Osmunda claytoniana , Z., O . cinnamomea , Z. 55 
and the basal segment extends from the upper to the lower 
surface of the prothallium. It is first divided by a transverse 
wall into a dorsal and ventral cell, the latter being usually the 
larger, and sometimes divided again by a second transverse 
wall before any vertical walls are formed. The subsequent 
divisions are more or less irregular, but the limits of the 
segments can usually be followed without difficulty for some 
distance back of the growing point. 
The lateral segments seem to divide only by walls in two 
planes, and contribute to the formation of the marginal parts 
of the prothallium. 
It is almost impossible to determine with certainty the 
number of cells that are properly to be regarded as initials in 
the older prothallia, as the young lateral segments are often 
indistinguishable from the real initials, and the central ones 
are from time to time divided into apparently equal cells that 
push aside the outer cells that have apparently been of equal 
rank with them, but which now cease to divide with the same 
regularity as before. Vertical sections rarely show more than 
two or three cells that can properly be called apical cells — that 
is, that show the characteristic form and regular method of 
division. 
On comparing the method of growth here described with 
the liverworts, the closest resemblance is found in Dendroceros , 
a genus allied to Anthoceros , but having a thallus of almost 
precisely the same structure as the prothallium of Osmunda , 
except that it branches much more freely, and, according to 
Leitgeb 1 , the form of the apical cells and their method of 
division is the same. This in connection with the fact that 
Dendroceros, like Anthoceros, has the most highly differen- 
tiated sporogonium of all the Hepaticae, confirms the view 
already expressed that the Anthoceroteae are the most nearly 
allied, among known Bryophytes, to the primitive Filicineae. 
As the prothallia grow, new root-hairs arise, at first only 
from the marginal cells, but later also from the lower cells of 
the midrib. They may be formed either by the simple 
1 L. c. Heft V. p. 30. 
