SECT. I. ORGANOGRAPHY AND GLOSSOLOGY. 69 



compound leaf (fig. 64>. a) and the simply expanded 

 petiole just described (i) ; the latter* being more dilated 

 in proportion as the leaflets of the limb are fewer in 

 number. These flattened petioles are termed "phyl- 

 lodia," and the character of their venation, corresponds 

 very closely with that 

 of the curvinerved 

 leaves of monocoty- 

 ledonous plants. The 

 non-development of 

 the limb is also com- 

 mon in some species 

 of Monocotyledons, 

 which are never- 

 theless, capable of 

 producing one. The 

 Sagittaria sagittifolia 

 (fig. 65.), an aquatic 

 of this class, has the 

 limb developed at the summit of those leaves only, 

 which reach above the surface of the water, all the rest 

 consisting merely, of strap-shaped expansions of the pe- 

 tioles. 



De Candolle considers the greater number of sheathing 

 leaves, which are not furnished with distinct limbs, to 

 be only petioles ; and although such are found in 

 several Dicotyledons, as in Ranunculus gramineus, La- 

 thyrus nissolia, the whole genus Bupleurum, and some 

 others, yet they are more especially characteristic of Mo- 

 nocotyledons, where he supposes the development of a 

 true limb to the leaf to be comparatively rare ; though 

 it certainly occurs in the Arum tribes, Sagittariae, and 

 some others. Some limbless petioles are cylindrical and 

 pointed like the leaves of a rush. 



(76.) Foliaceous Branches. The phyllodium is not 

 the only substitute which nature provides, to supply the 

 absence of a perfect leaf. In some plants, the leaf is com- 

 pletely abortive, and becomes a small dry scale, incapable 

 F 3 



