SCIENCE- GOSSIP. 



257 



DESMIDS. 



By Dr. G. H. Bryan, F.R.S. 



Collection. 



A LTHOUGH the desmids have been favourite 

 ■^^ objects with microscopists since the days when 

 attention was attracted to them by the writings of 

 Ralfs and others, they are very rarely met with in 

 collections of microscopic slides. During the ten 

 years that I was a member of the late Alfred Allen's 

 Postal Microscopical Society I only remember twice 

 seeing desmids in the boxes circulated round the 

 society, and a microscopic cabinet containing repre- 

 sentatives of these minute algae is quite as rarely 

 found as one that does not contain representatives of 

 their near relatives, the Diatomaceae. 



The reasons which make diatoms so much more 

 popular as " collectors' objects " are not far to 

 seek. (1) They are more frequently found in a state 



in which they are mounted, is a fairly stable com- 

 pound, certainly more so than such media as phos- 

 phorus, sometimes used for mounting diatoms ; (5) 

 desmids do not require the tedious adjustment of 

 illumination and the use of high powers, with con- 

 sequent fatigue to the eyesight involved in resolving 

 the more difficult diatom tests. 



The neighbourhood of Capel Curig, in North 

 Wales, is a well-known hunting-ground for desmids, 

 and, while this lies rather beyond the radius of an 

 ordinary afternoon outing from my present home, I 

 have obtained during the past three years a number 

 of good gatherings nearer at hand. The best locality 

 I have met with has been the Nant Ffrancon Valley, 

 between Bethesda and Ogwen Lake. Most desmids 

 have a great propensity for pools and ditches filled 

 with totally submerged plants of Sphagnum, and it 



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Fig. t. Desmids mixed with foreign matter. A, Micrasterias 

 rotata ; b b, Tetmemorits laevis ; c, Penium nagelii ; D, 

 Staurastrum ; e e, Cosmarium. 



(From micro-photographs by F. Noad Clarke.) 



of comparative purity ; (2) they can be preserved an 

 indefinite length of time before being prepared ; (3) 

 they can be cleaned by chemical means, while desmids 

 cannot ; (4) they can be permanently mounted in 

 balsam, or some equally stable medium, which is not 

 the case with desmids ; (5) their markings afford 

 tests for the highest powers of the microscope. 



As a set-off against these advantages it may be 

 said of the desmids that (1) in certain localities they 

 occur in great abundance and purity ; (2) the in- 

 destructible portion of the diatoms consists only of 

 their flinty envelopes, while in the desmids the whole 

 plant, including the cell contents, is preserved ; (3) 

 they can, as I hope to prove, sometimes be cleaned 

 almost perfectly by mechanical means ; (4) glycerine, 



Fell. 1900. No. 69, Vol. VI. 



Fig. 2. Micrasterias rotata, from a cleaned gathering. 

 (From micro-photographs by F. Noad Clarke.) 



is in such localities that the greatest variety and the 

 most beautiful forms of Micrasterias, Euastrum, 

 Penium, etc., are generally found. This fact limits 

 their distribution considerably, for on the mountain 

 sides the Sphagnum, though usually growing in wet 

 places, is most frequently not submerged in water, 

 and on the other hand it is often absent from pools 

 and ditches. In such cases desmids may be found ; 

 but this is less probable. There is often a kind of 

 sliminess about submerged plants of Sphagnum which 

 sometimes betokens desmids; while in other cases 

 they give a greenish tinge to the water. 



On lifting out the Sphagnum, and either squeezing 

 it into a wide-mouthed two-ounce bottle or a small 

 cup, which should then be rapidly emptied into the 

 bottle, the desmids, when present, will if large, He 



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