TT 
casionally by longitudinal walls and thus add to the number of 
_marginal cells of the prothallium; but the further growth of 
the secondary marginal cells is limited, while that of the apical 
cells is unlimited. From the bases of the apical cells segments 
are also cut off, which contribute to the formation of the 
cushion which bears the archegonia. 
Vertical sections of the apical cells are almost perfectly semi- 
circular in form (Figs. 7, 8, a), and comparing this with the sur- 
face view, or sections parallel to the surface, we see that these 
cells are shaped like a flat half-disk. As compared with the 
other cells they are large, and an examination of the segmenta- 
tion shows that the divisions occur with great regularity, The 
segments are cut off by a wall parallel with the inner wall of the 
apical cell, and reach from the upper surface to the lower one of 
the prothallium. (Figs. 7, 8). The segment is next divided by a 
wall at right-angles to the first formed, into two cells, of which 
the lower one (the upper one in the figure), is slightly larger, and 
divides more actively than the upper, so that the thickening on 
the lower side of the prothallium is more marked than on the 
upper, and results in the formation of the projecting cushion of 
tissue upon which are borne the archegonia. These seem to bear 
a definite relation to the segments. After the first division in the 
young segment, walls are formed in each semi-segment perpen- 
dicular to the surface of the prothallium, thus dividing the seg- 
ment into four cells, two of which are dorsal and two ventral. 
One of the latter forms at once the mother-cell of the young 
archegonium. (Fig. 7 ar.). 
In Osmunda observations were limited to young prothallia 
that had not yet formed archegonia, but as in this genus there is 
a thickened midrib traversing the whole prothallium almost from 
the first, it was to be expected that the method of growth would 
not deviate very much from that of the Polypodiacez. It is not 
at all infrequent to find in Osmunda (O. cinnamomea, and O. 
Claytoniana) that the two-sided apical cell of the young prothal- 
lium gives place to a single cell from which three sets of segments 
are cut off—two lateral and one basal. (Figs. 4,5). In the cases : 
observed this cell was considerably larger than the individual cells _ 
of the apex of the other fern-prothallia studied. 
