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W. B. Crow 
dition (see also ( 4 )). Where present, however, the character may be 
of systematic value as indicating the presence of a special substance 
in the mucilage comparable with the pigment of some sheaths, which, 
as we shall see, may disappear under certain conditions. Such a 
special substance is met with in the chi tin of Chroococcus macrococcus 
Rabenh., which accumulates in special concentric zones of an other¬ 
wise pectic sheath (i). 
The Pigment 
The phenomenon of mucilage-secretion is in some ways paralleled 
by that of pigment-secretion. It will be recalled that in the coloured 
plasma of all Cyanophycean cells three coloured substances occur, 
namely, chlorophyll, carotin and phycocyan, the two former in the 
form of minute granules, the latter probably in solution. In those 
forms in which a definite central body occurs, the pigment is usually 
absent from this part, although Chodat ( 5 ) found in Chroococcus 
turgidus (Kuetz.) Naeg. that the central region was often rich in 
pigment. In addition to the ordinary pigmentation of the blue- 
green cell certain species of Glceocapsa and Chroococcus are distin¬ 
guished by their different and more intense coloration. Kohl (12) 
has attempted to show that the pigmentation of these forms can be 
explained as the development of the three normal pigments in un¬ 
usual proportions. Thus the violet and purple-red coloration of the 
cell in the subgenera Rhodococcus Hansg. and Rhodocapsa Hansg., 
the orange colour in Chr. macrococcus Rabenh. and the yellow in 
Gl. paroliana Breb. may be explained as due to the preponderance 
of carotin with varying proportions of the other two pigments. No 
doubt this kind of variation is sufficient to explain the differences in 
cytoplasmic coloration. The latter may vary in one and the same 
species. Such is the case in Gl. conglomerata Kuetz. where the plasma 
varies between blue-green and brown, and in Chr. pallidus, Chr. 
decolorans and others where it varies from blue-green to yellow. The 
red pigment of Glceocapsa alpina Naeg. and of Glceothece rupestris 
(Lyngb.) Born, is found to disappear under certain circumstances ( 4 ) 
and we have noted a similar colourless state of the sheath as some¬ 
times occurring in Glceocapsa sanguinea (Ag.) Kuetz. Colour variation 
is known in Glceothece to depend largely on ecological factors and in 
this respect there is no reason for supposing that the Chroococcacece 
differ essentially from Gomphosphceria ( 14 ) and Oscillatoria ( 18 ) where, 
however, the variability may be more striking. 
But, whilst the variation in colour itself may be of little interest 
from a genetic point of view, the variation in its distribution may be 
