48 
P. O. BOWER. 
ferent circumstances ; e. g. the bunch of hairs at the apex of 
young Fucus plants is due to an unusual activity of tangen- 
tial division in the cells of the limiting tissue. This pheno- 
menon again appears at the apex of older branches of 
Fucus, whose activity of apical growth is in any way 
hindered (Reinke, loc. cit., p. 338). And Rostafinski (loc. 
cit. p. 9) notes how the cells of the limiting tissue of older 
branches divides by tangential rather than longitudinal 
walls. The activity of tangential division being then so 
variable, before asserting that the condition described in the 
case of Himanthalia is the normal one for that species, it 
wdll be necessary to compare the results here obtained with 
those drawn from materials collected at a different time of 
year. I think it quite possible that the plants I have used, 
being collected in August, were in a semi-dormant state 
previous to their period of active grow’th and reproduction 
in the winter. If so, the hair, as represented in fig. 10, may 
be compared with those at the apex of the young plant of 
Fucus, or of the older branch of Fucus living under diffi- 
culties. On this ground I shall not attempt to suggest 
which is the simplest form, or the typical mode of forma- 
tion of the conceptacle. 
This variation in the activity of tangential division ac- 
counts for the want of uniformity in number of the cells 
thrown off in different species, and even in the same species 
(this was especially noticed in Halidrys and F. platycarpus). 
In viewing the formation of the conceptacle of the whole 
group (as far as studied), we may say generally, that the 
differences in mode of development (in the early stages at 
least) depend upon the difference in activity of tangential 
division of the cells of the central series ; for it will be seen 
that upon the behaviour of the central series in relation to 
the surrounding tissue depends also the origin of the lining 
tissue of the conceptacle. 
In Fucus the terminal cell of the central series (initial 
cell), ceasing to divide tangentially and being left behind by 
the surrounding tissue (fig. 1), when that cell decays a 
cavity is formed, which extends further into the tissues than 
the base of the cells of the surrounding limiting tissue. The 
tissue lining the cavity is therefore in this case derived in 
its basal part from the cortical, in its upper part from the 
limiting tissue. In Himanthalia, however, the cavity thus 
formed only extends at most to the base of the cells of the 
limiting tissue ; the layer lining the cavity is thus derived 
only from the limiting tissue. This distinction of origin 
must not, however, be put on a level with that from Der- 
k 
