452 THE ANATOMY OF WOODY PLANTS 



degree of rigidity are most useful for the sectioning of hard mate- 

 rials of vegetable origin. Microtomes in general which provide 

 for the raising of the object by a vertical screw and elaborate 

 ratchet devices should be avoided, as they are very difficult to 

 keep in order when the sections are cut wet in alcohol, as is invari- 

 ably the case with the materials under consideration. A slicing 

 instrument which has proved itself to be of the greatest value for 

 the purposes here enumerated is the Thoma microtome, manufac- 

 tured by the Jung firm of Heidelberg. The so-called Naples 

 model is the best size and type and has the advantage of being 

 immune from rust by reason of its phosphor-bronze construction. 

 In this microtome the object is raised by sliding up an inclined 

 plane through the action of a micrometer screw which is accurately 

 gauged by means of an adjustable clicking ratchet device. The 

 horizontal position of this mechanism and its manipulation by 

 hand are great advantages, since by reason of these features it 

 escapes the corrosion which rapidly impairs vertical screws for 

 regulating the thickness of the sections. For cutting small objects 

 the carrier provided with the instrument which can be orientated 

 in two planes is suitable. The object-carrier is provided with 

 small cylinders which are primarily intended for use with paraffin. 

 The hollow of the cylinder, instead of being filled with paraffin as 

 in its ordinary mode of employment, is blocked by a well-seasoned 

 and accurately rounded piece of wood. This should be dicoty- 

 ledonous and rather coarse-grained. The smooth upper surface of 

 the block is varnished thoroughly (it is generally best to dip the 

 end of the block into the solution) with a 4 per cent solution of 

 nitrocellulose. If the blocks are being used for the first time, they 

 should receive a second coat of celloidin (or collodion) after the 

 first has had time to dry. The objects imbedded in celloidin (or 

 collodion) are now removed and examined with a lens to ascertain 

 the plane in which they are to be sectioned. When decision is 

 reached on this point, the surface of the hardened nitrocellulose 

 is sliced with a sharp knife in a plane parallel to that of the desired 

 sections. The smoothed surface should be perfectly flat. After 

 wiping, this surface, which should never actually expose the object, 

 is clipped into a 4 or 6 per cent solution of nitrocellulose and then 



