18 r.lJTTISH PESMIPIAPFJE. 



equal a 4 per cent, solution of acetate of potassium 

 (containing a trace of copper acetate). A solution of 

 picric acid will also answer as a preservative, and a Ol 

 per cent, solution of osmic acid may be utilised for 

 fixing the cell-contents. Flemming's solution and also 

 carbolic acid may be used as preservatives, but are not 

 to be recommended. 



When glass-stoppered bottles are not used, it is in 

 all cases advisable to previously soak the corks in 

 strong methylated spirit in order to destroy various 

 spores of fungi. 



EXAMINATION ANP SPECIFIC DETERMINATION. For the 

 examination of Desmids a compound microscope is 

 necessary with two powers of 70 to 100 and 400 to GOO 

 diameters respectively, and it is sometimes essential to 

 examine the smaller species under a magnification of 

 800 to 1000 diameters. In all cases accurate drawings 

 should be made to scale with a camera lucida. We 

 emphasize the word accurate, as great confusion has 

 been and still is caused by the rough, inaccurate 

 figures of Desmids which inexperienced authors have 

 published. Desmids appear to be of very different 

 forms according to the position in which they are seen, 

 and for this reason it is necessary to have two or three 

 views of most of them. They should be drawn in 

 three positions : (1) the front ricir, in which the two 

 longest axes of the Desmid and the full constriction 

 are seen ; (2) the side-view, in which the longest and 

 shortest axes and the constriction are seen ; and (3) 

 the vertical vieir, in which the Desmid is observed 

 from above. With practice an observer can obtain 

 these various positions with more or less ease. 



Many species can only be accurately identified from 

 empty cells or semicells in which the surface-markings 

 can be plainly seen, and therefore preserved material 

 is better for the identification of species than living 

 material. If few empty cells can be found, treatment 

 with a 5 per cent, alcoholic solution of potassium 

 hydroxide is often very useful. 



