122 METHODS IN PLANT HISTOLOGY 



In our own collection, there are more than 6,000 paraffin cakes. They 

 are filed in pasteboard boxes 28 cm. long, 10 cm. wide, and 2 cm. deep. 

 With the generic name written on the box, and the boxes arranged al- 

 phabetically or, preferably, taxonomically, it is easy to find anything 



in the large collection. 



CUTTING 



As soon as the paraffin is thoroughly cooled, it is ready for cutting. 

 Trim the paraffin containing the object into a convenient shape, and 

 fasten it upon a block of wood. Blocks of pine f inch long and f inch 

 square are good for general purposes. Put paraffin on the end of the 

 block so as to form a firm cap about | inch thick. Warm the cap and 

 the bottom of the piece containing the object, and press them Hghtly 

 together; then touch the joint with a hot needle, put the whole thing 

 into cold water for a minute, and it is ready for cutting. Cutting can 

 be learned only by experience, but a few hints may not come amiss: 



a) The knife must be sharp. This condition, which used to be the 

 most difficult, has become the easiest; for any paraffin section, up to 

 2 cm. square, can be cut with a safety razor blade. The holder shown 

 in Figure 3, with the shding microtome shown in Figure 2, with a hard 

 safety razor blade, preferably the Watts blade described on page 10, 

 will furnish relief from the tedious sharpening of microtome knives 

 which, at their best, are not equal to a good safety razor blade. The 

 "Gem" or "Star" blade is good for paraffin sections and is unequaled 

 for wood sections, but the back must be broken off to make it fit the 

 holder shown in Figure 3. Fortunately, The American Safety Razor 

 Corporation, Brooklyn, N.Y., will furnish the Gem blade, specially 

 tempered and tested, and with the back removed for microtome use, 

 at 50 cents for a package of 10 blades. 



Some students have trouble with safety razor blades. There must be 

 a good holder. The holders shown in Figures 3 and 4 eliminate any 

 trouble from this factor. The angle must be right. A study of Figure 5 

 should eliminate any trouble from this source. In general, the safety 

 razor blade should project farther beyond the holder, and the angle 

 between the blade and paraffin should be greater for thin sections 

 (1-5 m) than for thicker sections. The blade should project the least, 

 and the angle should be the least, for hard sections and thick sections. 



Find the thickness at which the paraffin and object cut best. When 

 in doubt as to the proper thickness, cut at 10 m- When the room 

 temperature is at zero centigrade, onion root-tips in 52° C. paraffin 



