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EVSOU.^TBI^G FRESHWATER ALGAE, MOSSES, ETC. 



By E. D. Evens. 



(Read December 14th, 1920.) 



Mounted specimens of freshwater algae are not often met with , 

 due to their generally unsatisfactory character. For the minute 

 study of their cell contents, stained and mounted specimens are 

 indispensable, but for the rapid recognition and identij&cation of 

 living plants these are not always satisfactory, and a series of 

 type-mounts of freshwater algae preserved in their natural green 

 condition without shrinkage would be very useful. 



A great many different fluid media have been recommended 

 for this purpose, most of them employing a copper salt for the 

 preservation of the chlorophyll. Chlorophyll is a complex 

 ester or mixture of esters, containing a small amount of magnesium. 

 Both acids and alkalis rapidly split it into its alcoholic and acidic 

 components, the former also removing the magnesium. Hence 

 fluids to preserve chlorophyll unchanged must be neutral. Mag- 

 nesium-chlorophyll is at best an unstable substance, but by treat- 

 ing it with solutions of various acetates the corresponding metals 

 may be made to replace the magnesium, producing much more 

 stable compounds (1) . The copper derivative is one of the most 

 stable, and this is the reason why cupric acetate is usually present 

 in fluids recommended for the preservation of green plants (2)) 

 (3), (4). Plants treated with copper salts have a rather bluish- 

 green colour and do not bleach out in bright light, but are found 

 to show a good deal of shrinkage unless the solutions used are 

 very dilute. The copper salts must be thoroughly washed out 

 and the object mounted in a neutral medium such as 5 per cent, 

 formalin which has been standing over chalk or, better, in pure 

 glycerine. If the mounting medium contains copper, sooner 

 or later round black or red globules are deposited all over the 

 cover-glass and object, varying in size from a mere point to half 

 the size of a pin's head. These sometimes form in a few days 

 after the mount has been sealed, sometimes they take several 

 months to appear. By reflected light the opaque red ones are 

 evidently metallic copper, and the brownish transparent ones 

 appear to be cuprous oxide, both being formed by the reducing 

 action of organic matter on neutral copper salts (cf. the 



