400 BOTANICAL GAZETTE [MAY 
USE OF CHLOROIODIDE OF ZINC IN PLANT HISTOLOGY 
Chloroiodide of zinc has fallen more or less into disuse in the botanical 
laboratory, perhaps because of its apparent vicissitudes. Since it is 
unequaled in usefulness in histological work, however, it is advantageous 
to workers who still believe in the practicability of the old-fashioned 
hand-section razor to obtain a working knowledge of this reagent. 
By the use of the “one solution’? mixture (Behrens: 25g ZnCl., 
8g KI, 1.5g I, 8cc water) negative results are often obtained, but when 
properly prepared and kept from deterioration, the solution may be 
used for a number of years. To insure uniform results, the two 
solution mixture, first suggested by Novopokrowsky, is recommended. 
Solution A, iodine potassium iodide 1:1:100; solution B, zinc chloride 
2 parts, water 1 part. Stain in solution A for a few seconds and then 
transfer to solution B. Keep object moving in a drop of this second 
solution until a bright blue color is obtained. To hasten the reaction 
and to intensify the color it sometimes becomes necessary to add a drop 
of solution A subsequent to the treatment with solution B. The 
rapidity of the staining and the intensity of the color obtained often 
depend on the nature of the membrane. Certain tissues will stain only 
after prolonged treatment, but most herbaceous material will react 
very readily. 
Since iodine dissolves in water very slowly, it becomes necessary to 
prepare the reagent some time before it is desired for use. When a 
section of a potato stem is stained by this method, for example, and 
the preparation viewed under the microscope, it is seen that the cellulose 
membranes are a bright blue, lignified, cutinized, and suberized walls a 
yellowish brown. Young phloem fibers and immature xylem cells, of 
course, take the cellulose stain. The staining reaction of the sieve 
tubes of the primary phloem groups is most striking in cross-sections. 
Their intenser stain and the heavier walls stand out conspicuously in 
contrast with the phloem parenchyma cells, which stain like ordinary 
parenchyma of pith or cortex. The pathological anatomist finds the 
chloroiodide of zinc especially useful in the study of necrotic tissues, 
since the double staining obtained by this method permits of a more 
searching inquiry into the nature of the cell wall changes than is possible 
with a one-sided differential stain like the classic phloroglucin-HCl 
reagent. The use of the chloroiodide of zinc stain may occasionally call 
for some patience, but the results obtained warrant and reward it in 
every case.—ERNsT ARTSCHWAGER, Cornell University. 
