LEAFY SURFACE OF PETIOLES. 



151 



from what we observe in common plants, we might suppose to be 

 the case. The petiole is sometimes expanded into a leafy sur- 

 face, and may even perform all the functions of the true leaves, 

 when the latter are deficient. Thus in a British aquatic plant, 

 known by the name of Arrow-head (technically Sagittaria sagit- 

 tifolia) which is common in running streams, we observe what 

 appear to be two kinds of leaves ; 

 some elevated above the surface, 

 and formed like the head of an 

 arrow (whence the name of the 

 plant) ; and others flattened, of 

 equal breadth throughout (tech- 

 nically termed ligulate or strap- 

 shaped), and not appearing above 

 the water. These last are in 

 reality the flattened petioles, 

 which perform the functions of 

 leaves, as long as they remain 

 under water; but as soon as any 



of them have Strength to elevate tioles beneath the water, and true 

 . , . . , , . . ,, leaves above. 



their summits above its surface, 



true leaves are developed from them, and tho petiole then con- 

 tracts into a rounded form. 



228, A corresponding structure ,is exhibited by some of the 

 Acacias of New Holland ; which are sometimes so completely 

 destitute of true leaves, as to be termed " leafless." When this 

 is the case, the petioles are flattened and expanded, and present a 

 leaf-like surface, which is adapted to perform the functions of the 

 true leaves ; from these they differ, however, in having the two 

 surfaces alike, and in their expansion being vertical, instead of 

 horizontal. The true leaves (which, like those of other Acacias, 

 are very compound in their character, see Fig. 61,) are only to be 

 found in young plants, or in old ones which have been freely 

 pruned ; and it is not uncommon to find many degrees of deve- 

 lopment, intermediate between those which exhibit the fully ex- 

 panded leaf with its narrow and cylindrical petiole, and those 

 which have no vestige of the blade, and present nothing but 



