RECENT WORK, ETC. 473 



the tissues of the entire plant from special germinal 

 layers. 



Passing on to the question of histogenetic layers in the 

 apical meristem of growing points, the problem is, of course, 

 different for plants which grow by means of a single 

 apical cell, and for those which have a small-celled apical 

 group. In the former case, as the apical cell gives rise 

 to all tissues alike, the question is whether the early 

 division walls of the sesfments 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 limit between inner 

 and outer cortex, and the second defines the central cylin- 

 der, in Osmunda and Todea the order is reversed (18). 

 In the former case endodermis and pericycle are more 

 nearly related to one another 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 monostelic 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 ana- 

 tomy. 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 piliferous 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 



