Osmunda claytonictna , L., and O. cinnamomea , L. 79 
of Osmunda, the apical cell of the root has regularly the 
tetrahedral form found in the Leptosporangiatae and Ophio- 
glosseae. 
Occasionally it is quite large (Fig. 90 A) and conspicuous, 
but usually it is smaller. At first, of course, its outer face is 
free (PI. VI, Fig. 91 r), and segments are cut off only from 
the lateral faces. As the apical cell at this stage is unmis- 
takeable, the primary root must be regarded as originating 
simultaneously with the other organs of the embryo, and 
morphologically equal to them, and of course, of exogenous 
origin ; very soon, however, a periclinal wall arises, cutting off 
the first cell of the root-cap, and from this time, with every set 
of lateral segments, a basal segment also is formed. When 
fairly established, it is found that the division of the segments 
corresponds in the main with that found in the other Filicineae 
having a single tetrahedral apical cell. Each segment divides 
first into two parts, by a vertical wall ; and each semi -segment 
then divides into two cells, a small inner one which is the 
initial for the plerome, and a larger outer one, which by further 
division gives rise to periblem and dermatogen, and in part, 
also, to the root-cap. 
When compared with the other Filicineae, it resembles most 
nearly Botrychium 1 , with which, it agrees in the small size of 
the apical cell, and the large size of the segments and their 
slow division at first, as well as the greater irregularity in the 
divisions and the bulky character of the whole root. 
Series of cross-sections through the apex and below it show 
much less regularity than is the case in the Polypodiaceae, and 
at a very short distance from the apex all trace of the limits 
of the segments is lost. Immediately below the apex, their 
boundaries can usually be made out, their zig-zag lines meeting 
at the centre. The sextant walls can also usually be made out, 
striking these at varying angles. Beyond this it is not possible 
to trace any invariable arrangement of the cell-walls. The 
majority of the walls are periclinal, and the result is a more or 
less perfect concentric arrangement of the cells. The limits 
1 Campbell, 1. c. 
