Diatoms 



FLUID WHOLEMOUNTS IN NONAQUEOUS MEDIA 



37 



fourth side is, however, made in such a 

 manner that a gap of about a millimeter 

 will be left between the cement and one 

 of the corners of the cover. This gap is 

 necessary to permit the heat-expanded 

 glycerol to escape. After the slide has 



thus been not quite sealed it is permitted 

 to cool and a rag moistened with 95% 

 alcohol is used to remove excess glycerol 

 from the httle vent which has been left. 

 This vent is then itself sealed with a drop 

 of very hot cement. 



Preparation of Diatoms in Bromonaphthalene 



Strewn slides of diatoms may be 

 mounted dry in the manner described in 

 Chapter 1. When it is necessary, however, 

 to resolve fine structure, they should be 

 prepared in a medium of high refractive 

 index, and no resin has yet been found 

 which is as satisfactory as bromonaph- 

 thalene. No one who has ever examined 

 diatoms mounted in bromonaphthalene 

 will ever wish to use any other medium 

 and, though the process is tedious, the 

 end result justifies the trouble taken. 



Before mounting, diatoms must be 

 collected and cleaned. The three great 

 sources are fresh-water, sea-water, and 

 fossil deposits. Diatoms occur in fresh 

 water as part of the plankton, but are 

 mostly found in the mud on the bottom of 

 ponds or attached to weeds. No attempt 

 should be made to separate diatoms from 

 the weeds in the field; the collection 

 should be taken back to the laboratory. 

 The first rough separation is then carried 

 out by cutting the plants into about ]>i- 

 inch lengths and putting them into a flask 

 with enough water to cover them, shaking 

 vigorously, and then straining this water 

 through coarse cloth into another con- 

 tainer. More water is then added to the 

 material, which is again shaken, and so on 

 until after four or five washings, all the 

 diatoms have been removed. These wash- 

 ings may be set on one side to settle for 

 further treatment. 



Diatoms may be separated from fresh 

 mud by taking advantage of their photot- 

 ropism. The mud, together with an ade- 

 quate quantity of the water from which it 

 was collected, is placed in a small saucer 

 and a thin layer of cheesecloth is spread on 

 the top. The mud should be sufficiently 

 Uquid to permit diatoms to pass through 

 readily, but sufficiently solid to prevent 

 the cheesecloth from*sinking into it. If 

 the dish be set in bright light for a day or 



two, the diatoms will migrate through tlie 

 cheesecloth and form a d;irk greenish 

 smear over its surface. The cloth is then 

 removed, washed, and the diatoms ac- 

 cumulated in a small quantity of water. 



The collection of diatoms from marine 

 plants may follow the technique used for 

 fresh-water plants, though the larger algae 

 are better scraped with a blunt knife. 

 These scrapings are then transferred to a 

 jar of sea water where the diatoms and 

 debris settle to the bottom. A rather large 

 number of marine diatoms are, however, 

 planktonic and can be collected from sea 

 water with a centrifuge. A plankton-con- 

 centrating centrifuge is described in Chap- 

 ter- 2 and with its aid large volumes of 

 water may be processed in a relatively 

 short space of time. If such a planktonic 

 centrifuge is not available, it will be neces- 

 sary to collect the samples by towing be- 

 hind the boat a long conical net of the 

 finest obtainable bolting silk to the end of 

 which is attached a small tube in which 

 collect those specimens which have not 

 passed through the net. Unfortunately the 

 diatoms form a relatively small bulk, even 

 though they may be numerous in quan- 

 tity, of the total material collected, so that 

 if there are many crustaceans among the 

 plankton it is desirable to have a double 

 net, the first layer of which will retain the 

 crustaceans without permitting the dia- 

 toms to pass. Whatever method of con- 

 centration is adopted, however, one ends, 

 as in the other processes described, by 

 having a mixture of dirty diatoms and 

 sludge accumulated in the bottom of the 

 dish. 



Diatoms also occur in guano, and in 

 many fossil deposits, and must be roughly 

 separated before cleaning. If the fossil de- 

 posit resembles guano, it is only necessary 

 to shake it up in water and pass it through 

 a coarse sieve to remove the sand and 



