BROWN: WooDs FOR MICROSCOPIC STUDY 139 
structure of woody tissues. This is true both for the relatively 
thick sections cut from unimbedded blocks and for the very thin 
sections obtained by the celloidin method. In these latter sec- 
tions, for example, it is possible with this stain to bring out such 
structures as the reticulated thickenings of pit membranes as well 
as similar features not visible by other means. 
Before staining, sections cut from the unimbedded blocks or 
imbedded sections on slides are washed in three or more changes 
of water to remove glycerine. The sections are then covered with 
a 2.5 per cent aqueous solution of iron-alum (ferric ammonium 
sulphate) for five to twelve hours. Next, they are washed in run- 
ning water for five minutes (or two minutes in case of slide 
material), stained with 0.5 per cent aqueous solution of haema- 
toxylin (Chamberlain, ’15, p. 41) for twelve to twenty-four hours, 
washed in water for three minutes, and then again treated with 
the iron-alum solution. The sections are left in the iron-alum 
until they become light gray, but, for best results, the material 
should be watched under the microscope, so that the process may 
be stopped the moment the clearest definition is reached. Sections 
are then washed in 1unning water for three hours or longer to 
iemove all trace of iron-alum. In many cases, this washing also 
serves to remove precipitates and other clouding matter from 
the tissues so that the clearness and transparency of the section 
is greatly improved. It is often well to extend the period of 
washing to twenty-four hours or longer. For this purpose the 
siphon apparatus shown in Fic. 3 is useful, since it allows a thor- 
ough percolation of water through the sections for any length of 
time without danger of mechanical injury to the tissues or of loss 
of material. The sections rise flat to the top directly in the out- 
flowing tap water which, from a public supply system, is usually 
sufficiently alkaline to give a clear blue color to the haematoxylin, 
hence being preferable to distilled water for this purpose. 
For best results, woody tissues stained as above should be 
counterstained with safranin. This stain is made up by combining 
equal portions of a 1 per cent solution of alcohol-soluble safranin 
in 95 per cent alcohol and a 1 per cent aqueous solution of water- 
soluble safranin. For extremely thin membranes, where a very 
weak counterstain is desirable, one to three minutes in the safranin 
