322 THE PRIMARY STEM 



The strands of such elongated cells which will eventually 

 form the vascular bundles are known as the desmogen 

 strands (Fig. 53, desm.), and these soon become distinct 

 from the surrounding tissue. The outlines of the tissues 

 which will be formed later from these often become clear 

 at an early stage of development, and the large cells 

 which will become the vessels of the xylem become 

 apparent. About the same time, or even earlier, 

 the limit between the vascular cylinder and the cortex 

 becomes evident, owing to differentiation between peri- 

 cycle and cortex, the cells of the latter continuing to 

 grow in width. 



Development o! Leaves and Axillary Buds. While 

 these changes have been taking place in the stem tissue, 

 the leaves, which first appear very close to the stem 

 apex as small papillae, or, where the adult leaves have 

 a broad insertion on the stem, as curved ridges of 

 meristematic tissue, have been growing in length and 

 breadth. In the axil of each leaf there arises, sooner or 

 later, another papilla of meristematic cells, the rudiment 

 of the axillary bud (Fig. 53) ; and upon the sides of this 

 there arise the rudiments of the first leaves of the lateral 

 shoot into which the bud may develop. The bud may 

 grow out at once to form a branch, not much behind the 

 main shoot in development. On the other hand, it may 

 remain a bud for a long time, even for many years ; and 

 in some cases it never develops further at all. Its fate 

 depends on various external and internal conditions. 



Differentiation of the Tissues. The first tissue to 

 attain the adult form is the protoxylem of the vascular 

 bundles. Close to the inner limit of the desmogen strand 

 certain elongated cells become spirally thickened (see 

 p. 280), and the thickening band rapidly becomes lignified, 

 the protoplasm then dying. This happens at first at 



