124 



METHODS IN PLANT HISTOLOGY 



h) Keep the microtome well cleaned and oiled. Xylol is good for 

 cleaning a microtome and the oil used for sewing machines is thin and 

 efficient. Three-in-one oil is all right if the microtome is in constant 

 use, but not so good if the microtome is to remain idle through a long 

 vacation. 



c) Trim the block so that each section shall be a perfect rectangle. 





B 



Fig. 23.— Ribbons 



A ribbon of sections hke that shown in Figure 23 A is much better 

 than one like B of the same figure, because sections will usually come 

 off in neater ribbons if the knife strikes the longer edge of the rec- 

 tangle, so that the sections are united by the longer sides rather than 

 by the shorter. Crooked ribbons are caused by wedge-shaped sections, 



and are always to be avoided, because 

 they make it difficult to economize 

 space, and also because they present 

 such a disorderly appearance. The knife, 

 which should be placed at a right angle 

 to the block and not obliquely, should 

 strike the wJiole edge of the block at once, 

 and should leave in the same manner. 



If sections stick to the knife, it may 

 be that the knife is too nearly parallel 

 with the surface of the block, as in 

 Figure 24 A. By inclining the knife 

 as in Figure 245, this difficulty is often obviated. A spUt or scratch 

 in the paraffin ribbon may be caused by a nick in the knife. Use 

 some more favorable position of the edge, or sharpen the whole knife. 

 A spHt or a scratch in the ribbon is often caused by some hard 

 granule which becomes fastened to the inner side of the edge of the 

 knife. This is the most common cause of the difficulty. Simply wipe 



B 



Fig. 24. — Position of the knife 



