142 



THE AMERICAN MONTHLY 



[August, 



served add move iodine solution, and 

 continue to do this until reduction be- 

 gins. Then the strength is right, and 

 the operation may be continued until 

 the desired eflect is obtained, pouring 

 the solution over the plate, held level 

 so as to flow evenly, and catching it 

 in the glass as it flows off, so as to use 

 the same mixture repeatedly. The 

 operation can be done in a tray also, 

 but there the progress of the reduction 

 cannot be so critically watched. 



Occasionally local reduction can be 

 applied to advantage. A method that 

 has been highly recommended is to 

 moisten a piece of soft cloth with 

 alcohol and rub it gently over the 

 dense parts of the negative. The 

 deposit is thus rubbed down, and 

 the cloth becomes blackened. We 

 have not experimented with this 

 process, but it is well spoken of 

 by reputable authorities. A method 

 we have used with some success is 

 the careful local application of the 

 cyanide and iodine solution men- 

 tioned above. The plate was thor- 

 oughly soaked in a tray of \\-ater and 

 the reducing agent applied with a flat 

 brush just where it was required to 

 act, frequently plunging the plate into 

 clean water to prevent the action from 

 spreading. By working slowly very 

 good results were obtained. 

 (To be conii'tiued.) 

 O 



Provisional Key to Classiiicatioii of 

 Algje of Fresh Water.— X. 



BY THE EDITOR. 



{^Continued from page /34-'\ 



Fatnily Nostocace^e — Continued. 



d. Nostocee^. Group. 4. 



Synopsis of Genera. 



Curved chains, in gelatin with a 



common external envelope. 



No s toe, 116. 

 Like A^c>.y/6)<:,butin indefinite gelat- 

 inous layers. Anabcena. 117. 

 Cylindrical, sheathless cells in fila- 

 ments, spores cylindrical. 



Aphanizomenon, 118. 



Spores on both sides of a hetero- 



cyst. Sphcerozyga. 119. 



Heterocysts terminal, spores sin- 

 gle. Cylindrospermuin, 120, 



Filaments curved, in distinct 

 sheaths, spores separated from heter- 

 ocysts. Aulosira, I3i. 



Filaments sheathed ; spores fus- 

 cous, golden yellow. 



Nodjilaria, 123. 



Cells orbicular, in sheaths, spores 

 unknown. Chrysostigma, 123. 



Cells cylindrical, sheathed, hetero- 

 cysts terminal. Coleospermimi, 124. 



Filaments curved, several in a 

 sheath. Hilsia, i2<^. 



116. Genus Nostoc Vaucher. 

 Trichomes composed of spherical 



cells, more or less curved and inter- 

 laced, with or without special gelat- 

 inous sheaths in a gelatinous ma- 

 trix of definite form, enclosed in 

 a common more or less firm mem- 

 brane or pirederm. Heterocysts 

 terminal or intercalate between the 

 vegetative cells. Resting spoi'es with 

 thick envelopes, about the same size 

 as the heterocysts. with granular 

 contents, green, bluish, or yellowish- 

 brown, formed from vegetative cells. 



[Li the course of vegetative in- 

 crease, portions of the common 

 gelatinous matrix soften, and some 

 of the vegetative series of cells are 

 set free, which for some time possess 

 an oscillaria-like movement. These 

 come to rest, and are not then to be 

 distinguished from filaments of a7ia- 

 bcena. New plants arise by the divi- 

 sion of these cells parallel to the axis 

 of the filaments, finally separating, 

 and then grow into new filaments.] 



117. Gtnus Attabcena ^ory. 

 Filaments resembling Nostoc, but 



in single or in indefinite slim}' masses. 

 Heterocysts terminal or intercalate. 

 Spores not contiguous to the hetero- 

 cysts. 



1 1 8. Genus Ap/ianizo»ie?ton Mor- 

 ren . 



Filaments composed of cylindrical, 

 vegetative cells without sheaths, vmit- 

 ed in free floating floes, blue or olive 

 color. Heterocysts intercalate. Rest- 

 ing cells cylindric, not contiguous to 

 the heterocvsts. 



