SECTION CUTTING AND PERMANENT MOUNTS. 109 



The object of killing the protoplasm is, as just stated, to make 

 it more opaque, and, at the same time, to preserve its structure 

 for a long time. The process is termed fixing. The protoplasm 

 is fixed or coagulated. 



After fixing the object must often be hardened for cutting. 

 There are various reagents for fixing and hardening, some of 

 which do both at the same time, while others only fix or kill. 

 (See under the respective reagents for fixing and hardening 

 tissues, page 103.) 



Although there are a number of such reagents, those actually 

 used, especially in botany, are few. Alcohol is used in nearly 

 all cases. Tender objects must never be placed at once in strong 

 alcohol. 



STAINING. It often happens that some objects, even after 

 fixing, are so transparent that their structure cannot easily be 

 made out. The more a transparent body approaches in its 

 refraction of light the medium in which it lies the more difficult 

 it is to be seen. Finally, when the refracting power is the same 

 as that of the medium, the object is invisible. Shells of 

 Diatoms in glycerin are invisible, the refractive indices of both 

 being 1.43. By staining or coloring the transparent parts of an 

 object these become visible and their structure is easily made 

 out. In a heterogeneous section, like that of a plant stem, for 

 example, the chemically different materials in it select, differ- 

 ent stains, so that by a contrast of colors a great deal more is 

 learned than by study of the unstained section. The different 

 stains require different lengths of time for action, which is best 

 learned by actual work with them. Some of them are perma- 

 nent, others only temporary. The stains fall into groups, as 

 aniline, carmine, haematoxylin stains, etc. 



CLEARING. In most cases, even after making very thin sec- 

 tions, these are too opaque and obscure for observation with the 

 higher powers of the microscope and, consequently, must be 

 made more transparent, that is, must be cleared. Some objects 

 are naturally very opaque, as pollen grains, spores; such objects 

 must always be cleared before studying them. It often hap- 

 pens that it is desired to study the cell-walls only of a section, 

 but this is impossible because of the dense mass of cell contents, 

 as starch, protoplasm, oil, resin, milk-sap, etc., which must first 

 be removed by special clearing reagents. (See the various 

 clearing reagents, page 104.) 



MOUNTING. If objects are not intended to be kept for a long 

 time they are mounted in water, alcohol, dilute glycerin or con- 

 centrated glycerin, or some other suitable material. For per- 

 manent mounting they are usually placed in a medium which 

 solidifies after a time, such as balsam, glycerin-gelatin, which 

 are commonly used. The handiest is the second, because of the 

 little preliminary treatment necessary. The object, in what- 



