84 STRUCTURAL BOTANY 



The vascular bundles begin to develop at some 

 little distance from the apex. The outer part of the 

 plerome divides up more actively than the rest, giving 

 rise to a small-celled tissue. Within this zone certain 

 strands of cells divide more frequently by longitudinal 

 than by transverse walls, so that they come to consist 

 of rather elongated cells. These constitute the pro- 

 cambial strands, each of which develops into a 

 vascular bundle, as already described. The inner- 

 most elements of each, strand thicken their walls and 

 become the protoxylem, while the outermost are 

 converted into the protophloem. The successive 

 development of the elements then goes on from 

 within outwards in the xylem, and from without 

 inwards in the phloem. But, as it is an open bundle 

 with which we are concerned, there is always a layer 

 of cells left between wood and bast, which remains 

 capable of dividing, and forms the cambium of the 

 bundle, by means of which it can increase the 

 number of its elements indefinitely. 



Only the dermatogen and periblem take part in the 

 formation of the leaves. To form a leaf, the outer 

 layers of the periblem grow out, and divide by walls 

 parallel to the surface, while the dermatogen follows 

 their growth (see Fig. 32, I). The latter gives rise 

 to the epidermis of the leaf only. All its other 

 tissues vascular bundles as well as mesophyll are 

 formed from the periblem. The plerome of the stem 

 takes no part in the development of the leaf. 



The growth of the leaf itself, unlike that of the 

 stem, goes on chiefly at the base, the lower part of 



