1 1 8 Brebner. — On the Prothaltus and Embryo of 
the prothallus, viz. long narrow hairs with a few transverse 
walls, the component cells being distinctly uni-nucleate. The 
figure of the rhizoid of the prothallus would do just as well 
for a root-hair of the young sporophyte. 
Apical Growth. 
The vexed question of the apical growth in stem, root, and 
cotyledon is not much cleared up by the stages secured in 
D. simplicifolia. The stem of the embryo, represented in 
Fig. 1 6, seems to have a well-marked single apical cell at st. 
This is represented on a larger scale in Fig. 17. There is 
probably a certain amount of obliquity in this section, as the 
young bundle is not seen passing into the cotyledon at this 
point, but in the fourth preceding section of the series. This 
much is certain, however, that the cell in question is absolutely 
the largest anywhere near the apex of the stem, and likewise 
possesses the largest nucleus. On account of its size, and of its 
position in relation to the cotyledon, it is practically certain 
that it is the apical cell. With regard to the embryo of 
Fig. 18, there is probably still a single apical cell in the 
growing-point of the stem, but rapidly approaching the time 
when it will be merged in a group of equivalent initials. It 
is already hardly, if at all, larger in size than one or two of 
the adjacent cells, but its nucleus is undoubtedly the largest 
and at the same time richest in chromatin (cf. Fig. 19); 
hence, in all probability, it is the apical cell. In a series of 
transverse sections of a plantlet, in which only the cotyledon 
had expanded, and of which the second leaf was still quite 
young, the stem had a single apical cell with a four-sided 
base. The cotyledon of this plantlet was the one already 
referred to as having a very feebly developed bundle, the 
xylem consisting of only two elements. In somewhat more 
advanced plantlets it was quite impossible to fix on any 
particular cell as the apical cell, and in these cases there 
would be little doubt that the apical meristem consisted of 
a small number of equivalent initials. 
With regard to the cotyledon, the somewhat late stages 
