CHAP. II. LEAVES. 113 



Jjoles, whicli, in some instances, even extend to the margin, 

 wlien the leaf becomes lobed. In this case it is difficult to 

 deny that the parenchyma developes and combines more to- 

 wards the edge of the leaf than in the centre ; while, on the 

 other hand, by a different direction and another mode of de- 

 velopement of the parenchyma, the contrary takes place in 

 the greater part of leaves. The fact, that divisions are the 

 deepest in those individuals of the same species whose vegeta- 

 tion has been least favoured by humidity and the nature of the 

 soil, is a confirmation of this theory. 



" Palm trees seemed to offer an exception to this mode of 

 accounting for the formation of lobes; but the recent ob- 

 servations of Mohl have demonstrated that those plants also 

 are conformable to the theoiy. The leaves of Palm trees 

 begin by being apparently simple, they then gradually divide 

 from the extremity to the base of the blade, and there are on 

 the edges of the divisions some ragged remnants which look as if 

 they indicated an actual rending asunder. But Mohl, by 

 observing these leaves microscopically, when first developing, 

 ascertained that these divisions never are intimately united 

 at their edges, and that they are merely held together by a 

 net of down. This may possibly depend upon the dry and 

 leathery texture of their leaves, which causes the bladders to 

 be converted into hairs instead of uniting in consequence of 

 their gi-eat approximation. If the adhesion is incomplete, it 

 is no wonder that the leaves should separate in proportion as 

 the veins diverge by the enlargement of the leaf. Palm 

 leaves, then, are not, as has been supposed, simple leaves which 

 divide into lobes contrary to what happens in other plants ; 

 they are divisions bordered by a parenchyma which has never 

 been united to that of the division next it, and which, in 

 consequence, does not tear, but only separates. 



" The unequal degrees of union of the parenchyma that sur- 

 rounds the veins, combined with the arrangement of the latter, 

 form the principles on which the nomenclature of divided 

 leaves has been contrived. 



" When the parenchyma between the primary veins is not 

 united, so that the blade is composed of several distinct parts 

 combined by the midrib only, the distinct portions oi' lobes 



I 



