Affinities and classification 39 



probable that the chromatin-substance which occurs in small particles and in small 

 quantity is not of precisely the same nature as that which is present in higher types of 

 nuclei. 



The centrally differentiated incipient nucleus of the Myxophycese appears 

 to have no exact counterpart in the Bacteria, and it furnishes clear evidence 

 that the protoplast of the Blue-green Algae is of a higher type. The cell- 

 walls, also, in the young cells, and in the heterocysts throughout life, consist 

 of cellulose, whereas the bacterial cell- walls never consist of cellulose. The 

 presence of the pigments chlorophyll, carotin, and phycocyanin, enabling the 

 myxophyceous protoplast to construct organic substance under the influence 

 of light, is also a feature of great importance, and the fact that these 

 pigments are restricted to the peripheral zone of the cell must not be lost 

 sight of 1 . The resting-spores of the Myxophycese are always formed by the 

 rejuvenescence of a vegetative cell, and are never of the nature of endospores 

 such as are produced in the Bacteria. On the whole, the distinction between 

 the Myxophycese and the Bacteria, although not absolutely sharp, is still 

 sufficiently wide to make the use of such a term as the ' Schizophyta ' 

 rather unwise. 



The group is primitive, and the living Myxophycere of to-day are the 

 descendants, probably but little changed, of a group of organisms which were 

 left aside very early in the evolution of plant-life. The complete absence 

 of sexuality is undoubtedly to be associated with the incomplete differ- 

 entiation of the protoplast and the consequent absence of a highly organized 

 nucleus. 



CLASSIFICATION. The different families of the Myxophycese are for the 

 most part well-defined and easily diagnosed. The only controversial points 

 have been those concerning the systematic position of the genera Chroothece, 

 Asterocystis and Glaucocystis. 



A careful examination of the original specimens of Chroothece- indicates 

 that this genus would be best placed in the Chroococcacese. Asterocystis 

 also appears to be a typical Chroococcaceous genus 3 , but Glaucocystis is still 



1 There are no known saprophytic Myxophycese, with the exception of Oscillatoria decolorata 

 G. S. West (Journ. Bot. 1899, p. 263; see also M e Keever in Trans. Edin. Field Nat. and 

 Microscop. Soc. Nov. 1911, p. 370). 



2 Some of the original material of Chroothece Richterianum Hansg. was issued in Wittrock 

 & Nordstedt's Alg. Exsic. in 1884. The author has to thank Dr Borge of the Botanical 

 Museum at Stockholm for kindly sending the specimens, and Miss Acton for making careful 

 preparations of them in the botanical laboratory of the University of Birmingham. 



3 The cytological structure has been carefully examined by the author in the following 

 species: A. africana G. S. West, A. antarctica W. & G. S. West and A. smaragdina (Reinsch) 

 Forti, and found to be similar to that of some of the larger species of Chroococcus. The cells of 

 A. halophila (Hansg.) Ferti also appear in the living state to be like those of Chroococcits, but 

 this species has not been examined cytologically. 



