APHANIZOMENON FLOS-AQU^. 23 



contents, which scarcely ever present any trace of granular 

 structure. Under the action of iodine the following structures, 

 fig". 6 b, may be seen in the heterocyst : — 1. The endochrome 

 contracted towards the centre of the cell, and presenting a 

 well-defined boundary. 2. External to this a delicate cell- 

 wall separated from the contracted endochrome by a transpa- 

 rent interval, and frequently presenting in its interior, at each 

 extremity, a minute spherical body, with considerable refrac- 

 tive powers. 3. An external very delicate, but well-defined 

 transparent investment. 



At first no other kind of cell beyond those now described 

 could be detected in the filaments, but in specimens gathered 

 somewhat later many filaments presented in some part of their 

 course a long cylindrical and slightly-dilated cell, fig. 4 6 i, 

 generally about two or three times the length of the lietero- 

 cysts ; occasionally a single filament presented two such cells. 

 They correspond to the cells named sporangia in the other 

 Nostochineae ; their contents are always minutely granular, 

 and under the action of iodine, fig. 7, separate from the walls 

 of the cell and contract towards the centre, where they present 

 a very definite boundary, in which a double outline can some- 

 times be distinctly seen ; while, external to this, and separated 

 from it by a clear space, a colourless investing membrane has 

 become very obvious ; but the second investment, so evident in 

 the heterocysts, could not here be satisfactorily demonstrated : 

 the little spherical body visible at each extremity of the cell 

 of the heterocyst could not be seen in the sporangium. Fila- 

 ments bearing sporangia were accompanied by those bearing 

 heterocysts, but whether the two kinds of cells ever coexisted 

 in the same filament was not manifest. 



That the sporangia are not simply enlarged cells, but the 

 result of the union of several ordinary cells, is highly probable. 

 The author has succeeded in observing what appears to be 

 intermediate stages of formation, in which the endochrome of 

 a group of ordinary cells had already begun to assume the 

 minutely-granular condition of that of the sporangium, the 

 septa being, at the same time, evidently in process of disap- 

 pearing, fig. 8. 



When the living plant is collected and placed in a jar of 

 water, the fasciculi will frequently be seen after some hours 

 to have broken up into their component filaments, which will 

 then rearrange themselves in elegant wavy curves parallel to 

 one another, and forming a nearly uninterrupted stratum on 

 the surface of the water, fig. 3. 



Aphanizomenon Flos-aquce, after the death of the plant, is 

 eminently sensitive to the action of light. Specimens dried on 



