THALLUS OF SPHAEROCARPOS DONNELLII AUST. 
187 
a cell at the apex of each lobe that has very much the appearance of an 
apical cell ; but this is the exception rather than the rule. The typical group 
of initial cells is shown in figures 26 and 30-34 (Pis. IX, X). It consists 
of a marginal row of small cells of dense contents, rather longer than wide, 
all substantially alike as to size and shape. As these cells continue to 
divide, not enlarging to any extent, the segments cut off from them on 
the outside outstrip them in growth and form two lobes, one on each side 
of the apical region. This results in the formation of an apical notch, which 
may be median (figs. 26, 32, 33) or lateral (fig. 34). The lateral position 
of the apical notch is a common appearance; it is noted in the subsequent 
history of the plant in the formation of a small and a large lobe on opposite 
sides of the growing point, and frequently in the formation of short branches 
when the growing point forks. 
After the young thallus has reached this stage in its development, it 
becomes more than one cell thick in the central portion. Just how this 
transformation is effected I have been unable to determine. Mounts 
in toto no longer prove satisfactory, owing to the density of the cell contents 
in the apical region ; and the plants are still too small to be handled readily 
by ordinary methods of fixing and sectioning. 
Apical Growth and the Formation of the Thallus Lobes 
Both Campbell (2) and Leitgeb (9) describe the growth of the thallus 
as being due to a single apical cell which cuts off right and left lateral seg- 
ments and dorsal and ventral segments. Campbell says that the lateral 
segments so resemble the apical cell in horizontal view that it is difficult 
to say with certainty that there is but one apical cell. Leitgeb describes 
the apical cell as lying at the deepest point of the notch in the forward 
margin of the thallus, and states that this notch is often so narrow that 
there is space only for a single small cell. He goes on to say, however, that 
sometimes the notch is broader, and more rarely it is quite wide, which 
appearance, he thinks, is a sign that forking is about to occur. The situa- 
tion throughout, according to Leitgeb, is practically the same as that found 
in typical Ricciaceae and Marchantiaceae. 
It is a difficult matter, in mature plants, to obtain sections passing 
through the growing point exactly in a longitudinal vertical direction, 
owing to the small size of the plant, and to the presence of a mass of in- 
volucres and lobes about the growing point. In many hundreds of slides, 
I have obtained two satisfactory series of sections of the growing point 
cut in this plane (figs. 38-54, Pis. X, XI), and some rather less satisfactory 
series cut in horizontal and in transverse vertical planes (figs. 55 -67). 
The plants which I studied were grown in a Wardian case in the green- 
house, and the shady conditions, the abundance of moisture, and the 
absence of any seasonal limitations of the growing period, were responsible 
for a more luxuriant growth of the thallus, with a corresponding increase 
