178 HISTORY OF BOTANY 



claimed to have demonstrated in the growing regions of 

 the Phanerogamic stem and root three definite histogenetic 

 layers of cells, comparable with the germinal layers of 

 the animal embryo, and to these he gave the names of 

 dermatogen, periblem, and plerome, giving rise respec- 

 tively to the epidermis, the cortex, and the vascular 

 cylinder. To these, in 1874, Janczewski added a fourth, 

 the calyptrogen, as the initial layer of the root cap. 



A more detailed examination of different plant types, 

 however, led to the discovery of various modifications 

 of Hanstein's original scheme, some of the germinal 

 layers being shown to have a common origin. In 1877 

 De Bary in his Comparative Anatomy of Phanerogams and 

 Ferns reviewed the whole subject, and came to conclusions 

 somewhat adverse to those of Hanstein and Janczewski. 

 His criticism was as follows : "To the question whether 

 in all cases only definite zones of meristem give rise to 

 definite sorts of tissues, the most 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 majority 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 

 negative of the constant genesis of different sorts of 

 systems of tissues 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 



