42 Botanical Microtechnique 



of teaching material, and loi many research tasks, razor blades are 

 economical and entirely satisfactory. 



For cutting very thin sections, uncommonly thick or large sections, 

 or for totigh materials, the heavy microtome knife is indispensable. 

 The greater rigidity of a knife ]:)ermits sectioning of material Avith 

 which a flexible razor would chatter, cutting sections of uneven 

 thickness. 



Ihe sharpening of a microtome knife is a laboriotis process, best 

 learned by obser\ing a demonstration. If a knife is badly nicked or 

 bowed in the center it is best to send it lo the mantifacturer for 

 grinding. A straight edge and a ne^v correct bevel of the cutting edge 

 are thus established. W'ith a properly grotuid knife, occasional strop- 

 ping restores the edge for a considerable time, depending on the 

 hardness of the material being cut. A honing back is a longitudin- 

 ally split metal cylinder that is slipjied over the thick back of the 

 knife for honing or stropping. The diameter of the cylinder deter- 

 mines the angle of the honed wedge. I'he metal of the cylinder is 

 usually softer than the knife and wears away faster by honing. "When 

 the cylinder is appreciably worn, or unevenly worn, a new one should 

 be fitted and a new wedge angle honed on the knife. It may be 

 necessary to hone the knife on a fine gray hone tising soap stids as 

 a lubricant. W'hen the fine hone and strop fail to restore the edge, 

 and it is not possible to have the knife machine-ground, a uvw -wedge 

 and cutting edge can be established as follows. Place the cutting 

 edge on the coarser yellow hone with the knife vertical, and make one 

 light stroke, remoA'ing the cutting edge compkteK. I hen la\ the 

 knife flat on the hone and stroke with long oblicpie stiokes. alternating 

 the two sides, imtil the two sides of the new wedge meet. Examine 

 periodically with a microscope. Hone on the fine gray stone initil 

 the edge consists ol uniform, minute serrations. Stro]) as usual before 

 using. 



Factors Influencing Sectioning 



Successful sectioning in paraffin depends ujkmi a nmnber of inter- 

 acting factors, the most important of Avhidi will i)e discussed briefly. 



QUALITY OF THE PARAFFIN 



The hardness ol I lie parallm should be appiopriate for the char- 

 acter of the tissues, the desired thickness of the sections, and for the 

 room temperature at which the cutting is done. 1 he paraflin shoidd 



