22 G. T. HARRIS ON THE COLLECTION AND 



mounting fluids so obviously have this end in view, that it was 

 only a matter of time for my resistance to break down and com- 

 mit me also to the quest of a formula that should achieve this 

 desirable end. I have already stated my indifference to the 

 preservation of colour so long as the cell contents remained un- 

 altered. The colour if lost or degraded can always be simulated 

 by interposing between the slide and radiant a green filter, which 

 really satisfies the colour sense almost as well as having it in the 

 desmid. At the same time it is somewhat of a tour de force if 

 the colour can be retained without sacrificing essentials in cell 

 structure. To fix and preserve histological detail in desmids is 

 undoubtedly far easier than to preserve their colour. Chloride 

 of gold, which is a most delicate and admirable fixative for des- 

 mids, stains the cell contents blue, but leaves them in perfect 

 condition for study. Osmic acid by itself colours them deep 

 brown, unless cleared by hydrogen peroxide ; Hermann's solu- 

 tion destroys some of the blue in the green of the chlorophyll, 

 leaving it rather yellow-green. In some species, Closterium cos- 

 latum, Closterium striolatum, and others naturally somewhat 

 yellow, the alteration in colour is negligible. The most promising 

 results as regards combined fixing and colour preservation were 

 got when I adopted the method of collecting already described. 

 Desmids collected and preserved by that method nearly twelve 

 months ago have retained their natural colour although killed 

 and fixed. This, I am aware, is not a period of time suggestive 

 of indefinite permanence, it is merely promising. When one has 

 seen the colour of mounted desmids disappear utterly in about 

 ten days, twelve months is, naturally, gratifying. I have always 

 found Micrasterias rotata to be a most impossible desmid to pre- 

 serve the colour of, and the only time I have approached success 

 with it has been when collecting and preserving it on the spot. 

 As it grows readily in a small tank I have usually a good supply 

 of healthy individuals upon which to experiment, and have done 

 so rather extensively. With IJ- per- cent, formalin the colour of 

 this species fades completely in two days, and even when mounted 

 in such a mild preservative as thymol water the colour gradually 

 disappears. Hence I look upon the fact of this species having 

 kept its colour for upwards of twelve months as promising useful 



