A Critical Study of certain Unicellular Cyanophyceae 87 
pure lines. The same applies to the species symbiotic in lichens, which 
were chosen for this reason, the members of each collection being 
measured from one and the same lichen thallus 1 . In such cases it 
will be seen that the variability, although actually small, is not 
altogether insignificant when compared with the actual dimensions 
of the cell. Nevertheless, as will appear in comparison with other 
characters to be dealt with subsequently, cell-size is one of the more 
constant specific characters in the Chroococcaceae. 
It will also be seen from the tables, especially from Table II, that the 
variability differs greatly among the different species. At first it might 
be thought that specific variability would be a fundamental property 
of the various species and useful as a criterion in classification ; 
further, that greater variability in certain species of Chroococcus 
indicated a more primitive position for these forms in the scale of 
evolution, since in general the more primitive members of a group 
show greater variability. But variation from the mean depends on 
the reaction or reactions caused by various stimuli, either external 
or internal to the cell. It has been shown by such work as that of 
Brand ( 3 ) on Glceocapsa and of Brunnthaler( 4 ) on Glceothece that organ¬ 
isms such as those we are dealing with, are, like most other simple 
forms, very sensitive to external stimuli, e.g. those of light and osmotic 
or chemical conditions. Hence it must be assumed that such ex¬ 
trinsic factors play the chief part in bringing about variation. Now 
the action of these stimuli will be to some extent dependent on the 
size of the cell. Thus cells of small mass will be acted upon to a less 
extent by certain chemic and other stimuli than those of greater 
mass, whilst those of small surface will respond less to photic, ionic and 
osmotic stimuli than will those of greater surface 2 . Hence there is no 
indication that the rather remarkable variability of some of the species 
tabulated in II is anything more than the effect of their larger cell-size. 
Internal Structure of the Cell 
A few facts regarding the minute internal structure of the Cyano- 
phycean cell are of interest from the present point of view, although 
the unicellular forms have been less studied than the filamentous 
ones and the whole subject must be approached with diffidence, 
owing to the very diverse conclusions reached by those who have 
investigated it. Most writers are agreed, however, and it can easily 
1 I have to thank Prof. Darbishire for the loan of specimens of lichens. 
2 It is here merely assumed that living cells are influenced by the ordinary 
physical laws of mass and surface action. 
