54 Campbell —On the Prothallium and Embryo of 
growth in the segments causes them to extend more and more 
beyond the apical cell, which thus comes to lie in a depres- 
sion, and the familiar heart-shape of other fern-prothallia is 
produced. 
From the first the prothallium is more elongated than is 
common in Polypodiaceae or in O. regalis 1 . The very broad 
form of prothallium, figured for the latter by Kny 2 , I have 
failed to find in either species examined by me. 
The single two-sided apical cell persists for a long time, but 
is finally replaced by one of different form, or by several similar 
initial cells. In a number of cases observed (Fig. 23), the tri- 
angular initial had been replaced by a four-sided one, a set of 
basal segments being formed as well as lateral ones, as in 
Pellia 3 . Whether this condition is ever permanent, is difficult 
to say positively. Usually the single four-sided initial is 
divided into equal parts by a longitudinal wall (Fig. 27), and 
the resultant cells may- divide again in the same way so as to 
form a row of similar marginal cells as in other ferns. 
Attention has already been called 4 to the remarkable re- 
semblance between the apical growth of fern-prothallia and 
that of certain liverworts, and these resemblances are especially 
marked in Osmunda. 
For a clear comprehension of these points, careful longi- 
tudinal sections are necessary, in addition to the study of 
surface views of the prothallium. Longitudinal sections 
(Plate IV, Figs. 25, 28) show that the apical cells are much 
larger than appears from surface views, being very deep. 
Each is in fact a semi-disc, whose edge only is seen from 
above, and its volume is much greater than that of the ad- 
jacent cells. On comparing the two species it is found that 
O. claytoniana has the apical cells larger and less convex 
than O. cinnamomea , in both respects coming nearer the 
Polypodiaceae. As in the latter, the inner wall is nearly flat, 
1 L. c. Pi. I. fig S . 1 6, 17. 2 L. c. PI. II. fig. 1. 
3 Leitgeb, Untersuchungen fiber die Lebermoose, Heft III. p. 7. 
* Campbell, A Study of the Apical Growth of the Prothallium of Ferns ; Bull, 
of Torrey Botanical Club, Vol. XVIII. no. 3. 
