PRESfiEVATION OF DESMJDS. 25 



be seen that the desmids are mounted in their own bog water, 

 plus a small proportion of fixing agent ; and if " sphagnol " 

 is a laboratory fact they also have whatever benefit it confers. 

 As far as the preservation of the green colour of the plant is con- 

 cerned, nothing that I have tried- has given me such good results, 

 but twelve months is far too short a period to be a satisfactory test, 

 and it may be that this method also will be merely a contribution 

 to the melancholy collection of unreliable formulae. 



One hint I may give to intending mounters of desmids which 

 I have learned in the very practical school of experience ; it is, to 

 avoid the use of too shallow a cell for mounting them in. As a 

 good number of species require the use of fairly high powers, there 

 is the temptation towards the use of very shallow cells ; at least 

 this was so in my own case, and the result has been that although 

 the cells were carefully sealed the extremely small amount of 

 fluid has evaporated or been absorbed, and many slides made at 

 great trouble are now finding their way to the waste box. Deeper 

 cells containing the larger species and an appreciable amount of 

 fluid seem exempt from this trouble. 



I cannot conclude these hints on collecting and mounting des- 

 mids without allowing myself the reflection that it is unlikely 

 that a systematic student of the Desmidiaceae will spare the time 

 necessary for making slides of these plants. The amount of time 

 involved in getting even fairly decent slides of the various species 

 is very great, and it is a question if mounted specimens are of 

 much value to the student. Specific determination of the bulk of 

 desmids can only be satisfactorily made by studying them in 

 three aspects, and unless a cell of considerable depth were used 

 this could not be ensured. A former president of the Quekett 

 Microscopical Club asserted that the best record of a desmid was 

 obtained " by the free use of camera lucida," and I imagine that 

 this dictum has to be accepted even now. Moreover, however 

 attractive specimens may be which retain their life-like appearance 

 when mounted, the fact that the cell wall cannot be adequately 

 studied when the cell is filled with its endochrome seriously dis- 

 counts the value of a mounted specimen. Identifying specimens 

 in the genus Cosmarium is almost impossible unless the empty cell 

 can be obtained, and the same may be said of a large number of 



