CHAP. II. 
LEAVES. 
135 
regular figure, as, for instance, oval, rounded, elliptical, &c. 
Their regularity, however, is never mathematical ; and there 
are certain leaves, like those of the Begonia, the two sides of 
w’hich are most remarkably unequal. 
“ Leaves are either entire^ that is, without toothings of any 
kind; or toothed in various ways upon their edges; or divided 
more or less deeply into lohes^ which leave void spaces between 
them, which we call recesses [sinus). 
Differences of this kind only become intelligible when one 
starts from the idea that a leaf is a mere expansion of tissue, 
in which the parenchyma is more or less extended, according to 
the divergence of the vessels that compose the veins, and to the 
degree of vegetating vigour of every species upon all points of 
its surface. In this expansion, which constitutes vegetation, 
it may be understood that a cellular tissue, mingled with firm 
parts like veins, ought to assume, especially at the edges, very 
different appearances. Each vein is to be considered as sur- 
rounded with parenchyma as well as the ligneous fibres of 
the stem. When this parenchyma stretches a great deal be- 
tween the principal veins, and unites them completely up to 
their extremities, the leaf is entire; but when the separation 
of the principal veins is greater, and the cellular tissue is com- 
paratively less extended, the union of parenchyma takes place 
in only an imperfect manner, and thus lobes and openings are 
produced in the middle of the leaf, or various kinds of 
toothings in its circumference. 
‘‘ In support of this theory, which has originated with M. 
De Candolle, it must be remarked that the bladders of cel- 
lular tissue have a great tendency to grow together when they 
come in contact in a young state. The fluids which tissue 
secretes are more or less viscid ; the growth of the bladders in 
diameter causes them to press against each other ; they are 
extremely homogeneous in different parts of the same organ ; 
all these may be supposed to concur in producing the pheno- 
menon of which the grafting of one plant upon another is the 
most striking example. The structure of flowers depends 
upon the existence of this tendency, as will be showm hereafter. 
With regard to leaves, Dracontium pertusum affords a verifi- 
cation of this theory in the irregular holes pierced through 
K 4 
