Tropistic Movements of Plants, 
79 
The filter-paper should be as small as possible, and the fluid should drain 
off thoroughly. Now ammonia and 0.3 cc. of silver solution are again 
added to the filtrate. If any reduction occurs, CaCl 2 and (NH 4 ) 2 C 0 3 are 
again added and the solution filtered, and this process may be repeated 
three or four times. If at last no distinct precipitation of silver occurs, 
it is necessary to see whether a precipitate of silver chloride appears on 
adding HC 1 so as to give a distinct acid reaction. If a white cloud appears 
there must have been a slight excess of silver solution. In this case 10 cc. 
of the original filtrate are measured off and treated as before, except that 
the silver solution is diminished by o*i to 0-2 cc. If no excess of AgN 0 3 
can now be detected, the right value must lie between the two results of 
titration. Since the limits of error cannot be more in this method than 
+ 0*2 cc., the homogentisinic acid can be estimated within + 1 m g- The 
colour and the quantity of the precipitate in the reduction tests of the ex- 
tracts prepared from root-tips can be exactly distinguished even if the tests 
differ in concentration so little that the difference corresponds to 0*3 cc. 
silver solution; usually the limit of error is found to be about +0-2 cc. ~ 
silver nitrate. 
In such experiments 5-0 cc. of ^ silver nitrate are usually required for 
xoo root-tips. Of course by far the greater part of the silver solution 
consumed is not reduced by the homogentisinic acid, but by precipitation 
of proteids and other substances. Therefore the coefficient of conversion 
calculated by Baumann (4*1 mg. homogent. acid — 1 cc. ~ AgN 0 3 ) 
cannot be applied here. It is, therefore, necessary to determine by weighing 
the share in the entire silver consumption taken by homogentisinic acid 
in the before mentioned experiments. First of all it had to be made out 
whether the whole of the reducing substance could be obtained in solution 
by alcoholic extraction of the dried root-tips. A hundred root-tips were 
dried in Hofmeister’s weighing-glasses and weighed, then powdered with the 
glasses in a mortar, extracted with absolute alcohol, then the remainder 
after drying and weighing was extracted with distilled water. The alcoholic 
and the watery extract were titrated with ^ AgN 0 3 . Two such tests, 
together with a control experiment without root-tips, gave the following 
results : — 
I. 
II. 
Control. 
1-3634 g- 
1-5843 g- 
0-0209 g. 
16240 g. 1-0315 g. 
1-6421 g. 1-0315 g. 
0-0181 g. — 
The weight of the glass . . 
The bowls with 100 root- tips . 
The weight of the root-tips . 
