A Critical Study of certain Unicellular Cyanophyceae 89 
vacuoles and which may occur both in plankton and benthon (M. 
pulverea (Wood) Migula, M.fuscolutea (Hansg.) Migula). A transition 
between the two groups is found in M. ochracea (Brand) Lemm. and 
M. elahens (Meneg.) Kuetz., these being at first benthonic and with¬ 
out pseudovacuoles, later developing these structures and becoming 
planktonic. The important feature is that under given ecological 
conditions, e.g. in freshwater plankton, they are fairly constant for 
the species. An examination of a large quantity of material of several 
species of Microcystis from the freshwater plankton of Ceylon tends 
to emphasise the constancy of the occurrence of pseudovacuoles in 
the plankton species. The only case where individual variation occurs 
with respect to presence and absence of these structures is found in 
species (M. ceruginosa Kuetz., etc.) which normally show them but 
where, in occasional colonies, some cells do and others do not possess 
them. However, the cells where these pseudovacuoles are absent 
show other signs of an abnormal, probably senescent or pathological 
condition (e.g. giving a coloration with corallin soda in distinction 
from the other cells which do not) and, moreover, traces of degene¬ 
rating pseudo vacuoles themselves are often revealed under the highest 
magnifications. Such cells are never found in a state of division. 
They are fairly constant in some forms of Microcystis and may per¬ 
haps be compared with the necridia of filamentous Cyanophyceae. 
Since they do not reproduce, it follows that the cells of new colonies 
will possess pseudovacuoles. 
A definite cell-membrane, distinct from, and within the mucilage 
sheath, is generally stated to occur in the cells of the Chroococcaceae 
(e.g. (11)). It corresponds in position with the “ innerfinvestment ” of 
the sheath of filamentous forms, a layer which is insoluble in 33 per 
cent, chromic acid solution and which is said to consist of gelatinised 
form of protoplasm. Gomont (io> points out that unlike the mucilage 
of the surrounding sheath the inner investment has a constant com¬ 
position among these higher Cyanophyceae. The various species of 
Chroococcaceae vary appreciably with regard to the distinctness of 
the development of the cell-membrane. In Glceocapsa alpina (Naeg.) 
Brand ( 3 ) was unable to find it in normal cells, but observed quite 
freshly divided products of a division that were abnormally con¬ 
nected by a tubular bridge, and hence concluded that there was a 
definite cell-membrane in this species, although too fine to be visible 
in the ordinary state. Fritsch(8) found the membrane of Glceocapsa 
visible in older cells having a definite cell-sheath, but invisible in the 
young state. The same difficulty of demonstrating a membrane is 
