MAKING SECTIONS 53 



describe the thickness of a section is a "micron" which is one thousandth of a 

 millimeter, but hand sections are rarely cut less than 10 microns thick and 

 are usually better at two or three times this. 



Methods of Holding Material. The material, although it may be suitable 

 for cutting, is rarely of a size and shape which may be gripped in the holder 

 of the hand microtome without additional support. Therefore, it must be held 

 in some substance which itself will cut readily and which may be shaped easily 

 to support the material to be cut. It is perfectly possible to embed the material 

 in wax before cutting a hand section, but if one is to go to this amount of 

 trouble it is usually better to employ a complex microtome of the type described 

 on page 61. Vegetable tissues are generally used to support objects for hand 

 sectioning. The two best known are "elder pith" and carrots. Elder pith has 

 the advantage that it may be stored indefinitely and cuts with a clean crisp 

 action. Unfortunately, the pith of the American elder (Sambucus canadensis) 

 does not appear to be as suitable for the purpose as the pith of the European 

 elder (S. nigra). This difference between the two species may account for the 

 disfavor in which elder pith is held in the United States, for in the author's 

 experience it is far more convenient than the carrot. The disadvantage of the 

 carrot is that it must be absolutely fresh and, even if it is kept in water overnight, 

 it loses much of that crispness which is necessary for the production of a 

 good section. 



Almost all hand sections are cut from botanical material, the majority of 

 them from leaves or stems. To support a leaf, merely cut a cylinder of the 

 right diameter to fit in the microtome from either elder pith or carrot, split 

 it down the middle, insert the leaf (Fig. 18), and then tighten the holder. 

 Stems, however, cannot be held by this means, so that one must obtain a hol- 

 low cylinder, having an outer diameter convenient to the microtome being 

 employed and an inner diameter slightly less than that of the stem to be gripped. 

 This hollow cylinder is split, the stem inserted, and the section cut (Fig. 19). 

 Of course, a few substances, such as cork or stiff plant stems, may be cut with- 

 out any other support; these are, however, in the minority. 



Hardening and Fixing Materials for Cutting. Many objects which are in 

 themselves unsuitable for sectioning by hand may be rendered more suitable 

 if they are fixed and hardened in some chemical reagent. If, however, one is to 

 go to the trouble of hardening and fixing material in a formula designed to 

 preserve the structure of the cells, it is usually worth while going to the 

 additional trouble of embedding the material and cutting paraffin sections. It 

 is generally sufficient for material, which is to be hand sectioned, to be pre- 

 served in 90 per cent alcohol, a process which is equally applicable to the stems 



