DOES NOT DETERMINE ORGANOGENY 



177 



segments of the apical cell ancl the appearance of leaf-primordia : Schwen- 

 dener has even been able to show that where the arrangement of the 

 leaves is spiral, the spiral of leaf-arrangement may be antidromous to that 

 of the successive segments, and he states that the latter condition is 

 almost as common as that where the two spirals are homodromous. It 

 thus appears that, in those Pteridophytes in which the apical segmentation 

 is most regular, no constant relation exists between the formation of 

 segments and the origin of the appendages : Naegeli's conception of the 

 apex as a dominating influence in this matter is not supported by the 

 facts. And here it may be noted that even in the embryo of the Higher 

 Plants there is evidence that the first cleavages in the embryo do not 

 define the position of the parts : for it has been found by Westermaier 1 

 that the primary median wall of the embryo of Cruciferae has no strict 

 relation to the position of the subsequent cotyledons. 



e -j t , r 



FIG. 92. 



Scheme of the succession of cells in the apex of the root of Equisetum hicmale, after 

 Naegeli and Leitgeb. A, longitudinal section. B, transverse section at the lower end of 

 A. 7z = principal walls. j = sextant walls. <r = the first, = the second, rihe third 

 tangential wall. In A the figures I. -XVI. denote the successive segments. = dermatogen. 

 k, I, m, n,p successively older portions of the root cap. From Sach's Text-book. 



A somewhat similar Idea to that above discussed was initiated also 

 in relation to the internal differentiation of tissues. Naegeli and Leitgeb 

 established early the relation of the outer limit of the central vascular 

 cylinder to the first periclinal wall in segments at the apex of the root 

 in Equisetaceae, Marsiliaceae, and Polypodiaceae (Fig. 92). Subsequently 

 Hanstein's study of the meristems in certain well-defined cases of the 

 Higher Plants led him to distinguish formative tissues giving rise respec- 

 tively to epidermis, cortex, and vascular cylinder: these he designated 

 dermatogen, periblem, and plerome. As the study of the tissues became 

 more exact, and took form in the stelar theory of Van Tieghem, the 



1 Kef. Bot. Cent., vol. Ixxvii., p. 122, 1899. 

 M 



