77 
Tropistic Movements of Plants. 
Enzymes decomposing tyrosin with formation of homogentisinic acid 
have been frequently prepared in recent times from animal and vegetable 
tissues. Such a tyrosinase Bertrand (4) discovered in Hymenomycetes 
{Russula), in the tubers of Dahlia, in the root of beet. Gonnermann (5) 
showed lately, in the case of the tyrosinase of the beet-root, that the 
substance produced in the fermentative alteration of tyrosin is identical 
with homogentisinic acid. 
The effect of tyrosinase on tyrosin is oxydation accompanied by the 
formation of ammonia and carbon dioxide. The formation of homogentisinic 
acid from tyrosin is expressed by the equation : 
c 9 h u no 3 +o 3 - C 8 H 8 0 4 +NH 3 + C0 2 
Tyrosin Homogentisinic acid 
There remains the question, whether the enzyme called tyrosinase is 
a distinct undivisible substance or a mixture of an oxydizing and desami- 
dizing enzyme. 
An important fact established in my laboratory is the occurrence of an 
oxydizing enzyme in root-tips, which changes homogentisinic acid into 
a substance not able to reduce silver nitrate. The homogentisinic acid, 
therefore, cannot be regarded as a final product of metabolism from the 
tyrosin. 
The result of our experiments shows, therefore, that a short time after 
the beginning of the geotropic induction, there appears a retardation of the 
normal destruction of tyrosin, to be recognized by an accumulation of 
homogentisinic acid. We have in the following report to confirm it more 
exactly and to describe the methods of investigation. 
II. 
The substance reducing AgN0 3 can easily be extracted from the 
ground root-tips by treating them with alcohol of 96 per cent. After 
evaporating the alcoholic solution and dissolving the remainder in water 
a brown solution is obtained, weakly acid to litmus, nearly free from sugar 
and reducing ammoniacal solution of AgN0 3 very strongly. The solution, 
even when weakly acid or neutral, assumes a dark colour on being allowed 
to remain in contact with air. Crystals I could never obtain from such 
extracts. The reactions of those solutions were nearly the same as those 
described by Baumann for solutions of pure homogentisinic acid. Treated 
with alkali the solution changes to a reddish brown or a dark brown 
according to the concentration ; ammoniacal solution of AgN0 3 is imme- 
diately reduced on warming ; cold Fehling’s solution is not reduced, but 
is feebly reduced on boiling. Iron chloride gives a green colour, iron 
vitriol a blue-violet. Millon’s reagent gives a yellow colour ; acetate of 
lead gives a precipitate. To identify homogentisinic acid (which is difficult 
