DOES NOT DETERMINE ORGANOGENY YZ 
segments of the apical cell and 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! 
that the primary median wall of the embryo of Cruciferae has no strict 
relation to the position of the subsequent cotyledons. 
Fic. 92. 
Scheme of the succession of cells in the apex of the root of Zguzsetum: hiemale, after 
Naegeli and Leitgeb. 4, longitudinal section. JZ, transverse section at the lower end of 
A. f=principal walls. s=sextant walls. c=the first, e=the second, ~=the third 
tangential wall. In A the figures I.-X VI. denote the successive segments, o=dermatogen. 
k,l, m, n, p=successively older portions of the root cap. From Sach's Text-dook. 
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 Ref. Bot. Cent., vol. Ixxvii., p. 122, 1899. 
M 
