THALLUS OF SPHAEROCARPOS DONNELLII AUST. 1 89 
The existence of a group of initial cells is similar to the condition in 
Marchantia as usually described. Mottier (11), however, inclines to the 
view that in Marchantia also only one apical cell is present. 
In longitudinal vertical section these initial cells (a, figs. 38-54) are 
regularly wedge-shaped, cutting off narrow segments mostly, but not 
always, in alternate succession. Archegonial initials may be seen, as 
described by Leitgeb (9), Douin (5), and others, only one or two cells 
distant from the initial cell or cells, each apparently formed from an entire 
dorsal segment of the latter {ar, figs. 38-54). From the ventral segments 
mucilage hairs grow out, each consisting of a row of cells, the terminal cell 
being large, spherical, filled with dense contents, and provided with a large 
nucleus {s, figs. 38-54). In the preparations these mucilage hairs become 
more or less shrunken and torn. 
Sections cut through the growing region in a horizontal and in a vertical 
transverse plane are less satisfactory than those just described, owing to 
the even greater difficulty of orienting the plants in the paraffin for this 
purpose. Figures 55-61, Plate XI, illustrate a series of horizontal sec- 
tions, starting at the ventral side. The small cells marked a in figures 59 
and 60 probably represent the apical group. They are noticeably more 
dense in content than any other cells in the sections, except those of the 
mucilage hairs. They are obviously dividing with considerably rapidity; 
and the varying length of the same cells in succeeding sections (x, figs. 59, 
60) makes it probable that they are bounded posteriorly by an inclined wall, 
and therefore correspond to the initial cells shown in the vertical sections. 
The sections suffer from the fact that they are cut at a slight angle from the 
horizontal. 
Figures 62-67, Plates X and XI, illustrate similar groups of cells as seen 
in vertical transverse section. The first series (figs. 62-64) is taken from 
a very young thallus, consisting of a simple plate one cell thick in all parts 
save in the center, where the cells are smaller, denser, and obviously em- 
bryonic. At the beginning of the series (fig. 62) there are two plates of cells, 
one cell thick, separated by an empty space — the apical notch — instead of 
by the group of initials. At one side in the drawings {x, figs. 63, 64) there 
are several large hyaline cells which disturb the otherwise symmetrical 
arrangement. This is probably due to a fold in the thin thallus-blade. 
Figures 65-67, Plate X, illustrate a similar series from a mature plant. 
Here also a group of similar embryonic cells (a) is present ; but in this case 
they are cut slightly on a bias, so that at one side they seem to merge into 
a mature lobe (/), while at the other side there is no corresponding structure. 
It is unfortunate that none of the sections except those cut in the longitudinal 
vertical plane showed the young lobes so evident in the latter; unless the 
cells marked y, figures 57-60 (PI. XI), may be interpreted as such. 
A second method was used in order to determine the origin of the 
lateral lobes of the thallus. Single plants were isolated and grown in 
