STRUCTURE.] DE CANDOLLE S THEORY. 269 



kind ; or toothed in various ways upon their edges ; or divided 

 more or less deeply into lobes, 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 surrounded with parenchyma as 

 well as the ligneous fibres of the stem. When this parenchyma 

 stretches a great deal between 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 comparatively 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 pro- 

 ducing the phenomenon 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 shown hereafter. With regard to leaves, Dracontium 

 pertusum affords a verification of this theory in the irregular 

 holes pierced through the middle of its blade between the 

 veins. The more weak the development of this leaf has 

 been, the larger are the holes, which, in some instances, 

 even extend to the margin, when the leaf becomes lobed. In 



