210 31. C. COOKE ON SOME DESMIDS 



frond is often of superior value to one filled with endoclirome ; it 

 permits you to see the puactse or markings of the segments, which 

 are obliterated whilst the endoclirome remains, and in the genus 

 Cosmarium this becomes daily of greater importance than ever. 



I admit that for the study of the endoclirome alone, its presence 

 is, of course, the most important, but this can be done, and draw- 

 ings made from the plant in the living state, and if specimens can 

 be mounted with the endoclirome unchanged and uncontracted, so 

 much tlie better, but as yet I lia^e seen no method entirely satis- 

 factory. For twelve years I have kept some slides mounted in the 

 silicates of potash and soda, but these were not satisfactory, as half 

 the slides would persist in deliquescing. Far more successful was 

 my friend Mr. Wills, for his slides, mounted much longer even than 

 mine, surprised me. Of course I enquired the medium, and learnt 

 that he simply used the water in which the Desraids had been 

 collected, and was careful never to leave them exposed to the day- 

 light. 



One great difficulty in mounting objects with such thin and deli- 

 cate cell walls as Desmids is to employ a medium of no greater 

 density than the cell contents. If a denser medium, such as 

 glycerine, be employed, the endoclirome immediately contracts, and 

 never expands again as before. Water, or water containing a little 

 camphor, is of equal density, and no change can be detected. 



After all, the preservation of the endoclirome is of less im- 

 portance than the perfect contour of the cell. If there is any con- 

 traction or collapse the objects are useless. 



Supposing, therefore, that there is no necessity to preserve the 

 endoclirome, there is another feature to be remembered besides the 

 preservation of contour, and that is that the medium employed 

 should not render the delicate cell walls so transparent as to become 

 ultimately invisible. I commend this to your notice as a warning. 

 In simple water I encountered no difficulty in discerning the 

 structure of the cell walls, after a period of not less than twelve 

 years. So much cannot be said for glycerine. As a hint, I may 

 add that I have seen empty fronds stained of various colours, both 

 of Desmids and of Volvox, and these exhibited all the details in an 

 unexceptionable manner. 



Let me commend these little plants to those of you who are 

 getting tired of Stephanoceros and Melicerta. There are plenty of 

 discoveries yet to bo made. 



