THE LEAVES OF VASCULAR PLANTS. 245 



visible, and often denuded of parenchyma ; b\it fre- 

 quently, also, the petiole is sheathing and foliaceous ; we 

 see this particularly in the Graminese, where it bears the 

 name of Sheath. This cylindrical sheath surrounds 

 the stem in a considerable part of its extent : it is most 

 frequently split throughout its whole length, because 

 the two borders remain free. It bears externally at its 

 extremity a limb with parallel nerves, separated from 

 the sheath by a kind of hard strangulation. The apex 

 of this sheath is prolonged interiorly into a short, scaly, 

 and, most frequently, erect lamina, which has received 

 the name of the Ligule. The Cyperaceae only diifer 

 from most of the Graminese, with regard to their foliage, 

 in this: 1st, that their sheath is almost always entire; 

 that is to say, that the two borders are joined together 

 so as to form a true cylindrical tube ; 2d, that the 

 ligule is often absent ; and 3d, that the limb is less dis- 

 tinct from the sheath. 



There are examples in which the simultaneous and 

 constant existence of the limb and petiole hardly leaves 

 any doubt upon the nature of either of them ; but there 

 are doubtful cases which deserve particular mention. 

 If we examine the common Sagittaria, we shall find 

 that, when it grows out of the water, all its leaves have 

 the limb and petiole distinct ; when it grows in water, 

 its limb is nearly always abortive, and its petiole, in- 

 stead of being triangular or cylindrical, takes the appear- 

 ance of a flat foliaceous ribbon, terminated by a small 

 callosity, analogous to that which is observed in the 

 petioles of Dicotyledons, where the limb is abortive ; it 

 is not rare to find plants which bear these two kinds of 

 leaves at once. The same phenomenon happens in the 

 Potamogetons, where the leaves floating on the water 

 have a similar limb, whilst the submerged ones are 

 reduced to a membranous petiole. The comparison of 



