CHAPTER VII. 

 MICROSCOPIC TECHNIQUE ANp REAGENTS. 



MAKING OF SECTIONS.- In order to examine objects by means 

 of the compound microscope they must be relatively thin and 

 transparent ; furthermore, they must be mounted in water or other 

 mounting fluids. In material consisting of single cells, or, at 

 most, a layer of a few cells, the specimen may be mounted directly 

 in water. This manner of mounting may also be used in the exam- 

 ination of pollen grains, hairs, and thin organs, as petals. Usually 

 in the examination of the latter some clearing agent, as solution of 

 hydrated chloral, is necessary in order to make the specimen trans- 

 parent. As most objects consist of a large number of cells, it is 

 necessary to examine small portions of them, and these are termed 

 sections. They are made with a razor and correspond to the shav- 

 ings made by a carpenter's plane. As each object has three dimen- 

 sions, it is necessary that three different kinds of sections be made. 



1 i ) A transverse or cross section is one made horizontally through 

 the object, therefore its plane lies at right angles to the long axis. 



(2) A radial-longitudinal section is one made at right angles to the 

 cross section and it lies in the plane of the radius, so that in a 

 dicotyledonous stern the section would be made parallel with the 

 medullary rays. (3) A tangential-longitudinal section differs 

 from the preceding in that it lies parallel to the outer surface of 

 the object, or in a plane tangent to the cylinder. These several 

 forms of sections are readily understood from the adjoining illus- 

 tration (Fig. 414). 



Sections of roots, stems, barks, and many fruits and seeds 

 can be made directly without embedding the material, and while 

 sections can be made holding the material in the hand, between 

 the thumb and three fingers, the hand microtome for holding 

 the material may be used by those who are less experienced. In 

 the sectioning of leaves and other material that is not firm, and 

 fruits and seeds which are too small to hold in the hand, the 

 material should be embedded in some substance which will hold 

 it and give it stability. When the tissues are not too hard the 



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