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 imits, 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 



