94 Botanical Microtechnique 



sectioned. The device used on the sliding microtome for this purpose 

 is known as a freezing attachment. The various freezing attachments 

 are described in the catalogues of the manufacturers of microtomes. 

 The device replaces the usual tissue clamp or object carrier of the 

 microtome. Freezing is accomplished by the evaporation or expansion 

 of a freezing agent — ether, CO^ gas, or solid COo (dry ice) . The piece 

 of tissue to be frozen is usually enveloped in a fluid or semifluid 

 medium, which on freezing affords additional sup})ort. 



Gum arable is probably the best-known supporting medium. Make 

 an aqueous solution of gum arable of thick, sirupy consistency. Add 

 a few crystals of carbolic acid. Dip the specimen to be cut into the 

 gum. Freeze a 2- to 3-mm. layer of gum on the supporting disk of the 

 freezer. Place the specimen on the disk, wrapping a generous quantity 

 of the gum around the specimen. Turn on the freezer, and, as the gum 

 begins to congeal, wrap more gum on the specimen until the material 

 is well supported. Proceed with the sectioning. 



Gelatin is another satisfactory supporting medium. Make a gelatin 

 solution that is semifluid at room temperatures. Add 0.1 per cent 

 carbolic acid as a preservative. Warm on a water bath for use. and 

 use in the manner described above for gum arable. A semifluid 

 solution of agar is another excellent medium, used like gelatin. 



Freehand or microtome sections can be mounted in a drop of 

 water or 50% glycerin and studied with the microscope. If glycerin 

 is used, the water can be evaporated and the cover glass sealed with 

 lacquer or paraffin, making a semipermanent mount. The mounting 

 media described on ])age 102 also can be used. Sections of dark- 

 colored woods, or other materials having adequate coloration, can 

 thus be made into scmij)ermancnt or jxTmancnt slides without 

 staining. 



Staining 



The staining of sections of uiKinbeddcd tissues is essentially the 

 same as the staining of celloidin sections. I'he worker \\li() has had 

 previous experience with paraffin sections can foll()\\- tlie staining 

 charts in Chap. 7, making the necessary modifications. For the benefit 

 of workers who wish to use the present chaj)ter without having had 

 previous experience, an outline of some simple, practical processes is 

 offered. Sections cut in alcohol should be progressi\c]\ transferred or 

 run down to water before staining. Add an e(iual Nolume of water 

 to the alcohol containing the sections. Mix genily. jK)ur off half of 

 the liquid, and add an equal volume of water. Pour off all the liquid, 



