CHAP. II. 
LEAVES. 
113 
holes, wliicli, in some instances, even extend to the margin, 
when 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 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 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 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 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 or lobes 
I 
