MOUNTING FRESHWATER ALGAE, MOSSES, ETC. 227 



and preserves it better than aqueous media, but it is often diffi- 

 cult to get delicate objects into glycerine without shrinkage, 

 e.g. Vaucheria and its zoospores. In this case, formalin or thymol 

 water or the formalin-zinc-acetate fixing solution may be used as 

 the mounting medium. The green colour does not fade in either 

 case in the dark, but the aqueous mounts bleach out quickly in 

 direct sunlight, while those preserved in glycerine keep their 

 colour perfectly, even after weeks of exposure to bright daylight. 

 In a few cases this method, as well as the copper one, fails com- 

 pletely, the colour changing to a dirty brown during fixation, 

 e.g. in some delicate leafy liverworts, some fern prothallia and 

 often in Spirogyra and Zygnema. 



In some cases (Micrasterias, Cladophora) small dark spots are 

 deposited on the object during fixation, much resembling those 

 occurring in the copper method, only rather smaller ; and I 

 find it very difficult to conjecture of what they can consist. The 

 only other zinc salt which I have tried is zinc salicylate, and this 

 produces the same effect. In such cases it is advisable to cut down 

 the time of fixing as much as possible and to make the subse- 

 quent washing very thorough. 



The above method is suitable for algae, mosses, liverworts, 

 prothallia of ferns and equisetaceae, selaginella and higher 

 plants, green hydrae, etc. 



It is a great advantage with mosses and other aerial plants if 

 the water used for diluting the concentrated formalin for the 

 fixing liquid has been recently boiled and cooled, as it will then 

 quickly dissolve any air about or within the leaves, etc., and 

 allow the fixing fluid to penetrate rapidly. It must not be boiled 

 after the addition of the zinc acetate, as this causes hydrolysis 

 with the precipitation of white gelatinous zinc hydroxide. 



I strongly recommend the boiling of all botanical fixing solutions 

 that will stand it, such as chromacetic acid, as it increases the 

 penetrative power. The object should be placed in a bottle and 

 the cooled fluid poured in until it is overflowing, when the cork 

 may be pushed home so as not to include any air. The material 

 floats at first, but sinks as the entangled air is dissolved. 



The neutral formalin required in the above method may easily 

 be obtained by keeping a lump of chalk in the bottle of strong 

 formalin. 



The fixing and staining of algae have been very ably dealt with 



JouRN. Q. M. C, Series II.— No. 87. 16 



