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THE TAXONOMY AND VARIATION OF THE 
GENUS MICROCYSTIS IN CEYLON 
By W. B. CROW 
A recent examination of the collections of freshwater algae from 
the plankton of the inland waters of Ceylon, made by Prof. 
F. E. Fritsch in 1903, showed that the members of the genus Micro¬ 
cystis form probably the most important constituents of the phyto¬ 
plankton during the period (Aug. 21st to Nov. 10th) at which the 
collections were made 1 . The species evidently attained a remarkable 
abundance in many localities frequently forming water-bloom. Along 
with this abundance in number of individuals was found an extra¬ 
ordinary diversity of form, the various types being linked together, 
however, by intermediates. In fact, whenever abundant material 
occurs the precise limits of the different species become impossible 
to define. This is particularly true of those species which possess 
pseudovacuoles, since these species are found in the greatest abund¬ 
ance. The same phenomenon has been observed for three species 
from tropical Africa and has led Ostenfeld(6) to consider all the forms 
with pseudovacuoles as belonging to the same species. Transitional 
forms connecting not only these three species but also several other 
types of a very varied nature will be recorded below. This kind of 
continuous variation is being discovered to a greater and greater 
extent in Cyanophycece and Chlorophycece. That it is not fluctuation 
directly dependent on environment is shown by the fact that forms 
which are quite distinct, together with transitional types, may exist 
for considerable periods, side by side, under the same ecological 
conditions. It is probable, therefore, that there is some genetic 
difference between the types distinguished as species below. 
In order to emphasise the more essential specific distinctions 
among the members of the genus the following key is given. Although 
any such scheme must be largely artificial its primary divisions are 
based on cell characters as being more fundamental than colony 
characters, which have been too greatly relied upon by previous 
investigators. The key includes several species which are not known 
from Ceylon but also the two new species described below. As the 
1 The material was preserved in tubes of dilute formalin and that dealt with 
here was in an excellent state of preservation. 
