SUGGESTIONS FOR COLLECTING AND PREPARING 



DIATOMS. 



By Albert Mann, 

 Custodian, Section of Diatoms, Umted States National Museum. 



The study of the diatoms has taken on a much greater importance 

 than formerly, because of the now generally recognized fact that the 

 role they play in aquatic life, especially in the food supply of fishes, 

 is a very great one. In their fossil state also new uses for diatom earth 

 have been discovered and the amount of this material now supplied 

 to the various industries is enormous. I have been asked to give some 

 simple suggestions on the best methods for collecting these organisms 

 and preparing them for microscopic study. 



The living forms, inhabiting all waters, fresh and marine, will be 

 first taken up. The presence of diatoms in any considerable quantity 

 can be detected in quiet waters by the rich brown or yellowish-brown 

 color of the film composed of diatoms, which coats the bottom of such 

 places or clothes the submerged stones, sticks, and other bodies. By 

 carefully removing this pellicle, for which a bent piece of tin or a 

 clamshell are good instruments, the collector secures a rich gather- 

 ing, very slightly mixed with dirt, sand, or other matter. 



But there are few such places met with, especially in the collecting 

 of marine diatoms. Two very distinct methods of securing material 

 must be used, because we have to do with two unusually different hab- 

 its of life of the diatoms. Some of these are plankton, that is to say, 

 they live suspended in the water. The presence of the plankton forms 

 is generally unsuspected, because they are not readily visible in the 

 water, although generally present, and that, too, at times in immense 

 quantities. The plankton diatoms are secured by means of a plank- 

 ton net composed of silk bolting cloth with a mesh of about No. 18. 

 This net is attached to a wire ring having a diameter of about 15 to 

 18 inches and is conical in shape, but should be as far as possible 

 without folds, having a length of 2^ to 3 feet, and with the 

 lower end or tip slightly rounded. This absence of folds can be 

 secured by cutting the bolting cloth and carefully sewing up the seams 

 with a double line of stitches so as to avoid any flaps. Attached to a 

 stout cord by three lines tied to the wire rim at ec{uidistant points, the 



No, 2410— Proceedings U. S. National Museum. Vol. 60, Art. 15. 



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