Tropistic Movements of Plants, 81 
Not stimulated. Stimulated. Control. 
Titration of I 0-75 cc. 0*7500. i*oo cc. i-oo cc. ^ AgNO s 
Titration of II 0-75 cc. 0*75 cc. 0-75 cc. 0-75 cc. „ 
The increasing of the amount of AgN 0 3 in the titrations after geotropic 
stimulation therefore depends only on the portion soluble in alcohol, and 
the substances in the root-tips which are not soluble in alcohol do not 
participate in augmenting the reducing power. 
Therefore the view is a very probable one, that silver-reducing sub- 
stances other than homogentisinic acid take no share in the alterations of 
metabolism caused by geotropism. The amorphous yellow residue of the 
alcohol extract certainly cannot be regarded as pure homogentisinic acid, 
and may probably contain other distinct silver-reducing substances. But 
so far as reliance can be placed on calculations from the small amounts 
available, the reducing power of the residue from the alcoholic extract does 
not differ much from the results obtained by Baumann for pure specimens 
of homogentisinic acid. 
If the amounts obtained from the control experiments are subtracted 
from the average amounts for the portion of alcoholic extract soluble in 
water, the average weight of this portion can be calculated as 3-87 mg. for 
100 root-tips ; the average value of titration with - 5 - AgN 0 3 as 0*875 cc. 
Therefore 1 cc. silver solution can be taken as equivalent to 4-478 mg. 
‘ homogentisinic acid.’ Baumann gives 4-1 to 4-2 mg. for pure homogen- 
tisinic acid. 
Homogentisinic acid was also prepared from the upper parts of 
seedling roots by converting the tyrosin contained in them by means of 
the root enzyme (tyrosinase) by autolysis in chloroform water, and by 
extracting the product with alcohol. Ten cc. of the alcoholic extract, 
being a solution of crude homogentisinic acid, reduced 6-3 cc. of AgN 0 3 
and gave 26*4 mg. dried residue ; 20 cc. of the same solution reduced 
12-4 cc. silver nitrate and gave 51*1 mg. residue. It follows from this 
result that 1 cc. of the silver solution is equal to 4*1 mg. dried residue, i. e. 
crude homogentisinic acid. 
The absolute amount of homogentisinic acid contained in root-tips can 
also be calculated from the results before mentioned. Since the average 
weight of dry substance of 100 root-tips of Lupinus albus according to ten 
estimations can be taken as 22-7 mg., the 3-87 mg. homogentisinic acid 
contained in the dry substance equal 17 percentage of the dry substance. 
After a geotropic stimulation this amount is increased by one-quarter and 
rises above 20 percentage of the dry substance. 
By means of the method described the increase of silver-reducing 
G 
