24 



Clarence J. Elmore 



unfavorable to the growth of diatoms. It is hkely, however, that 

 some diatoms were growing when inorganic sediment was being 

 formed, but in the large amount of this sediment they have been 

 overlooked. 



COLLECTING AND PRESERVING 



Few directions are needed for collecting diatoms. Any vege- 

 table growth in water or in damp places is likely to contain them. 

 All that is necessary is to get the material in any way that is most 

 convenient and to carry it in whatever may be at hand for the 

 purpose. 



When it is desired to preserve the protoplasmic contents of 

 diatom cells, they may be placed in formaldehyde or other pre- 

 servative in the same manner as other algae. For careful cyto- 

 logical work they may be killed in hot mercuric bichloride solution, 

 and then, after washing and dehydrating, mounted in balsam. 



The determination of species depends mostly on the siliceous 

 shells and when they are collected for that purpose no preservative 

 need be used. The material containing them may be wrapped in 

 paper and allowed to dry. Diatoms preserved in this way are in 

 no way inferior to those preserved by more careful methods for 

 purposes of identification when it is only the shells that are to be 

 examined. 



MOUNTING 



The markings on the shells are often very delicate and difficult 

 to see. In many of the smaller species they can not be seen when 

 in their natural condition and mounted in water. They are more 

 easily seen when the protoplasm is removed from the shells. This 

 may be removed by boiling in a test tube in nitric acid. This 

 destroys the cell contents, leaving only the siliceous shells. Water 

 is then added, and after the diatoms have settled it is poured oflf. 

 This process is repeated until the acid is all removed. A drop of 

 the liquid containing them is placed on a coverglass and allowed 

 to dry. 



The method usually used liy the writer, however, is to dry a 

 small portion of the material containing the diatoms on a cover- 

 glass and to heat until all of the organic material has been burned 



