TEACHERS DEPARTMENT. 
157 
that the geotropic stimulus causes a retardation in the metabolism 
of the plant. 
As is well known, the only evidence we have that a plant has 
received a stimulus, as from gravity or light, is by observing some 
reaction, e. g., curvature of the root or shoot in response to the 
stimulus. In case the stimulus acts and no movement results two 
conclusions only have hitherto been possible. Either (1) the 
stimulus was not received, or (2) the movement, for some cause, 
could not be executed. In cases where the perception of the 
stimulus and the responding movement take place in different 
portions of the plant, as when the root-tip perceives the stimulus 
of gravity and the curvature takes place in the growing (motor) 
zone back of the tip, we have hitherto had no means of detecting 
what goes on in the sensitive zone. The principal explanation 
offered up to the present time as to the means by which the 
plant detects the stimulus is the statolith hypothesis of Nemec, 
which postulates the displacement of starch grains in young cells 
of the root cap as the mechanism of the plant’s perception. 
Czapek has already* determined the presence in the root tips 
of Vicia Faba major of one of the inorganic ferments or enzymes 
known as oxydases, which aid in the oxidation of various sub¬ 
stances. He has also demonstrated that these root tips have the 
power of reducing silver nitrate. 
In the present research, when the action of the oxydase was 
inhibited by chloroform, crystals of tyrosin appeared in the tissues 
of the root, and the oxydase has been identified as tyrosinase. 
Tyrosinase decomposes tyrosin into ammonia, carbon dioxide and 
an acid known as homo gen tisini c acid. It is this acid that causes 
the reduction of silver nitrate when the root tips of Vicia Faba 
major are placed in it. This oxydase is further capable of chang¬ 
ing homogentisinic acid into a substance that will not reduce silver 
nitrate. The complete destruction of the tyrosin by the enzyme 
includes, therefore, the destruction of the homogentisinic acid. 
But when roots are placed horizontally there appears, a short 
* Uber einen Behind an geotropische gereitzen Wurtzeln, Ber. Deutsch. 
bot. Gesell. 15: 516. 1897, und Weitre Beitrage zur Kentniss der geo- 
tropisclien Reizersclieinungen, Jahrbiich. wissenschaftl. Bot. 32 : 207. 1898. 
