18 
THE TANNINS. 
The tannins may be defined as a class of amorphous, 
astringent, vegetable substances, the members of which 
produce a blue or green color and precipitate with salts 
of iron, and with animal membrane form an insoluble 
and imputrescible compound called leather. 
The tannins are widely distributed throughout the 
vegetable kingdom. The richest deposit appears to be 
in galls, of which they comprise from twenty-five to 
seventy-five per cent., according to the variety. The 
next most abundant source is the inner bark of certain 
trees, notably of the oak species; they are also found 
in leaves, unripe fruits, woods, and some buds and 
flowers, but rarely and in small quantity in seeds. 
Many theories have been advanced by different in¬ 
vestigators to explain the uses and method of forma¬ 
tion of the tannins in plants. It has been observed 
that starch is always present with tannin in barks; 
that in many respects the tannins resemble the resins : 
the members of both classes, when heated with potas¬ 
sium hydrate, yield the same or similar products of 
decomposition, and, further, when seeds which are free 
from tannin germinate, it may then be detected. It 
has been found that the amount of tannin in a plant 
varies greatly with the time of year, and in some cases 
changes with the variation in the amount of starch 
present, as shown by Peacock in Heucliera Ameri¬ 
cana. He determined the amount of tannin to be 
greatest in October and least in May, while the starch 
was greatest in March. In Geranium maculatum it has 
© 
been found by the author that the tannin is present in 
largest amount in April, just before the plant blooms, 
and rapidly decreases until it reaches a minimum in 
