CHAP, ii The Differentiation of the Tissues 199 



general answer according to our present knowledge is 

 a distinct negative. To be sure this negative does not 

 hold for all single cases. For instance, for the large maj ority 

 of roots, not only does each of the layers of meristem 

 correspond to a definite section of a definite system of 

 tissue, but even the separate parts of each of these sections 

 may often be traced back to its separate initial cells in the 

 apical meristem. . . . But even in roots exceptions occur. 

 The epidermis, for instance, in the Gymnosperms does not 

 originate from a distinct dermatogen layer. . . . The nega- 

 tive of the constant genesis of definite sorts of systems of 

 tissue from definite zones of primary meristem holds to 

 a much greater extent in leaf-forming shoots. Here also 

 it is true there are such relations. The system of vascular 

 bundles of many stems of Phanerogams, for instance, is 

 derived exclusively from the plerome cylinder. But exactly 

 the opposite also occurs. . . . And the whole of the tissues 

 and tissue-systems of the leaves, which are continuous 

 with the similar and synonymous tissues of the plerome of 

 the stem are formed, according to the data in hand, out- 

 side the plerome, being derived, as is the whole leaf, from 

 the periblem and dermatogen, or from the layers of meri- 

 stem corresponding in position to these.' 



Proceeding to discuss Famintzin's position, then only 

 recently published, we find him saying : 1 



' On the share taken by the dermatogen in the develop- 

 ment of tissue, there can be no two opinions ; the main 

 question is, therefore, whether the system of vascular 

 bundles arises universally, i. e. in the whole plant, from the 

 same primary layer of meristem. If we ignore isolated 

 cases of controversy, the plerome or a certain region of it 

 is in stem or root the initial part for the system of vascular 

 bundles, or for the greater part of them. The question, 

 therefore, is whether the parts of the system of vascular 

 bundles, which pass from the stem system into the leaves, 

 and which belong to the latter, also arise from the plerome 

 at the apex of the stem. This could not be otherwise 

 effected than by outgrowths of the plerome pushing between 

 the other layers of the young forming leaf, and growing 



1 loc. cit., p. 23. 



