Sept., 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



235 



The only time at which spray infj can be of use is during 

 the migration time, i.e., from the middle of May to the 

 middle of June, and then, unfortunately, it is undesir- 

 able on account of the blossom. On the whole, hand- 

 pickiny is the only reliable method, and this should be 

 done when they have g-ot into the new buds, for then 

 their numl>er is reduced to a minimum, and the re- 

 moval of all the new infested buds in July would appar- 

 ently clear the plants of the disease. When black cur- 

 rants are gfrown extensively, hand-picking- is a serious 

 consideration, yet it is the best method that can be 

 recommended. If this method be adopted annually the 

 disease can be reduced tO' a negligible quantity within 

 three years. The picked buds should always be burnt 

 carefully, and not "dug in." 



CleaLiiing Desmids. 



The cleaning of Desmids is generally somewhat 

 troublesome, and many of my readers may be glad to 

 know of a simple method of procedure adopted by 

 Professor G. H. Bryan, and communicated tO' the 

 American "Journal of Applied Microscopy." . Tlie 

 method to be described is particularly applicable to 

 material obtained from mountain bogs containing sub- 

 merged plants of Sphagnum, among which specimens 

 of the genera Micrasterias, Euastnim, Clostcrium, Vcniitm, 

 and others abound. To' collect desmids from this 

 source, a good plan is to squeeze the Sphagnum mXo a 

 wide-mouthed bottle, but the majority of desmid 

 gatherings appear amenable to the same method of 

 cleaning. The apparatus recjuired consists of one or 

 two shallow porcelain saucers or photographic dishes, 

 an old pomatum pot being useful among the number, 

 and a tapered glass tube with a rubber cap, such as a 

 "filler" for a fountain pen. A gauze strainer for a 

 coffee pot is useful for straining out any large pieces of 

 dirt, tlie stuff left behind being examined for filamentous 

 desmids. The strained material is run intO' one of the 

 porcelain dishes, and after a short interval — not more 

 than half a minute — the dish is inclined to one side 

 and gently rocked. Any desmids in the gathering will 

 be seen to collect in a bright green line, or patch, at 

 the edge of the receding water, and can then be readily 

 picked up with the pen-filler in an almost pure state. 

 On working round the edge O'f the dish, the desmids 

 may be drawn intO' green patches in almost any desired 

 part of the vessel, and one lot after another picked up 

 until there are none left worth troubling about. As the 

 desmids are removed, they are transferred to the poma- 

 tum pot, where a drop of Zenker's fixative suffices to 

 fix them. (The formula for Zenker's Fluid is Ka Cro O7, 

 2.5 grms. ; Na.j So^ i grm. ; Hg CI,, 5 grms; glacial 

 acetic acid, 5 cc. ; water ad 100 cc. Dissolve the Hg Cl^ 

 and K. Crj O7 in the water, with the aid of heat, and 

 add the acetic acid in proper proportions as required, 

 as it evaporates readily.) By repeating the rocking 

 process, the desmids are again collected and transferred 

 from the fixative to another dish containing clean 

 water. They are deposited in a patch in the water near 

 the edge of the dish, and by repeating the rocking, the 

 fixative is gradually washed away, together with any 

 remaining foreign matter. The water should be 

 changed at least once. This method of washing in- 

 volves less loss of specimens than the ordinary decanta- 

 tion method, provided that care is taken each time to 

 deposit the desmids as close together as possible, for 

 the few desmids that are not picked up in the first 



attempt are easily collected and picked up subsequently. 

 The whole process takes but half an hour or an hour, 

 so that the fixative is removed before it has time to 

 injure the colour of the specimens. 



If much foreign matter is mixed with the origin.al 

 gatherings, the whole may be left in a wide-mouthed 

 bottle in the light for a day or two, when the desmids 

 will collect on the top of the sediment, where they will 

 increase and multiply. The surface layer, containing 

 the desmids, may then be syphoned off and cleaned a; 

 before. Even in poor material it is often possible, by 

 the rocking process, tO' collect with the pen-filler suffi- 

 cient desmids to mount one or twO' slides. A some- 

 what similar rocking process is useful for separating 

 Foraminifera from sand, but the rocking must be a little 

 more violent, and the sand is left behind, unlike the 

 flocculent matter in the desmid gathering, which is 

 swept forward by the water. 



To' mount the desmids, Profesor G. H. Bryan takes a 

 small piece of parchment paper, say, i^ by i inch, or 

 less, such as is used for packing tobacco, and folds it 

 into a little box. The water with the desmids i.s placed 

 in the box, which is then floated on glycerine. In two 

 days the water will have diffused intO' the glycerine, and 

 ! sufficient glycerine to penetrate the desmids will have 

 passed through the parchment intO' the bo'X. The 

 desmids are now ready for mounting in glycerine, and 

 have undergone no contraction. 



Some desmids, notably Closterinm, have a tendency to 

 adhere tO' the bottom of the dish, and then float on the 

 water, but this tendency to float gives similar difficulties 

 when they are washed by decantation. As species < f 

 this genus multiply rapidly by self-division, it is usually 

 possible to start with sufficient material to allow of a 

 mall loss by flotation. 



Action of Wood on Photogra.phic Plates. 



In a recent number of the proceedings of the Cam- 

 bridge Philosophical Society, I'rofessor H. Marshall 

 Ward refers to W. J. Russell's recent memoir in the 

 " Transactions of the Royal Society," in which is de- 

 scribed the action of a number of different woods on a 

 photographic plate in the dark. Russell had suggested 

 hydrogen peroxide as the active agent, this re-agent 

 having a definite action upon photographic plates, and 

 the resin in the wood as probably the indirect causal 

 agent, adducing in support the experimental result 

 that while gum-like bodies are inactive, those of a more 

 resinous nature are active. Professor Marshall Ward, 

 as the result of numerous experiments, concludes that 

 the activity is not merely due to resin or resin-like 

 bodies, but that tannin and tannin-like bodies, 

 as well as some others, may also be responsible. 

 It is at least clear that some body or bodies in the 

 liquified cell-walls reduce silver salts in the plate, and 

 that these bodies are either shot off, as if volatile, or 

 diffuse readily, seems clear from the want of sharp- 

 ness in the microscopic details. Readers will find no 

 difficulty in carrying out the process if they care to 

 try it. The sensitive film of the dry plate is merely 

 placed in contact with the smooth dry face of a wood 

 block and left in darkness for periods of varying length, 

 and the plate on development should show an image 

 of the wood, knots, for instance, being particularly well 

 marked. 



[Communicatioin and enquiries on Microscopical matters should be 

 addressed to F. Shillington Scahs, "Jersey," St. Barnabas Road, 

 Cambridge.] 



