APPARATUS AND METHODS. 17 



to penetrate the specimen or reach only its edges, before 

 concluding that the result of irrigation is negative it is 

 advisable to raise the cover-glass and apply a drop of the 

 reagent to the specimen directly. 



15. Clearing Reagents for Temporary Mounts. 



Sometimes it is difficult to see the cell- walls in a section 

 on account of the dense cell-contents ; or it may be desired 

 to make an entire leaf transparent. For any such pur- 

 pose various clearing reagents are used ; the mode of action 

 of such reagents differs in different cases, but the result is 

 to make the specimen more transparent. 



(1) Glycerine is frequently used instead of water for the mount- 

 ing of specimens, partly because it does not evaporate and partly 

 because it makes sections more transparent, hence it is a clearing 

 reagent as well as a mounting medium. 



(2) Caustic Potash causes swelling and partial disorganisation 

 of the cell-contents, and is especially useful with such preparations 

 as sections of growing-points, embryos in situ in ovule or archego- 

 nium, etc. A 5 per cent, solution in water answers for most pur- 

 poses, but for denser tissues a concentrated solution in alcohol may 

 be used. If the solution does not quickly make the tissues trans- 

 parent warm the slide. If the specimen becomes too much swollen, 

 so that even the cell-walls are not seen clearly, check the action of 

 the potash by treating the specimen with 10 per cent, acetic acid. 

 In any case, it is as well to rinse the specimen in water after treat- 

 ment with potash. 



(3) Eau de Javelle (see Appendix) is often preferable to caustic 

 potash. Either mount the specimen in this reagent and put it aside 

 for a few minutes, or warm the slide to hasten the action, then 

 wash with water, followed by acetic acid, and mount in glycerine. 

 Eau de Javelle has much the same action as potash, but it does not 

 cause so much swelling, and the cell-walls are left more distinctly 

 visible. 



(4) Chloral Hydrate (see Appendix) is a useful clearing re- 

 agent for pollen -grains, embryos, fairly thin entire leaves, etc. 

 The specimen may be either left in the solution overnight, in a 

 covered vessel, or may be heated in the solution to hasten the action. 

 See also Chloral Hydrate Iodine in Appendix. 



(5) Carbolic Acid (Phenol) is sometimes used for clearing. 

 It may be used instead of chloral hydrate for such specimens as 

 entire leaves which have been decolorised in alcohol ; the leaves 

 are transferred from the alcohol to either pure carbolic acid or a 

 mixture of three parts turpentine and one part carbolic acid. 

 Pollen-grains, etc. , may be cleared in this way. 



P. B. 2 



