112 ouganography. book t. 



are certain leaves, like those of the Begonia, the two sides of 

 which are most remarkably imequal. 



" Leaves are either entii-e, that is, without toothings of any 

 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 the 

 degree of vegetating vigom* 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 prinipal 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 tlieory, 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 f/raftinq of one plant upon another is the 

 most striking example. The structure of flowers depends 

 upon the existence of this tendency, as will be shewn hereafter. 

 With regard to leaves, Dracontium pertusum affords a verifi- 

 cation of this theory in the irregular holes pierced through 

 the middle of its blade between the veins. The more weak 

 the developement of this leaf has been, the larger are the 



