10, The Preparation of Whole Mounts and Smears 



Gross preparations may be roughly classified as follows: 



1. Dry preparations: herbarium sheets, Riker mounts, and bulk speci- 

 mens. These methods are not within the scope of this manuaL 



2. Wet preparations: museum-jar preparations; bulk material for dissec- 

 tion: whole mounts, smears, and macerations for microscopic study. 



Wet preservation may be used for a wide range of subjects in all 

 major categories of the plant kingdom. Many subjects, such as the 

 algae, can be preserved for critical study only by wet preservation. 

 These methods can be adapted to furnish material for gross specimens, 

 for dissection, and for microscopic preparations. Entire plants or 

 parts such as leaves, flowers, and fruits can be preserved in fluids that 

 kill the cells, prevent decay, preserve the material in firm condition, 

 and possibly preserve the natural colors. 



The best-known preserving fluid is ethyl alcohol, usually used at 

 a concentration of approximately 70 7^. This fluid preserves even the 

 bulkiest objects. Considerable brittleness is produced, but preserved 

 material can be made flexible for dissection by soaking it in water. 

 Cells are plasmolyzed by alcohol, but this is not objectionable in 

 dissections or freehand sections, in which the condition of the proto- 

 plasm is not important. Ethyl alcohol is difficult to obtain in some 

 institutions, its use at field stations may be undesirable, and shipment 

 of materials in alcohol involves legal technicalities. Regardless of 

 these objections, ethyl alcohol is firmly established as a preserving 

 fluid. 



Formaldehyde is an excellent preservative. This reagent is 

 obtained as an aqueous solution containing 37 to 40% formaldehyde 

 gas by weight. The U.S. P. (United States Pharmacopoeia) grade is 

 adequate for preserving bulk materials. The most useful concentration 

 for bulk preservation contains 5 parts formaldehyde solution in 95 

 parts of water. For massive objects the concentration must be doubled. 

 At these concentrations the material does not become brittle, and 

 some materials become pulpy after prolonged storage. Formaldehyde 



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