G8 LEAVES. 



inucli greater than on the other, thus producing the oblique 

 or one sided leaf, (iSg. 62.) 



A want of development and hardening of the 

 parts often produces deviations from the usual 

 forms. The existence of spines at the ex- 

 tremities of the lobes of the Holly, is owing to 

 these causes, and in some radiated leaves the 

 veins seem to be converted unto spines, and in 

 a species of the Prosopis *' one half of the leaf- 

 lets contract into a spine while the other half 

 remains leafy. But the most singular instance 

 of this kind of deviation occurs in a palm 

 called the Desmoncus, in which the upper leaflets of its pin- 

 nated leaves contract and curve into scythe shaped hooks by 

 which the desmoncus climbs, while the lower leaflets retain the 

 usual appearance of leaves." 



73. The Petiole is sometimes entirely wanting, but at others 

 excessively developed. In sessile leaves it is absent, but in 

 the NymphcEa odorata, a water lily, it is even six or eight i^eet 

 long. In the Palm, Palmetto and other tropical plants, it is 

 much longer, and assumes more the appearance of a branch 

 than a petiole. The lamina varies also from the size of the 

 minute scale-like leaves of the Moss to prodigious dimensions 

 on some tropical leaves. These organs in general bear no 

 proportions in size to the plants on which they are found. — 

 On some species of the Oak, of the most sturdy kind, the 

 leaves are small, but feebly corresponding with the gigantic 

 tree itself, while the leaves of the Rheum Rhaponticum (Pie 

 plant,) are exceedingly large, the plant itself, divested of its 

 leaves, is comparatively, in size, an insignificant plant. 



74. The duration of leaves is various. In some they fall 

 almost as soon as developed, and are then said to he fugacious, 

 at others the}^ remain till the end of the summer and fall with 

 the cessation of vegetation, when they are called caducous. 

 In others they remain during the winter, and are denominated 

 persistent, and produce the various evergreens of our forest. 

 Various hypotheses have been formed to account for the fall 

 of the leaf but the most satisfactory one to our mind, is that 

 given by Professor Lindly, which is, that while the stem and 

 leaf are both increasing in size, there is an exact adaptation of 

 the base of the leaf to the stem and no interruption takes 

 place ; but when the leaf becomes perfectly developed and is 

 susceptible of no further increase, the stem continues to en 

 large by the deposition of new matter from the leaves above, 

 which breaks the joining vessels, and the leaf of course falls. 

 The breaking of the vessels may be easily observed in the 



