206 BOTANICAL MICROTECHNIQUE 



The material, imbedded in the paraiSn cake, is out out in a small block, 

 which is fastened by heat to a metal holder for the rotary microtome or to 

 small wooden holders for the sliding ones. The block should be arranged so 

 that it will be cut as nearly as possible in the correct plane. The paraffin 

 is then trimmed around the object so that the cutting edge is square or rec- 

 tangular, with parallel edges. The block is then adjusted by a mechanism so 

 that the face which is to strike the knife is exactly parallel to its edge and so 

 that the object will be cut in the desired plane. 



Cutting in paraffin is not successful unless the sections run oft the knife 

 edge in an unbroken ribbon. There are a number of conditions necessary 

 to obtain this result. The knife must be very sharp and the edge without 

 nicks (at least where the cutting is done), which will split the ribbon 

 lengthwise. It is useless to attempt to cut with a poor or dull knife. If a 

 ribbon after running smoothly begins to split or show conspicuous lines, 

 draw the finger upwards along the edge of the knife. The difficulty may 

 have been caused by some hard particle lodged against the edge, which is 

 thus removed. The edge of the knife must be clean ; grease or paraffin may 

 be removed with xylol applied by a brush or with a soft rag. 



The ribbon should run sti-aight. If it begins to curve, trim the block 

 unevenly so that it will come off the knife straight ; a curved ribbon is 

 generally due to differences In the texture of the two sides of the object. 

 Sometimes sections roll up or fail to stick together in a ribbon. This gen- 

 erally means that the paraffin is too hard for the temperature of the room. 

 The cutting must be done in a warmer room or the material reimbedded 

 in a softer paraffin. Cutting the sections in the sunshine of a window 

 instead of in the shade will often remedy the difficulty. A more fre- 

 quent difficulty is a crushing of the sections together. This means either 

 that the knife is not sharp, that its edge is not perfectly clean, or that the 

 paraffin is too soft for the temperature of the room. If the paraffin is too 

 soft (as is commonly true in summer temperatures), the block and knife may 

 be cooled in ice water or the material reimbedded in harder paraffin. It is 

 time wasted to attempt to cut when the ribbon is not running smoothly; 

 find out where the ti-ouble lies and remedy it. 



The finest quality of hone is necessary for sharpening microtome knives, 

 and it should never be used for any other purpose. The Belgian stones are 

 ■ the best. There are also some good carborundum hones. The razor is 

 gently passed back and forth on the hone with the edge forward, and strop- 

 ping is not necessary or desirable if the hone is of the best quality. Soapy 

 water is one of the best lubricants of the hone. 



Sections are best cut from 7-10 micromillimeters thick for general histo- 

 logical work, but must be cut 5 or less for the finest details of protoplasmic 

 structure. A micromillimeter (also called a micron) is a thousandth of a' 



