112 



Methods in Plant Histology 



b) Keep the microtome well oiled and clean. 



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

 A ribbon of sections like that shown in Fig. 22, 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 



B 



FIG. 22. The ribbon. 



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 

 whole 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 Fig. 23, A. By inclining the 

 knife as in Fig. 23, B, this difficulty 

 is often obviated. In using the safety- 

 razor blade in a handle, it must be remembered that the blade is 

 sharpened from both sides; the angle must be sufficient to let the 

 paraffin block clear. A split 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 split 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 



A B 



FIG. 23. Position of the knife. 



