100 
W. B. Crow 
Denser types of mucilage may show individual cell-sheaths. The 
culmination of this type in Glceocapsa and Glceothece, where each 
cell-generation shows its individual sheath, may be regarded as 
secondary, since so many of these forms are only semi-aquatic or 
even completely terrestrial. In general, the larger colonial forms 
tend to develop small cells (see Table II). When not united to form 
definite colonies, however, the plant-body may sometimes attain a 
moderate size in the form of separate cells ( Chr. macrococcus (Kuetz.) 
Rabenh.). 
In agreement with the general principle that homogeneity, either 
physiological or morphological, gives rise to heterogeneity in the 
course of evolution, we can trace various phylogenetic stages in the 
differentiation of the protoplast and in the distribution of pigment. 
The latter may accumulate in both protoplasm and envelope or may 
be confined to either. Where only plasma-pigmentation occurs the 
colouring matter may be equally diffused or concentrated either in 
the peripheral or the central plasma. The type having a peripheral 
chromatophore has survived in all the higher Cyanophyceae. The 
type with central coloured plasma may have led to those forms which 
have a definite central star-shaped chromatophore. 
In conclusion the writer wishes to thank Professor F. E. Fritsch 
both for the loan of material and for several helpful suggestions and 
Professor R. C. McLean who has also taken considerable interest 
in the above work. 
Table I 
Variation in cell-diameter in collections from single habitats 
The headings indicate the dimensions of the classes, measured in microns. 
Under the dimensions of each class is given the number of individuals in the 
class. For each species ioo individuals were measured. 
Species 
i 
2 
3 
4 
5 
6 
7 
8 
9 
10 
Glceocapsa crepidinum (Rabenh.) 
Thu ret 
17 
50 
31 
2 
Gl. Ralfsii (Harv.) Lemm. 
- 
3 
28 
55 
13 
I 
— 
— 
— 
- 
Gl. polydermatica Kuetz. (large form) 
- 
- 
11 
20 
28 
25 
11 
5 
Glceocapsa in thallus of lichen Psoro- 
tichia sanguinea 
_ 
_ 
_ 
55 
35 
10 
_ 
_ 
_ 
_ 
Glceocapsa in thallus of lichen Jen- 
mania Gcebeli 
4 
23 
54 
16 
3 
_ 
_ 
Chroococcus cohcerens (Br6b.) Naeg. 
— 
2 
12 
26 
32 
16 
6 
6 
— 
Merismopedia elegans A. Br. 
- 
— 
- 
7 
54 
35 
4 
- 
- 
- 
Ccelosphcerium sp. 
- 
- 
- 
1 
16 
56 
22 
3 
2 
Microcystis ceruginosa Kuetz. 
- 
- 
48 
5 ° 
2 
- 
- 
- 
- 
- 
M. pallida (Farlow) Lemm. 
2 
29 
45 
18 
6 
