276 Hooker.—Hydrotropism in Roots of Lupinus albus . 
compartment and the average for each position calculated. In every case 
the experiments 
were made with the roots in 
Table III. 
their customary positions. 
Distance 
Average 
Distance 
Average 
from the 
percentage 
from the 
percentage 
filter- 
relative 
filter- 
relative 
paper. 
moisture. 
paper. 
moisture. 
3 cm. 
98*2 
18 cm. 
94*9 
8 cm. 
97 -o 
23 cm. 
9 2 '7 
13 cm. 
96-1 
28 cm. 
90-1 
This shows that the fall in moisture is 
greater at the dry end of 
apparatus. The psychrometric difference would consequently be greater 
there than at the moister end of the compartment, which explains the 
greater intensity of reaction observed. 
The results of numerous experiments similar to those described in 
detail has been summed up in the following table. Only those roots were 
considered that reacted, and, moreover, only those in trays I, II, and III, in 
order that the results might be comparable. The intensity of the stimulus 
was calculated from the differences measured by the hygrometers at 20 0 C. 
and is expressed in percentages of relative moisture per centimetre. The 
bending was positive unless otherwise stated, and the averages were reduced 
to round numbers. 
Intensity of 
Table IV. 
No. of roots 
Average angle 
stimulus. 
observed. 
of bending. 
0*1 
45 
o° 
0*2 
• 32 
o° 
o *3 
86 
30° 
o *4 
60 
6o° 
o -5 
20 
IO° 
o*6 and 
34 
negative 
above 
The simpler apparatus for obtaining hydrotropic bending was rotated 
horizontally about its vertical axis on a clinostat. The action of geotropism 
was thus eliminated, while the hydrotropic stimulus was in no way affected. 
Of twenty roots so rotated, seventeen reacted and with the same intensity as 
the controls. This showed that geotropism was not a factor in determining 
the intensity of hydrotropic bending. 
4. Reaction time. Under optimum conditions, i. e. when the hygro¬ 
scopic difference near the root is equivalent to a fall of 2 per cent, in 5 cm. 
and when the absolute amount of moisture is above 90 per cent., roots 
require six hours to start a hydrotropic reaction. The bending proceeds 
for one to two hours, whereupon a reaction sets in. and the root-tip regains 
a vertical position. Under favourable circumstances the entire reaction, 
from the time when the psychrometric difference is established until the 
root-tip regains its vertical position, may be completed in eight hours. 
