Osmunda claytoniana , L., and O. cinnamomea , L. 53 
As germination proceeds, the chloroplasts separate and in- 
crease in size and distinctness. They are often arranged in 
lines extending from the nucleus to the periphery of the cell. 
The nuclei are large and distinct, and in the rapidly growing 
young prothallia are often met with in division (Fig. 4 a). No 
especial observations were made upon them, but they do not 
seem to offer any noteworthy peculiarities. 
Growth of the Prothallium. 
As soon as the apical cell is established, the divisions occur 
with considerable regularity. Segments are cut off alternately 
from the sides, and the limits of the segments can usually be 
traced for some time. The first wall in the young segment 
usually divides it into a marginal cell and an inner one. The 
former undergoes division mainly by longitudinal walls, and 
from it arise the marginal cells of the prothallium. The 
divisions of the inner cell are both longitudinal and transverse, 
and very early nearly horizontal walls are formed in the inner- 
most cells so that the axial part of the prothallium, even in 
its earlier stages, is several cells thick, and the beginning of 
the midrib, so conspicuous in the older prothallia, is thus 
formed. 
In O. cinnamomea , when a cell-surface is formed at once, it 
sometimes happens that for a short time both of the upper 
cells divide alike, and it is impossible to say which is to be 
the permanent apical cell (Fig. 16 x\ but usually this is 
established while the prothallium is still very small. 
In the earliest stages the apical cell is comparatively broad, 
and the segments remain undivided, or divide slowly, so that 
the outer wall of the apical cell projects beyond them ; soon, 
however, the young segments grow more rapidly, and cell- 
division proceeds faster, so that the outer cells are pushed 
forward and soon reach the level of the apical cell (Fig. 11), 
whose outer wall then becomes nearly straight, and the pro- 
thallium has the form of a narrow wedge, with the apical cell 
occupying the middle of the base. A continuance of the 
