280 THE ALGAE 



cells. In the process of development they become depleted of their 

 cell contents (Fogg, 195 1). They have a thickened wall of two 

 layers, the inner and more prominent of cellulose, and the outer of 

 pectic material. There is usually a pore at one end (if terminal) or 

 both ends (if intercalary). It has been shown (Fogg, 1949) that 

 their formation is inhibited in the presence of ammonium nitrogen, 

 and it has therefore been suggested that they are only formed when 

 a specific inhibitor (involving ammonium nitrogen) falls below a 

 critical level. The widespread occurrence of heterocysts, especially 

 terminal ones, and their speciaUzed structure indicates that they 

 must fulfil a significant function. Various suggestions have been 

 made as to their function, and in some cases they do seem to deter- 

 mine 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 fila- 

 mentous types. In the past it has been suggested that the hetero- 

 cysts are a form of food storage organ or that they represent archaic 

 reproductive organs that are now functionless. It has, for example, 

 been reported that in Nostoc and Anabaena these cells may occa- 

 sionally behave as reproductive bodies in that they are capable of 

 germination and thus producing new threads. In a more recent 

 contribution, Fritsch (1951) suggests that the heterocysts, during 

 the vegetative period, secrete substances that stimulate growth and 

 cell division. As a result, the concentration of these substances 

 (which may contain ammonium nitrogen — see above) falls off in 

 the cells that He further and further away from the heterocysts and 

 then the development of new heterocysts is promoted. He also sug- 

 gests that at the time of reproduction the nature of the secretion 

 changes and as a result akinetes are formed. It is evident that a 

 variety of untenable hypotheses have been put forward in the past 

 and we may be no nearer the solution of this enigma than were the 

 ear Her botanists. 



Hormogones, besides being cut off by heterocysts, may also be 

 produced by the development of biconcave separation discs at 

 intervals along the filament, or they may break off from the ex- 

 tremities of the trichomes. The hormogones, together with certain 

 of the filamentous types, e.g. Oscillatoria, and a few Chroococcales, 

 exhibit a slow forward motion. Active and continual secretion of 

 mucilage along the sides of the filaments is associated with this 



^ The trichome or trichomes and the containing sheath is termed a filament. 



