Heterocysts 



17 



manner between the vegetative cells of the filament (h'g. 11 C), or they 

 may be terminal (fig. 11 F and G), Sometimes they are seriate as in 

 some species of Tolypothrix, Calothrix, Nostoc, etc. As a rule, they are 

 a little larger than the vegetative cells, and in most cases stand out 

 conspicuously among the other cells of the filament. There is almost 



Fig. 11. Various Blue-green Algae showing different positions of heterocysts. A and B, portions 

 of two branched filaments of Nostochopsis Goetzei Schinidle, with stalked and sessile lateral 

 heterocysts. C, fragment of filament of Nostoc carnenm Ag., showing intercalary heterocyst. 

 D and E, small portions of filaments of Nostoc autun-ticn W. & G. S. West, showing 

 seriate intercalary heterocysts. F, Cylindrospermum tropic um W. & G. S. West with 

 basal heterocyst and papillate spore. G, Gylindrospermum indentatum G. S. West, with 

 basal heterocyst and smooth indented spore. In all the figures the pellucid heterocysts are 

 represented without shading. The hair-like appendages around the heterocyst in G are 

 slender bacteria radiating in the colourless mucilaginous envelope. A, B and F, x520; 

 CE, and G, x 500. 



a complete absence of pigment from the heterocysts, in consequence of 



which they present a very pellucid appearance, although they are often 



of a pale yellow-brown or yellow-green colour. The walls of fully-formed 



w. A. 2 



