152 
AMERICAN JOURNAL, OF BOTANY 
[Vol. io. 
The plants fixed in the field August 11 and 12, 1921, near Lake Waubesa 
form the basis of the studies reported in this paper. This material was 
supplemented by greenhouse plants of both Wisconsin and Florida stock, 
but, except as otherwise stated, all figures were drawn from the material 
of these collections. The plants were imbedded in paraffin, cut into sections 
10 m thick, and stained either with Flemming’s triple stain, Heidenhain’s 
iron-alum haematoxylin, or with safranin and light green. For the ob¬ 
servations recorded in this paper, the last-named combination proved most 
satisfactory. 
The Apical Cell and Segmentation 
Miss Clapp’s (1912) description of the position, form, and segmentation 
of the apical cell is in harmony with the earlier accounts of Kny (1863) and 
Leitgeb (1877). The apical cell is a large, wedge-shaped structure with 
right and left cutting faces from which segments are cut off alternately 
(Clapp, 1912). These two cutting faces are convex and meet above, below 
and behind, thus constituting all the surface in contact with other cells 
of the thallus. The free (anterior) surface of the apical cell is relatively 
small and approximates an ellipse of which the longer axis is the one per¬ 
pendicular to the surface of the soil or other substratum on which the thallus 
is growing (fig. 4, PI. XVI). Transverse sections through the apical cell 
show this same elliptical outline. Figures 4-8 show serial sections of the 
same apical cell (a) sectioned transversely, each section being 10 m in thick¬ 
ness. This cell tapers abruptly to a blunt point in the next serial section, not 
shown in the figures. (Compare also the horizontal section shown in figure 1, 
Plate XVI, and Miss Clapp’s figures 5, 6, and 11.) The relative lengths 
of the different axes of the apical cell vary considerably, but the horizontal 
transverse (right-left) axis is always much the shortest of the three prin¬ 
cipal axes (figs. 1, 4-8; Miss Clapp’s figs. 5, 6, 11). In the vegetative 
branch the vertical and horizontal axes of the apical cell are approximately 
equal (figs. 3, 10). The apical cell a shown in figures 4-8 appears in six 
vertical transverse sections, each io.m in thickness, thus having an antero¬ 
posterior length of about fifty microns, approximately equal to the length 
of its median vertical axis (fig. 6). 
The division of the apical cell is vertical and almost longitudinal (figs. 
1, 11), cutting off a segment which at first approximates the shape of the 
apical cell (figs. 1, 9-11). Such segments are formed alternately from the 
right and left cutting faces of the apical cell, as all previous workers agree 
(see especially Kny, 1863, PI. VI, figs. 4, 5; Leitgeb, 1877, PI. I, fig. 1). 
The segment appears to exceed the apical cell in size, and the side of the 
segment adjacent to the apical cell soon becomes markedly concave, as the 
apical cell resumes its biconvex shape. 
Leitgeb (1877), Campbell (1905), and Miss Clapp (1912) differ in their 
accounts of the division of the primary segment. Leitgeb (p. 41) says that 
the primary segment divides like a two-faced apical cell, cutting off segments 
