92 Bofanical Microtechnique 



Sectioning Unembedded Tissues With the Microtome 



Rigid materials or large objects from ^vhich it is difficult to obtain 

 complete sections freehand should be cut with a microtome. Any 

 sliding microtome, from the inexpensive table microtome to the most 

 elaborate precision microtome, may be used. A fresli twig of white 

 pine, basswood, or cottonwood makes an excellent sufjject. Clamp a 

 3-cm. length of twig into the microtome with 1 cm. projecting above 

 the clamp. Set the knife so that it makes a long slanting cut, and make 

 sure that there will be amjjle clearance between the tissue clamp and 

 the knife carriage. Keep the twig and the knife flooded ^\■ith water 

 while cutting. Remove the slices from the knife, and float them in a 

 watch glass of water. Examine the floating sections with a binocular 

 or a hand lens, discard imperfect sections, and continue sectioning 

 and selecting until enough satisfactory sections have been cut. The 

 ratio of perfect sections obtainable by this method is much less than 

 is possible by the celloidin method. Nevertheless, the cjualit) and 

 output by sectioning fresh material are an agreeable surprise to 

 workers who give it a fair trial. 



The above method can be used witli unembedded tissues that 

 have been killed in 70<';{ alcohol and stored in that fluid, or witli 

 materials that have been killed in an\ fixing fluid and dehydrated to 

 70% alcohol. This degree of dehydration is usually necessary to make 

 the tissues sufficiently firm. However, woody tissues may be killed in 

 FAA, rinsed in several changes of SOO^ alcohol to eliminate nuidi of 

 the acid, and sectioned as above. When cutting tissues from a wet 

 preservative or when cutting partly dehydrated tissues, keep the 

 tissues and the knife flooded with water or with ilifute alcohol of 

 approximately the same water concentration as the jjrcser\ati\e. Float 

 the sections in a dish of the fluid in which they were cut. 



Sections of living tissues ficciuently do not fia\e satisfactoiy 

 staining reactions, especially witli tfie stains used tor j)ii maiuiil sfides. 

 Furthermore, these staining processes usually induce severe plasmolysis 

 and alteration of the j)rot()pfasin. if good preser\aii()n of tfic jjroto- 

 plasm is dcsiicd, transfer sections cut from fixing materiaf into a 

 killing lluicf. in \vhich the protoj>fasm is fixed aii(f hardened. For 

 sections of woody twigs oi firm lu'rl)aceous stems, I'AA is recom- 

 mended. Foi critical studies try Craf II or III (C^hap. 3) or an 

 experimentally determined modification. Ihe c|uality of preservation 

 produced by a fornuda can be ascertained b) examining sections in a 

 drop of the fluid. Finn sections are killed and hardened almost 



