[ 8i ] 
A CRITICAL STUDY OF CERTAIN UNI¬ 
CELLULAR CYANOPHYCEAE FROM THE 
POINT OF VIEW OF THEIR EVOLUTION 
By W. B. CROW, M.Sc., F.L.S. 
(University College of South Wales and Monmouthshire) 
(With i figure in the text) 
nlike many of the groups of unicellular organisms the class 
V_J Cyanophyceae, Myxophyceae or Blue-green Algae seems remark¬ 
ably well defined. Whilst no two authors are agreed, for instance, 
as to the limits of that vague assemblage of motile types generally 
known as Flagellata, yet a review of the literature shows that modern 
systematists are in harmony in their definition of the Cyanophyceae. 
The systems of classification proposed for the group by Forti( 7 ), 
Tilden(iG), Lemmermann ( 14 ), Bessey(2), West(i8), and Kirchner(ii) 
not only agree in containing the same genera but also recognise the 
same families. It is true that Lemmermann divides the Nostocaceae, 
whilst according to Bessey certain genera of Bacteria should be 
admitted. But the system remains essentially the same. The only 
other modification that has been proposed in recent years is the 
exclusion of Porphyridium from the Chroococcaceae since this genus 
was placed by Brand in the Bangiales. This, at first sight an unimpor¬ 
tant change, is of considerable significance, for it raises the question 
whether other supposed Chroococcaceae may not be reduced members 
of this Red Algal series or of related groups. Whatever views may 
be held with regard to the Rhodophycean origin of Porphyridium 1 , 
as will appear subsequently, many unicellular Cyanophyceae are best 
regarded as relatively primitive forms, bearing a somewhat similar 
relation to the filamentous Cyanophyceae as the Tetrasporales to the 
filamentous Chlorophyceae. 
It is not necessary to enumerate here the many characters which 
are common to all the Cyanophyceae and which distinguish them so 
sharply from all other Thallophyta. A great deal of attention has 
been paid to these common characters; the less striking differences 
which exist between the species and genera have been almost neg¬ 
lected except by purely taxonomic writers and have not been treated 
1 See Staehelin, Ber. d. deutsch. hot. Ges. 1914; Kuffenrath, Inst. bot. Ldo 
Errera, 1920. But also Brand, Berichte. 1917. 
Phyt. xxi. 11. 
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