17 



a brilliant green. With respect to size, a very large specimen is jnst 

 comfortably visible to the naked eye, the British species ranging from 

 l-40th of an inch down to 1-2, 400th. To make the latter measurement 

 apparent, it may be added that when magnified "500 diameters" or 

 250,000 times, the length of the specimen will then only appear equal to 

 21-lOOths of an inch. They are formed always in clear water, foul or 

 muddy water knows them not ; they prefer open moorland pools and slow, 

 bright streams, in which they are seen in masses adherent to the water 

 plants, stones, or the sandy or peaty bottom, the masses looking like filmy 

 pale green clouds. 



These plants are thoroughly cosmopolitan, many of the species being 

 known to exist in the Temperate, Torrid, and Arctic zones alike, apparently 

 being unaifected by extremes of heat or cold. 



In the Yorkshire district, Goole, Adel, Cullingworth, Strensal), and 

 Scarborough Mere are well-known haunts of the tribe. Like low forms of 

 algse, the Desmids multiply and increase by " cell-division " ; a strictly 

 vegetative process, in which the semi-cells divide and each throws out a 

 new segment, which rapidly (often in four to six hours) grows like the old 

 one, thus constituting new fronds ; but the true reproduction of the family 

 is by " zygospores," which are each formed by the fusion of the contents 

 of two cells, these (like the fronds) are curiously diverse in form, being 

 globular, oval, angular, smooth, with plain or forked spines, &c. These 

 spores remain under water for at least one winter season (probably two), 

 and then burst, the contents growing into fronds or cells like the original 

 ones. In illustration of the species numerous forms were shown (which 

 had been photographed and were exhibited as lantern slides) from the 

 original and most exquisite drawings made by the lecturer. In conclusion, 

 the lecturer begged the members present to study these interesting little 

 plants, as there is much work to be done, and is needed with respect to the 

 British species. 



As regards the preservation and preparation of these (and other 

 fresh-water algae) for micro-examination, the most successful mode of 

 "mounting" seems to be in shallow tin cells, not over Twth inch thick, 

 cemented to the slide with shellac dissolved in cold, strong alcohol. Many 

 preservative fluids have been proposed, which all agree in being more or 

 less ineffective ; indeed it seems hopeless to attempt to keep Desmids in 

 their pristine beauty, as the brilliant emerald of the chlorophyll soon 

 becomes more or less diilled in time, and should a fluid of heavier specific 

 gravity that I'l (water being unity) be applied, the cell-contents contract 

 into shapeless masses. A weak solution of glycerine is useful for many 

 species, but it is hardly a " safe " medium, as glycerine cells have a bad 

 habit of leaking without apparent cause. A useful fluid is — distilled water 

 70 parts, camphor water 20 parts, saturated solution of acetate of alumina 

 10 parts. For the red or brown algse, leave out the camphor water and 

 add three drops per ounce of acetic acid. As Desmids mostly occur in 

 peaty water, the material should be washed repeatedly with clean water, 



B 



