108 VEGETABLE HISTOLOGY. 



(2 to 12 hours), and then in melted paraffin (not warmer than 

 55 C.) on a water-bath for 15 to 30 minutes (too high a tem- 

 perature or two long a bath causes excessive shrinkage) . Some 

 of the paraffin is then poured' into a small paper box or into ad- 

 justable metal frames. The object is transferred to it, and 

 after the mass has begun to set it is placed in cold water until 

 quite hard. It is then cemented (by paraffin) to a square piece 

 of cork and placed in the microtome. In cutting the knife is 

 kept dry. The sections should be fixed on the slide by the col- 

 lodion method. (Collodion mixture consists of 1 part of ether- 

 collodion and 3 parts of oil of cloves. In mounting sections a 

 slide is smeared with the mixture by means of a camel's hair 

 brush, the sections laid on and the slide placed on a water-bath 

 for a few minutes to evaporate the oil of cloves. The slide is 

 then placed in turpentine (to dissolve the paraffin), then 

 drained, after which a drop of balsam is placed on the section 

 and a cover glass put on.) 



CELLOIDIN METHOD. After dehydrating the object thoroughly 

 in alcohol, soak it 24 hours in a mixture of equal parts of alcohol 

 and ether. Make a thick solution of celloidin in the same mix- 

 ture and soak the object for some hours in it. It may then be 

 imbedded as follows : Dip the smaller end of a tapering cork 

 in the celloidin solution, allow it to dry for a moment (blowing 

 on it if necessary), and then build upon it a mass of celloidin, 

 allowing it to dry a moment after each addition. Transfer the 

 object to the cork and cover it thoroughly with the celloidin. 

 Then float the cork in 82 to 85 per cent. (0.842 specific gravity) 

 alcohol until the mass has a firm consistency (24 hours). It 

 may then be cut in the microtome, the knife being kept very wet 

 with alcohol of above strength. Keep the sections in 85 per cent, 

 alcohol until ready to mount them, then soak them for a minute 

 in strong alcohol, transfer to a slide, pour on chloroform until 

 the alcohol is removed, drain off the liquid, quickly add a drop of 

 balsam and cover. 



OPERATIONS INVOLVED IN MAKING A PERMANENT MOUNT OF A 



SECTION. 



FIXING AND HARDENING. In many cases plant tissues are sec- 

 tioned and examined in the fresh state. But often the contents 

 of cells in the living state are too transparent to be seen dis- 

 tinctly and must be killed in order to make them more opaque 

 and easily seen under the microscope. Again, portions of 

 plants may be too soft to be cut without first being put through 

 a hardening process. In all cases where it is desired to make 

 permanent mounts or slides of tissues the protoplasm is first 

 killed and then hardened, unless the tissue consists of cells 

 already dead, for example, wood cells, stone cells of nuts, etc. 



