Hooker.—Hydrotropism in Roots of Lupinus albi/s. 275 
Experiment V. 
No. of 
Distance from 
Average 
Growth 
tray. 
fi Iter-paper. 
growth. 
per hour. 
I 
5 cm. 
18*2 mm. 
0*90 mm. 
II 
10 cm. 
i6‘0 mm. 
0’8o mm. 
III 
15 cm. 
13-3 mm. 
0*67 mm. 
IV 
20 cm. 
io*8 mm. 
0*54 mm. 
The experiment was continued twenty hours. A tray with dilute 
sulphuric acid (10 per cent.) was placed in the dry end of the compartment. 
The hygrometers measured 88 and 93 per cent, respectively. Only one root 
(in III) bent towards the filter-paper. The others remained straight. 
This last experiment showed that if the hygrometers measured 
a difference of only 5 per cent., hydrotropic reaction was very nearly 
eliminated. If this difference was less, no reactions occurred. The hygro¬ 
meters were 25 cm. apart, so it may be said that a fall of at least 0-2 per 
cent, in 1 cm. is necessary to induce hydrotropic reaction. This represents 
approximately the minimum intensity of the stimulus. If the other extreme 
is obtained, the roots are affected by changes in turgidity and bend away 
from the source of moisture as in Experiment IV. The exact point where this 
occurs could not be determined ; the hygrometers were too inaccurate. The 
phenomenon is familiar, however, and was mentioned by Molisch ( 24 ). The 
optimum reaction was found to be obtained when a psychrometric difference 
of 0-4 per cent, for every centimetre was measured. This means a difference 
of but 0-04 per cent, between the opposite sides of a root 1 mm. thick. 
Repeated experiments with the zinc apparatus showed that the roots 
in tray I bent normally but 20° to 30° from the vertical, while those in 
tray V bent from 50° to 6o°. To explain this difference a detailed study 
of the moisture content of the air in the compartment was necessitated. 
When the compartment was empty the fall in moisture content was found 
to be more or less uniform. The presence of the roots, however, had 
a significant influence on this, so that experiments having any bearing on 
the question had to be made with the roots in their accustomed positions 
within the zinc box. Water must evaporate from the surface of a root 
suspended in moist air, unless the air is saturated. Consequently each 
root acts as a source of moisture and tends to increase the moisture content 
of the air about it. The same was observed in the experiments made to 
determine the minimum relative moisture. As a result of this increase, the 
fall in the relative moisture of the moister end of the zinc compartment, 
which contained more of the roots during these experiments, was diminished. 
The data for the following table were collected from ten experiments in 
which the hygrometers at the two ends of the apparatus measured approxi¬ 
mately 98 per cent, and 90 per cent. The relative moisture was then measured 
by hygrometers at intervals of 5 cm. throughout the entire length of the 
T 2 
