THE LEAVES OF VASCULAR PLANTS. 259 



of nourishment renders the leaves more divided, and a 

 very substantial aliment gives the parenchyma sufficient 

 development to fill up the intervals of the lobes ; thus, 

 most plants with divided leaves tend to have them more 

 entire in rich soil or in gardens. 



Among leaves with simple or curved nerves, divisions 

 are much more rare ; and if the Aroideae be excepted, 

 the leaves of which approach the preceding class, it may 

 be said that they all have the limb entire. 



The Palms seem to form, in this respect, a great 

 exception, but the nature of their leaves presents a cha- 

 racter which is peculiar to them : they often appear 

 divided either in the manner of pinnatisected leaves, or 

 resembling palmatisected ones ; but they are really torn 

 into strips, which follow the direction of the lateral fibres. 

 This tearing, natural and regular, is very evident in fol- 

 lowing the Palms from their young state ; for then their 

 leaves are entire, and the tearing is gradually seen to 

 take place, commencing from above, and extending to 

 the principal bundle of fibres. 



If the natural tearing of the leaves of Palms appears 

 to me to be a fact beyond a doubt, the cause of this 

 phenomenon, or the mechanism which produces this 

 rupture, is very difficult to be assigned. We see that the 

 lateral fibres converge in their young state towards the 

 apex of the leaf; and as they diverge a little towards the 

 base, it is likely that, according to the degree of diver- 

 gence and the rapidity of growth, there arrives a period, 

 in each leaf, when the fibres, developing by the base, 

 force their upper parts to separate, so as to form the 

 strips of which I have spoken : these strips are distri- 

 buted to the top of the petiole, when the lateral nerves 

 spring from it ; and on both sides of a common bundle, 

 when it exists throughout their whole length. The 

 depth of the strips varies like those of ordinary lobes. 



s 2 



