A Critical Study of certain Unicellular Cyanophyceae 97 
if the cells were capable of dividing in more than one plane but in the 
ordinary filament were inhibited from doing so. It is not possible to 
offer a complete explanation of this inhibition, nor is this the place 
to deal with these interesting filamentous Cyanophyceae. But it may 
be noted that where cells are so closely approximated as in these 
filamentous forms, by reason of their association in one dense muci¬ 
lage sheath, that interaction of the cells on one another is to be 
expected. Most writers are agreed that passage of substances in 
solution may occur in the filamentous forms ( 18 ). 
c 
N r 
Fig. 1. Diagrams, mostly constructed from Camera-lucida drawings. 
Protoplasm black. 
A. Aphanothece sp. Final stages in the division of a cell to show elongated 
character of daughter cells and shifting of their axes on completion of 
division. 
B. Merismopedia elegans A. Br. View of edge of colony to show elongation 
of cells. 
C. Merismopedia sp. Young colony without individual membranes. 
D. Merismopedia sp. Old colony to show individual membranes. 
E. Microcystis viridis Lemm. Colony to show variation with regard to pre¬ 
sence and absence of individual membranes and in distance apart of cells. 
F. Holopedium irregulare Lagerh. (after Kirchner). Two cells from side. 
Summary and Conclusions 
From the foregoing it would appear possible to draw certain con¬ 
clusions bearing on the classification and evolution of the group of 
the Chroococcaceae. Many of the features studied here have played 
an important part in the taxonomic treatment of the family. It 
would appear that certain characters, such as the orientation of the 
planes of division, the form of the cell, the size of the cell, can be, 
Phyt. XXI. II. 
7 
