108 SPECIFIC EXAMPLES OF SLIDE-MAKING 



indefinite period. The differentiating solution, which is 1 per cent acetic acid 

 in absolute alcohol containing 0.1 per cent iodine, is also stable. The staining 

 solution should be placed in a small corked vial or stoppered bottle, and the sec- 

 tions transferred to it from the water. They should remain in stain for about 

 24 hours. It is recommended that they should not be left longer than 36 hours 

 because they may suffer from a precipitate over the surface. For this reason it 

 is desirable to accumulate as many sections as possible before starting the proc- 

 ess. When the staining period is concluded, the contents of the vial should 

 be tipped out into a large watch glass. It will usually be found that some of 

 the sections remain behind, stuck to the side of the vial from which they are 

 removed. Under no circumstances should the vial be rinsed with anything ex- 

 cept the staining solution, which should be poured back from the watch glass 

 (the sections will have settled to the bottom), swirled around, and returned 

 to the watch glass. In this way in a short time all the sections may be removed 

 from the vial to the watch glass. In a second watch glass or even a small crystal- 

 lizing dish is placed an adequate quantity of the differentiating solution. Each 

 section is removed individually with a section lifter from the stain and placed 

 into the differentiating solution, where it may be watched under the low power 

 of a microscope as the dish is rocked gently from side to side. Differentiation 

 will usually take place within two or three minutes and may be determined 

 without the least difficulty when the lignified tissues are found to be of a bright 

 clear green, leaving a bright red in the nonlignified tissues. This process of dif- 

 ferentiation is also one of dehydration, so that the sections may now be re- 

 moved with a section lifter from the differentiating solution and placed in a 

 clearing agent. The author's preference is for terpineol which has all the ad- 

 vantages of clove oil without the disadvantage of making the sections brittle 

 so that they crack on mounting. All the sections may be passed through the 

 differentiating solution and accumulated in terpineol. They may remain in ter- 

 pineol until removed to a slide, where they are covered with balsam and 

 mounted in the normal manner. 



Sections prepared in this manner are permanent, and the process is so simple 

 that it can be recommended most warmly as an introduction to plant section- 

 staining techniques for an elementary class. The sections, however, are differ- 

 entiated clearly enough to be used for instructing a class at any level, and 

 generally they will be found much better for this purpose than the complex 

 quadruple-stained sections in which the cytological detail all too often tends 

 to obscure the clarity of morphological detail, the chief requirement in this 

 type of teaching. 



