THE FUNCTION OF TANNIN. 
273 
By Hanstein* aniline violet lias been used, colouring 
tannin fox-red. I have similarly used fuclisine, by which 
(after passing the sections through dilute sulphuric acid) 
tannin contents of the cells are coloured purplish red, the 
colour being retained for some months. 
Kraus f has suggested that tannin may he recognised by 
means of its relations with glycerine, under the influence of 
which it forms drops which later disappear. In this, however, 
it is not recognisable from glucose, which reacts in the same 
way. 
The reagent which is used in the hulk of the following 
experiments, and in all those which I considered crucial or 
upon which I have founded any conclusions, is a watery 
solution of potassium bichromate, first recommended by 
Sanio.J When portions of the tissue to be examined are laid 
for from two to seven days in this fluid, the tannin contents 
of the cells form with the reagent a compact moderately dark- 
brown gelatinous mass, either in one lump filling the cell, or 
in several lumps or balls of various sizes ; or, if only a trace 
of tannin be present, a few reddish small granules. This 
compound is insoluble, and sections of the tissue (rejecting 
the external ones) can be cut and preserved as preparations for 
at least two years or so, showing definitely both the position 
and the quantity of the tannin, though not showing to which 
of the tannin groups the particular one belongs. It is quite 
true that certain other cell contents are liable to reduce bi¬ 
chromate of potash, and produce a colour reaction ; the tannin 
reaction is, however, distinguished more even by form and 
appearance than by colour, and no one with experience is 
likely to go astray. 
I have found that osmic acid also is (in one per cent, 
solution) a reagent for tannin. The solution turns black by 
reduction. Provided the cells are unbroken, this colour 
reaction can also be preserved, but unfortunately certain other 
cell contents reduce osmic acid, and discrimination is not 
always easy. 
Distribution.— This will be specially considered hereafter. 
Function.— As to the part which tannin plays in the meta¬ 
bolism of the plant, the opinions of observers have widely 
differed. 
* Hanstein, Bot. Zeitung, 1868. 
f Kraus , “ Ueber das Verlialten des Zuckersaftes der Zellen gegen 
Alcohol und Glycerin, u. s. w.” Stzb. d. Naturforscli. Ges. zu Halle, 
1876 (Bot. Zeit., 1876, p. 644). 
{ Sanio, 1. c. Bot. Zeit., 1863. 
