92 



THE ART OF MAKING MICROSCOPE SLIDES 



Wood 



width as the cylinder. The cyUnder of 

 carrot is spht, inserted into the holder of 

 the microtome, and the leaf pushed down 

 into the center of the cyUnder (Fig. 35). 



The holder of the cj'hnder is now tight- 

 ened and the razor used to slice off as 

 much of the leaf and cork as projects 

 above the level of the plate (Fig. 36). The 

 micrometer screw at the bottom of the 

 cylinder is then turned as far as is neces- 

 sary to advance the carrot and leaf the 

 thickness of the required section above the 

 glass plate, and a section shaved off. The 

 razor must not be pushed straight across 

 the material, but must be drawn side- 

 ways, so that the whole length of the razor 

 is used to cut the section. In Fig. 36 the 

 razor is placed in the correct position for 

 the beginning of the cut; but by the time 

 it has passed through the block, the oppo- 

 site end of the razor will be opposite the 

 section. Notice also that the material is 

 being sectioned with its thin edge, not its 

 breadth, against the knife. This is neces- 



sary whether one is cutting sections by 

 hand or by any other means, because the 

 less material cut at the same time, the less 

 is the chance that it will be torn from its 

 support. 



The section on the knife is now brushed 

 off into one of the stender dishes of alco- 

 hol, the micrometer screw advanced the 

 same amount, another section cut, and so 

 on. About twice as many sections should 

 be cut as are ultimately required, for at 

 least half of them will either be damaged 

 or will have one end thicker than the other. 



The sections are therefore examined un- 

 der the low power of a microscope and 

 those which are not considered satisfac- 

 tory are thrown away. 



Stains used for materials of this type 

 are given in Chapter 21 (DS 21.15), to- 

 gether with a specific example. After the 

 sections have been stained as there de- 

 scribed, they should be dehydrated ex- 

 actly as if they were wholemounts and 

 mounted in balsam in the manner de- 

 scribed in Chapter 6. 



Preparation of a Section of Wood 



The last process described is one of the 

 easiest preparations which may be made 

 in microtomy, whereas the present is one 

 of the most difficult. The sectioning of 

 wood belongs properly in this chapter on 

 the preparation of freehand sections, be- 

 cause wood does not need to be embedded. 

 The difficulty of cutting it hes in the fact 

 that it is too hard to be cut by regular 

 methods while being in general too soft to 

 be cut by the method for ground sections 

 given in Chapter 10. Some plant materials, 

 such as the wood of the fignum \'itae, or 

 the ivory nut, may be cut by grinding 

 techniques and make better sections b}' 

 this method than by any other. A few 

 woods, such as white pine cut parallel to 

 the grain, are sufficiently soft to be cut in a 

 hand microtome without any preparation. 

 The present example deals with such 

 woods as maple or oak, which fall between 

 these two extremes. 



The first thing to be done in the prepa- 

 ration of any section of wood is to cut a 

 block, one end of which is of the size of the 

 required section. A section size about ^i 

 inch square is usually adequate to demon- 



strate the structure of the wood, and a 

 number of blocks this size should be pre- 

 pared with due regard to the plane of the 

 section. 



The wood must next be softened. This 

 would be relatively simple if mechanical 

 means alone could be emploj'ed. Unfor- 

 tunately, however, many woods, particu- 

 larly oak and teak, contain silica, which 

 can only be removed by treatment with 

 hydrofluoric acid, and this acid naturally 

 cannot be made to penetrate the wood 

 until all air has been removed. The blocks 

 of wood are therefore boiled in distilled 

 water for about ten minutes. A disk of 

 glass — or of wire gauze — which will just 

 fit inside the beaker, is placed on top of the 

 pieces of wood, and a sufficient weight 

 added on top to cause the blocks to sink 

 to the bottom. The beaker is now trans- 

 ferred to some type of vacuum equipment 

 and exhausted until bubbles are seen to 

 cease leaving the wood. The vacuum is 

 then released, the beaker returned to the 

 flame, and again boiled for ten minutes. 

 This process of alternate boiling and evac- 

 uation is continued until the wood no 



