672 Blackman.—The Primitive Algae 
as Ophiocytium (including Sciadium ), Peromelia, Characiopsis, 
Bz., Chlorothecium, Bz., and Mischococcus which previously 
had been in the Tetrasporaceae. The genus Conferva is also 
included here, and on the above characteristics is separated 
widely from Microspora which has none of them, but with 
which it had always been closely associated. The higher 
ranks of this cohort include three rather divergent types, 
Bumilleria , Botrydiopsis and Botrydium. 
In 1897 Bohlin published an interesting paper containing a 
detailed study of the cytological characters of this group. 
With regard to the structure of their cell-walls it had long 
been known that filaments of Conferva and Microspora break 
up across the middle of the cells into short cylindrical units 
consisting of the halves of two adjacent cells with a septum 
separating them. These are generally known as H-pieces 
from their appearance in optical section. Those of Microspora 
are homogeneous and of pure cellulose, but in Conferva 
(Fig. 14, a) they consist chiefly of pectic acid derivatives, and 
on swelling with potash show a special layered structure. 
On investigating filaments in all stages of division the growth 
of the cell-wall revealed a new type. The unit of growth 
consists of the halves of two adjacent cells, and thus the two 
halves of any given cell are of different age. On transverse 
division the new septum with the new ring-thickening at its 
periphery constitutes the starting-point for a new H-unit, and 
while it thickens by obvious apposition layers, these also 
extend and elongate the cells on either side of the septum 
and so push two older units apart. In the diagram of the 
swollen wall structure in Fig. 14, b is a young cell and c an 
old one. Considerable interest attaches to this procedure in 
itself, but this is much increased on its being shown that 
Ophiocytium, which Borzi had brought from a far-off family 
to join Conferva, possesses an interesting variation of this 
very type of growth. Ophiocytium when mature is a long 
curled cylindrical plurinucleate single cell which breaks across 
just near the top to liberate spores. This top cap is found 
to be the unaugmented half of the originally very short cell. 
