270 DE CANDOLLE'S THEORY. [BOOK i. 



this case it is difficult to deny that the parenchyma developes 

 and combines more towards the edge of the leaf than in the 

 centre; while, on the other hand, by a different direction 

 and another mode of development 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 vegetation 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 obser- 

 vations of Mohl have demonstrated that those plants also 

 are conformable to the theory. 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 develop- 

 ing, 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 great 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 

 surrounds 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 or lobes 

 are called segments. They differ from the leaflets of more 

 compound leaves merely by the circumstance of not being 



