CHAP, ii The Differentiation of the Tissues 195 



he observed a circular band traceable up to near the actual 

 growing point, and found that in the inner portion of this 

 belt the structures originate which ultimately lead to the 

 development of the bulk of the axis. This intermediate 

 zone constitutes the thickening ring spoken of, the struc- 

 tures arising in it being the vascular bundles. Thus he 

 distinguished, what had till then been obscure or not appre- 

 ciated, that the true cambium is not identical with the 

 zone of origin of the primary bundles ; so he made possible 

 the correct view of secondary formations. He showed how 

 the cambium itself is formed later in and between the 

 vascular bundles, and how it produces the secondary layers 

 of both wood and bast. 



Sanio made a very thorough investigation into the com- 

 parative anatomy of the stems of the groups alluded to, 

 studying the various elements of the wood and their rela- 

 tive distribution. He also inquired into the seat of origin 

 of the foliar structures and the relation of their bundles 

 to those of the axis. He .found them to originate in the 

 zone overlying the true thickening ring. 



Very little further progress was made for several years, 

 but an effort to place the subject on a satisfactory basis 

 gave us Hanstein's scheme of 1868. His proposals were 

 based on the recognition of definite and well-recognizable 

 generative or histogenetic layers of cells in the growing 

 part of the axis in both stem and root of the Angio- 

 spermous plants. He claimed to find three such layers 

 alike in the embryo and the axial growing points. These 

 regions were traceable further along the axis in most 

 cases, and gradually led to areas which were charac- 

 terized by very complete differentiation from each other. 

 He put forward the view that in the meristem also these 

 regions are differentiated and constitute a system con- 

 sisting of three definite areas, often only one cell in depth, 

 each independent of the others, and giving rise to a par- 



N 2 



