52 METHODS IN PLANT HISTOLOGY 



balsam and a cover. Since the xylol is very volatile, this last step must 

 be taken quickly. If blackish spots appear they are usually caused by 

 the drying of sections before the balsam and cover are added; if there 

 are whitish spots or an emulsion-like appearance, the clearing is not 

 thorough; this may be caused by poor xylol (or other clearing agent); 

 by absolute alcohol which is considerably weaker than its name implies 

 (the absolute alcohol must test at least as high as 99 per cent, and 

 ought to test as high as 99.5 per cent, if xylol is to be used for clearing) ; 

 or by passing too quickly through the absolute alcohol and xylol, or 

 even by moisture on the cover glass. The last danger is easily avoided 

 by passing the cover quickly through a Bunsen or alcohol flame before 

 laying it on the balsam. 



Delafield's haematoxylin is the most generally useful stain in the 

 haematoxylin group. It brings out cellulose walls very sharply, and 

 consequently is a good stain for embryos and the fundamental tissue 

 system in general. With safranin it forms a good combination for the 

 vascular system, the safranin giving the lignified elements a bright red 

 color, while the haematoxylin stains the cellulose a rich purple. It is a 

 good stain for chromatin, and the achromatic structures show up fairly 

 well, but can be brought out much better by special methods. Arche- 

 sporial cells and sporogenous tissue are very well defined if proper care 

 be taken. Lignified and suberized walls and also starch and chromato- 

 phores stain lightly or not at all. Whenever you are in doubt as to the 

 selection of a stain for general purposes, we should advise the use of 

 Delafield's haematoxylin. 



Mayer's haem-alum.— Haematoxylin, 1 g., dissolved with gentle 

 heat in 50 c.c. of 95 per cent alcohol and added to a solution of 50 g. of 

 alum in a hter of distilled water. Allow the mixture to cool and settle; 

 filter; add a crystal of thymol to preserve from mold (Lee). 



It is ready for use as soon as made up. Unless attacked by mold, 

 it keeps indefinitely. Transfer to the stain from water. It is seldom 

 necessary to stain for more than 10 minutes, and 4 or 5 minutes is 

 generally long enough. As a rule, better results are secured by diluting 

 the stain (about 1 c.c.-lO c.c. of distilled water) and allowing it to act 

 for 10 hours or overnight. 



This is a good stain for the nuclei of filamentous algae and fungi, 

 since it has little or no effect upon cell walls or plastids. Wash thor- 

 oughly in water, transfer to 10 per cent glycerin, and follow the Vene- 

 tian turpentine method, as described in chapter viii. 



