, PRESERVATION OF DESMIDS. 21 



dox, I fear) that to seek success by mounting a desmid in a preser- 

 vative many times denser than its natural medium was a some- 

 what fatuous quest ; also I saw no reason why desmids should be 

 exempt from another laboratory process, that of killing and fixing. 

 After trying several agents prescribed by the textbooks for fixing 

 vegetable structures, I adopted Hermann's platino-aceto-osmic 

 solution with very satisfactory results. It fixes and preserves 

 the histological elements in an admirable manner, and at the 

 present time there is no shrinkage or alteration of form in some 

 mounted eighteen months ago. There is, naturally, a little altera- 

 tion in colour, the chlorophyll becoming browner, but very much 

 less so than when osmic acid itself is used. Personally, as long 

 as the histological elements are well preserved I do not attach 

 much importance to alteration of colour. A few drops of Her- 

 mann's solution are added to the desmids in half a watch-glass 

 of water, and allowed to act for two or three minutes, then the 

 desmids are transferred with a pipette to fresh water two or three 

 times and washed, and are finally mounted in a saturated solution 

 of thymol. This is merely distilled water which has taken up as 

 much thymol as it will dissolve. With this fixing agent I have 

 been able to get perfectly satisfactory mounts of Netrium Digitus, 

 which appears to be a very difficult desmid to obtain well fixed, 

 its chloroplasts becoming distorted and the cell contents contract- 

 ing on the least interference with it. Hoping to avoid the tedious- 

 ness of fixing and washing, w^ith the almost inevitable loss of speci- 

 mens when transferring from one watch-glass to another, for a 

 short period I mounted the desmids straight away into IJ-per- 

 cent. formalin. Most of the slides then mounted became useless 

 from one cause and another — in some species the colour bleached 

 quite out, in other species the histological elements broke up ; so I 

 decided that formalin was useless, except perhaps for a few robust 

 species. Ripart's solution of copper acetate and chloride, which 

 at one time I used extensively, is another excellent fixing and pre- 

 serving agent, and the colour of some species is well preserved by 

 it. A few drops added to the desmids in half a watch-glassful 

 of water seems to be sufficient. 



So much insistence has been laid by mounters on preserving 

 the green colour of desmids, and the formulae published for 



