A Critical Study of certain Unicellular Cyanophyceae 93 
Many species of Chroococcaceae are characterised by the possession 
of envelopes which are not homogeneous. The heterogeneity is due 
generally to the differences in consistency and hence in optical 
properties of the successive layers of the envelope. Such stratified 
envelopes occur in nearly all the species or forms which have very 
dense and firm mucilage. I11 the simplest cases the alternating in¬ 
tensities of mucilage secretion producing such stratification are due 
to the alternating phases of vegetative and reproductive activity of 
the cell. Hence in certain colonial envelopes, such as those of Glceo- 
capsa and Glceothece distinct layers of the envelope can be recognised 
as belonging to distinct cell-generations. But in the degree of de¬ 
velopment of such distinct sheaths there is very great variation, even 
in one and the same habitat. Brand ( 3 ), working on Glceocapsa alpina 
Naeg., found that this organism could pass over into a status 
soluius in which it was indistinguishable from an Aphanocapsa. 
Fritsch(8) observed that in Merismopedia where cell-division is very 
rapid the primary sheaths formed an envelope round several cells, 
whilst when division was slower the envelope only enclosed a pair 
of cells. We have noted similar variation in Merismopedia, some¬ 
times correlated with the size of the colony and hence probably with 
the consistency of the mucilage (see Fig. 1 c, d). The colonies of the 
plankton species of Microcystis, too, are of interest in connection 
with the variability of the individual sheaths. In collections from 
the freshwater plankton of Ceylon, colonies of Microcystis viridis 
(A. Br.) Lemrn. were sometimes found with very compact mucilage 
sheaths. In some of these the cells each had their own sheath, whilst 
in others they had not, and variation in this respect was found in 
one and the same colony. The more diffluent colonies showed no 
individual cell-sheaths. 
Besides the lamellation of the membrane due to the existence of 
more or less individualised membranes for each cell-generation 
some envelopes appear, in optical section, to consist of numerous 
concentric layers. These may be formed by alternating periods of 
mucilage secretion of unequal intensity, but it is difficult to account 
for such periodicity. More likely they are due to the concentration 
into definite zones of some substance impregnating the mucilage. 
The phenomenon is probably almost identical with that of the for¬ 
mation of Liesegang’s rings in gelatin. It is very variable in one and 
the same species. Thus in Glceocapsa polydermatica, a species which 
generally shows the layers in a very striking manner, we have 
observed all transitions from the stratified to the unstratified con- 
