CHAP, ii The Differentiation of the Tissues 203 



The recognition of these layers as histogenetic was 

 opposed by many botanists later in the century, though 

 it received a certain support from Van Tieghem. The 

 weakness of the position was pointed out by Scott in 1894 

 in the following passage : 



' The question is whether the early division walls of the 

 segments mark out the boundaries between the principal 

 systems of tissue. Certainly the mere order of succession 

 of the walls has no constancy. Thus, according to Van 

 Tieghem, while in most Ferns the first tangential septum 

 marks the limits between inner and outer cortex, and the 

 second defines the central cylinder, in Osmunda and Todea 

 the order is reversed. In the former case endodermis and 

 pericycle are more nearly related to each other than are 

 the two layers of the cortex. From this and numerous 

 similar instances it is evident that the order of appearance 

 of the cell walls does not correspond with the relative 

 importance of the subsequent differentiation. In a mono- 

 stelic stem (e.g. a stolon of Nephrolepis) the first tangential 

 wall corresponds to the outer limit of the central cylinder. 

 In polystelic stems of other Ferns the same wall marks 

 the outer limit of the ring of steles, but here a great part 

 of the tissue to the inside of this wall forms part of the 

 cortex, according to Van Tieghem's use of the term. It is 

 evident that in this case the cortical tissue between the 

 steles is of common origin with the steles themselves, yet 

 the distinction between these regions is, according to Van 

 Tieghem, the most fundamental in anatomy. It may seem 

 trivial to dwell on such minutiae of development, but we 

 must remember that in other cases the most important 

 morphological distinctions have been based on equally slight 

 differences of origin. Thus in the development of the root, 

 the pilifero us layer arises in nearly all Dicotyledons from 

 the outermost part of the meristem, and is thus of common 

 origin with the root-cap, while in the Monocotyledons 

 generally, the piliferous layer is derived from the middle 

 meristematic group and is thus of common origin with the 

 cortex. . . . We may safely conclude that with the solitary 

 exception of the origin of the root-cap, the first tangential 

 cell walls do not constantly mark out the main histogenetic 



