20 
THE TANNINS. 
change; there is rather an increase in the amount of 
tannin in the rhizome through a production in the dark. 
Kraus inclined to the belief that the use of tannin in 
leaves is either to protect them from being eaten or to 
prevent rotting, etc. Fallen leaves contain as much 
tannin as they did during their best time of growth, 
indicating that the leaf tannin is of no value to the 
plant. During the germination in the dark of seeds 
containing tannin, as oak and horse-chestnut, there is 
no diminution, but an increase, in the amount of tannin. 
There is not yet sufficient evidence to show whether 
tannin is produced from non-nitrogenous substances, or 
whether it is formed in the conversion of nitrogenous 
compounds into albuminoids. Finally, Kraus con¬ 
sidered it probable that aromatic compounds may be 
formed in the production of albumen, some of which 
are used in the building up of albumen while others 
yield tannin. His investigations and conclusions rested 
on slight differences in the estimation of tannin, and 
Reinitzer claimed that the methods employed for the 
estimation of tannin are not sufficiently exact for phys¬ 
iological investigation. This is no doubt true of esti¬ 
mations made by different chemists, but one person 
should be able with the present methods to get results 
which are relatively accurate. 
It was early suggested to divide the tannins into two 
classes,—the iron-bluing tannins, or those which give 
blue precipitates, and the iron-greening tannins, or those 
which give green precipitates with salts of iron. It 
lias been found, however, that the same tannin may 
produce both blue and green precipitates with iron salts. 
Free acid, especially tartaric, causes the green color, 
