DEVELOPMENT. 139 



third leaf. Similarly throughout. While the germ of each 

 succeeding leaf thus arises, the germs of the previous leaves, 

 in the order of their priority, are changing their rude nodu- 

 lated shapes into flattened-out expansions ; which slowly put 

 on those sharp outlines they show when unfolded. Thus 

 from that extremely indefinite figure, a rounded lump, giving 

 off from time to time lateral lumps, which severally becoming 

 symmetrically lobed, gradually assume specific and involved 

 forms, we pass little by little to that comparatively complex 

 thing a leaf-bearing shoot. Internally, a bud under- 



goes analogous changes. The layer of substance which forms 

 the surface of the hemisphere, and in which these metamor- 

 phoses commence, consists of a transparent, irregularly -aggre- 

 gated mass of cells and centres of growth, not formed into a 

 tissue. Especially is this the case at the apex, where the 

 vital activity is the greatest. Here the primitive cellular 

 mass passes without any line of demarcation into the tissues 

 that are developing from it. While, by continued cell-multi- 

 plication this layer increases, and doing so most rapidly at 

 the apex thrusts outwards its lateral portions, these begin to 

 exhibit differentiations. " Gradually," says Schleiden, " se- 

 parate masses of cells, with a distinct and definite outline, 

 appear in this chaos, and they cease to partake of the process 

 of growth going on. At first the epidermis is separated, 

 then the vascular bundles, later the parenchyma." Similarly 

 with the lateral buds whence leaves arise. In the, at first, un- 

 organized mass of cells constituting the rudimentary leaf, 

 there are formed vascular bundles which eventually become 



%/ 



the veins of the leaf ; and gradually there appear also, though 

 in ways that have not been specified, the parenchyma and the 

 epithelium. Nor do we fail to find an essentially 



parallel i-et of changes, when we trace the histories of the in- 

 dividual cells. While the tissues they compose are separ- 

 ating, the cells are growing step by step more unlike. 

 Some become flat, some polyhedral, some cylindrical, some 

 prismatic, somn spindle- shaped. Tho.se develop spiral fibres 



