168 THE INDUCTIONS OF BIOLOGY. 



leaves, in the order of their priority, are changing their rude 

 nodulated 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 becom- 

 ing eymmetrically lobed gradually assume specific and in- 

 volved forms, we pass little by little to that comparatively 

 complex thing — a leaf -bearing shoot. . Internally, a 



bud undergoes analogous changes ; as witness this account : — 

 " The general mass of thin-walled parenchymatous cells 

 which occupies the apical region, and forms the growing point 

 of the shoot, is covered by a single external layer of similar 

 cells, which increase in number by the formation of new 

 walls in one direction only, perpendicular to the surface of 

 the shoot, and thus give rise only to the epidermis or single 

 layer of cells covering the whole surface of the shoot. 

 Meanwhile the general mass below grows as a whole, its 

 constituent cells dividing in all directions. Of the new cells 

 so formed, those removed by these processes of growth and 

 division from the actual apex, begin, at a greater or less dis- 

 tance from it, to show signs of the differentiation which will 

 ultimately lead to the formation of the various tissues en- 

 closed by the epidermis of the shoot. First the pith, then 

 the vascular bundles, and then the cortex of the shoot, begin 

 to take on their special characters." Similarly with second- 

 ary structures, as the lateral buds whence leaves arise. In 

 the, at first, unorganized mass of cells constituting the rudi- 

 mentary leaf, there are formed vascular bundles which 

 eventually become the veins of the leaf; and pari passu with 

 these are formed the other tissues of the leaf. Xor 



do we fail to find an essentially parallel set of chang'es, when 

 we trace the histories of the individual cells. While the 

 tissues they compose are separating, the cells are growing 

 step by step more unlike. Some become flat, some poly- 

 hedral, some cylindrical, some prismatic, some spindle-shaped. 

 These develop spiral thickenings in their interiors; and 



