PRESERVATION OF DESMIDS. 23 



results. When a large number of species from various genera are 

 collected and preserved at the same time and in the same medium 

 it enables one to make very useful comparisons, and I have prob- 

 ably learnt more with regard to colour preservation from looking 

 over the preserved material than from numerous, and I fear some- 

 what empirical, experiments. The following observations on 

 the present condition of desmids refer to material collected about 

 nine months ago and preserved on the spot. The species of the 

 genus Closterium keep both detail and colour remarkably well in 

 the preserving solution already given, with the exception of 

 Closterium Ehrenbergii, with which I have never been very suc- 

 cessful ; the colour is preserved excellently, but considerable 

 contraction of the endochrome takes place. The Euastra also 

 seem to retain their colour well. Species of Micrasterias are un- 

 certain. Micrasterias oscitans preserves excellently unless the 

 plants are aged. I have already alluded to the difficulty of fixing 

 Netrium Digitus satisfactorily, and it is rather a curious fact that 

 in the copper preservative solution the green of the chlorophyll 

 in this species nearly always turns some shade of red. It is, how- 

 ever, in specimens I have collected rarely a deep-green colour, 

 more often quite yellow-green. The filamentous genera, such as 

 Hyalotheca, and Desmidium, preserve excellently, both colour 

 and form. One of the species I have hitherto failed utterly with 

 is the beautiful Spirotaenia bispiralis. It is quite unrecognisable 

 a few weeks after placing it in the preservative. A fixing agent 

 of the nature of Hermann's solution might, however, effect its pre- 

 servation, but of course at the sacrifice of its delicate colour. I 

 gathered it frequently on Dartmoor in considerable numbers, but 

 have never come across a single specimen when looking over the 

 material, so I conclude that the fixing agent was unsuited to it. 

 The larger species of Cosmarium, Cosmarium Brehissonii, Cos- 

 marium Ralfsii, Cosmarium fyramidatum, etc., are generally well 

 preserved. The larger Staurastra also make good mounts, more 

 especially Staurastrum tumidum. Netrium interrwptum keeps 

 its fine colour magnificently, then flouts one by its endochrome 

 assuming the appearance of a pneumatic tyre with a puncture. 

 Tetmemorus Brehissonii is almost invariably finely preserved, and 

 its colour unimpaired ; in fact all the species of Tetmemorus 



