8 CYANOPHYCEAE 



probably do, these may be expected to differ from those of an 

 ordinary vegetative cell. Various suggestions have been made as to 

 their function, and in many cases they probably determine the 

 breaking up of the trichomes (or threads) into hormogones. These 

 hormogones are short lengths of thread which are cut off. thus 

 forming a means of vegetative reproduction among the filamentous 

 types. The heterocysts may also perhaps act as a food store, or they 

 may represent archaic reproductive organs which are now function- 

 less. It has been reported that in Nostoc and Anabaena these cells 

 may occasionally behave as reproductive bodies. Hormogones, 

 besides being cut off by the heterocysts, may also be produced by 

 the development of biconcave separation disks which develop at 

 intervals along the filament. The hormogones, together with certain 

 of the filamentous types, exhibit a slow motion, and although ciha 

 have been described for one species their presence has never been 

 corroborated. The active and continual secretion of mucilage along 

 the sides of the filaments is now regarded as the probable mechan- 

 ism for securing movement. Thick-walled resting spores, or 

 akinetes, occur in many of the filamentous forms, normally de- 

 veloping next to a heterocyst. The entire lack of sexuality must be 

 ascribed to the ancient cell structure and the absence of chromo- 

 somes together, possibly, with the lack of sterols. 



This type of cell structure naturally provides a problem for the 

 geneticist. There are tw^o possibilities because each cell may con- 

 tain one single gene or a number of genes (organized self-reprodu- 

 cing bodies which determine the properties of the cell and of the 

 organism). The genes must be separated from each other since 

 there are no chromosomes in which they could be situated, and 

 they will either be distributed generally throughout the cell or else 

 in a particular part of it. Since there is no special means of accurate 

 partition sexuality would be useless because it could not confer the 

 property of recombination but only of addition. 



Many of the forms aggregate into colonies, but in some of the 

 Chroococcaceae the plant mass is an association of such colonies 

 and not one large colony or thallus. The form which any colony 

 may take up depends on (i) planes of cell division, (2) effect of 

 environment which may determine the consistency of the mucilage, 

 uneven temperatures, for example, sometimes producing irregular 

 growth. It has been shown experimentally that the environment 



