A Critical Study of certain Unicellular Cyanophyceae 95 
of much greater importance. The distribution of pigment with refer¬ 
ence to the central region and peripheral protoplasm has already 
been referred to. The cases where, as in Chroococcus turgidus (Kuetz.) 
Naeg., the central portion of the protoplast may be as, or more, deeply 
tinged than the peripheral plasma represent a more primitive con¬ 
dition than those which have a central colourless region. For in the 
latter cases functional differentiation has become manifest between 
the layers of the protoplasm. If we assume that in allied Chroo- 
coccaceae evolution may have progressed along similar lines, except 
that the central plasma has become coloured and the peripheral 
colourless, then it will be possible to connect genera like Chroococcus 
with others having a central chromatophore ( e.g . Chroothece). In 
view of the plasticity of the chromatophore region in the Chroo- 
coccaceae there is no necessity to exclude organisms having a definite 
central chromatophore although some may equally well rank as 
reduced members of other Algal groups. 
Other types of pigment distribution may be mentioned to illus¬ 
trate the diversity of the group from this point of view. Whilst the 
majority of forms have their colouring matter diffused in the proto¬ 
plasm or at least in very fine granules, some species of Chroococcus 
often show shining globules of pigment within the cell. In Chr. 
macrococcus (Kuetz.) Rabenh., for instance, there may be several 
such globules, in Chr. rubrapunctus Wolle one in the centre of the 
cell (“nucleus” of Wolle). In the former species the cells are often 
without these globules. The distribution of the pigment in these forms, 
however, seems to form an essentially distinct type and indicates a 
difference in the properties of the pigment or perhaps the protoplasm. 
In other species of Chroococcus and in such species of Glceocapsa 
as Gl. rupicola Kuetz. and Gl. purpurea Kuetz. the red lipochrome is 
diffused throughout cell and sheath. This condition may be the most 
primitive one, comparable, in fact, with the cases where mucilage is 
secreted both within the protoplast and around it. But in most red 
species, and in all the brown species of Glceocapsa, the coloration is 
confined to the envelope. In some of these, at least, we are dealing 
with a pigment distinct from plasma-pigments. It has been given 
the name of gloeocapsin and is said to be a chemical indicator like 
litmus, becoming rose-red, reddish-orange or bluish-red with hydro¬ 
chloric acid, blue or blue-violet with caustic potash ( 19 ). In these 
Cyanophyceae it is important to distinguish between envelope and 
plasma-pigments, just as in the higher plants a distinction must be 
made between plastid and cell-sap colours. 
