Hooker.—Hydrotropism in Roots of Lupinus albus . 273 
2. Limits of the reaction. The first point that was investigated was 
within what limits of relative moisture hydrotropic reactions take place in 
roots. The upper limit is evidently saturation. The lower limit was found 
to be determined by the inability of roots to grow in air that is too dry. As 
no results have been observed in the literature which determined the amount 
of water vapour requisite for growth, the following experiments were 
made. A number of Erlenmeyer flasks were fitted with corks, and a hole 
was bored through each cork. Seedlings about 3 cm. long were selected, 
and the roots inserted through the hole. Only one root was placed in 
each flask, as several were found to alter materially the moisture-content of 
the enclosed air and to have a strong reciprocal influence. The vapour 
pressure within the flasks was regulated by sulphuric acid solutions. The 
roots were marked with ink and measured before and after the experiment, 
to determine the amount of growth during the twenty-four hours. The 
percentages of relative moisture were calculated from the vapour pressures. 
Table II. 
Percentage 
Relative 
No. of roots 
Average 
of//.SO, 
Vapour 
percentage of 
experimented 
growth 
in water. 
pressure. 
moisture. 
with. 
per hour. 
0*00 
17-0 
100 
30 
o*75 mm. 
7 *o 
16-5 
95 
35 
0*52 mm. 
14-0 
15-6 
90 
30 
0*22 mm. 
2 1*0 
14-7 
85 
30 
0*15 mm. 
27*0 
I 3‘9 
80 
25 
coo mm. 
In these experiments the cotyledons were left exposed to the laboratory 
air. If these are kept moist by wrapping them in wet cotton, the average 
growth per hour is greater, but the roots within the flasks stop growth and 
dry up if the relative moisture is reduced to 80 per cent. Seedlings grown 
in a compartment with saturated air may lengthen as much as 1*13 mm. 
per hour. 
Reaction to hydrotropic stimuli ceases, however, slightly above 80 per 
cent, relative moisture; evaporation then becomes so great and growth so 
slow that loss of turgidity is apt to come into play and to cover the effects 
of any hydrotropic stimuli, if such are perceived. Roots subjected to 
a psychrometrical difference at such percentages often bend away from the 
source of moisture. This is not to be confused with negative hydrotropism, 
which is not known to exist in roots ( 18 ). 
3. Intensity of the reaction. The effects of varying the intensity of 
the hydrotropic stimulus were next investigated. The zinc compartment 
and the hygrometers made from Stipa pennata grains were used to make 
the measurements. A noticeable difference in the rate of growth of roots 
situated at various distances from the filter-paper became evident before 
hydrotropic bending was observed. Each tray held five roots. 
T 
