2 5 8 



SCIENCE- GOSSIP. 



visible to the naked eye. From the contents of the 

 bottle, if the material happen to be rich, sufficient 

 desmids may be obtained for mounting some dozens 

 of slides for oneself and friends. 



Some species of Closterium are less partial to 

 Sphagnum, and seem to occur at times in large 

 quantities in tiny pools of water, forming thick green 

 patches. For collecting these I have found the best 

 instrument to be a "fountain-pen filler" or pipette, 

 a pointed glass tube with rubber cap. Other kinds, 

 notably certain species of Staurastrum, seem to grow 

 in ponds and rivers mixed with filamentous algae, 

 diatoms and infusoria. Up to the present I have 

 not been quite so successful in cleaning these as the 

 Sphagnum washings, but this I attribute to the lesser 

 abundance of the desmids in the gatherings. In cases 

 where desmids are scattered diffusely throughout the 

 water in which they grow, straining through linen is 

 recommended for the purpose of concentrating the 

 specimens into a smaller mass of water, and a bag 

 net is advocated by most writers for fishing desmids 

 out of pools, mountain tarns, lakes, and such localities. 



Preservation and Mounting. 



My mounts have hitherto been made from Sphag- 

 num squeezings, patches collected with the pen-filler, 

 &c, which I have brought home unstrained. Where 

 I have either devised new dodges or modified old 

 ones, it has lain chiefly in the modus operandi in 

 dealing with the material after collection, which falls 

 under the several headings of Fixing, Cleaning, Trans- 

 ferring to Glycerine, and Mounting. 



Fixing When the desmids have been allowed 



to settle, and as much as possible of the water drawn 

 off, the addition of a few drops of acetic acid is usually 

 sufficient. I have also been successful with acetate 

 of copper and Zenker's fixative. So much has been 

 written on the subject of fixing fresh-water algae in 

 botanical journals that I do not claim these are the best 

 reagents, but formulae for diluted fixatives containing 

 a given percentage of water are objectionable. This 

 is on account of the considerable amount of water in 

 which the desmids are contained to start with, and 

 the great loss of specimens that occurs when it is 

 attempted to reduce too much the bulk of this water. 

 Therefore let the fixatives be made strong, and the 

 water containing the desmids furnish the required 

 dilution. 



It is necessary to fix before cleaning, if the desmids 

 are mixed with filamentous algae, such as Spirogyra ; 

 otherwise the latter will get broken up and become 

 difficult if not impossible to remove ; besides, the 

 cleaning process is apt to disturb the cell-contents of 

 the desmids themselves. 



Acetic acid, and probably other fixatives, being in 

 time destructive to the colour of the desmids, should 

 be removed as soon as possible, whether before or 

 after the cleaning process to be next described. T 

 find after the more convenient, as most of the fluid is 

 left behind in the cleaning process and the desmids 

 can be then washed in a test-tube, which is repeatedly 



filled up with filtered rain-water, and the latter decanted 

 or drawn off with a syphon as soon as the objects 

 have settled. To prevent the specimens from lodging 

 against the sides of a tube I use a simple test-tube 

 suspender, consisting of an inverted U-shaped wire, 

 across the bottom of which an elastic band is stretched, 

 the test-tube being placed within this band and 

 hanging quite freely. The device is as useful for 

 diatoms, for which I have used it for many years, as 

 for desmids. 



Cleaning. — The fixed material must be strained 

 through wire gauze, and perhaps also through 

 muslin gauze. This will separate the free desmids 

 (unicellular forms) from Sphagnum leaves, bits of 

 grass or wood, or any of the numberless other 

 similar kinds of foreign matter, also from filamentous 

 algae and filamentous desmids. If there is much 

 residue, the material should be strained in two or 

 three batches, the residue in question being washed 

 off into a vessel of water to be examined for fila- 

 mentous desmids, such as Hyalothecd, or Sphaero- 

 zosma. A further washing and straining through 

 wire gauze will remove all traces of fixative from 

 this part of the material, and if the filamentous 

 portion wholly consists, as it often does, of desmids 

 or other algae, these may be fished out with a needle 

 from the dJbris with which they are mixed, and 

 transferred to a little clean water. 



The water that passes through the strainer should 

 be collected in a saucer, photographic dish, or other 

 shallow receptacle. It contains all the free desmids, 

 but as a rule these are mixed with a very large pro- 

 portion of " flocculent matter." What this flocculent 

 matter is, would be rather difficult to define briefly, 

 but its presence in slides, as exemplified in my 

 earlier mounts (fig. I), detracts considerably from their 

 beauty. It was only towards the end of last sum- 

 mer that I discovered how the desmids may be made 

 to separate themselves almost entirely from these 

 objectionable impurities. To this end, as soon as 

 the desmids have settled, the saucer containing them 

 should be gently tilted on one side, and at the same 

 time a slight rocking motion given to it. What 

 happens is a simple consequence of the well-known 

 physical properties of fluids, their viscosity, and 

 capillarity. 



In consequence of viscosity, the water in flowing 

 to the side of the saucer does not move in one even 

 stream, but the motion when slow consists of a 

 gliding of the upper layers over the lower ones, and 

 at the bottom, where the desmids are deposited, the 

 water has practically no velocity. The effect of this 

 motion is to roll the flocculent matter together into 

 lumps and carry it off with the bulk of the fluid, 

 leaving the desmids undisturbed till the edge of the 

 receding wave has reached them. At this point 

 capillarity comes in and draws the desmids along. 

 They will soon be seen to collect in a line at the edge 

 of the fluid. The slight rocking motion will help 

 the desmids to collect, and if the wave be caused 

 slowly to travel round the edge of the saucer, it will 



