Hooker.—Hydrotropism in Roots of Lupinus albus. 277 
The average time that elapsed before a reaction was visible was seven 
hours. In case of unfavourable conditions, due to a low stimulus intensity, 
or to slow growth, ten hours elapsed before bending could be detected. 
Roots were exposed to the influence of a moist filter-paper for periods 
varying from one minute to five hours, in the hope of determining a pre¬ 
sentation period such as exists for heliotropism and geotropism. The 
results were negative as no reaction was obtained unless the roots were 
exposed more than five hours. 
5. Localization of sensitivity. The greatest point of discussion in 
regard to hydrotropism has been whether the sensitivity is confined to 
the root-tip or no. Darwin, Molisch, and Pfeffer were of this opinion, 
while Wiesner, Detlefsen, and Jost gave evidence against the view. By 
the root-tip is meant one and one-half to two millimetres of the end of the 
root. Several methods were tried and found unsatisfactory before one 
was used which decided the question. 
Molisch’s experiment (24) was repeated, in which roots wrapped up 
to the tip in wet tissue-paper were exposed to a hydrotropic stimulus. 
Good results were obtained, although the roots reacted somewhat more 
slowly than under optimum conditions. 
A. Then the converse experiment was made, which Pfeffer had 
carried out in his laboratory (25). The root-tips alone were covered and 
the roots then exposed to a psychrometrical difference. Tissue-paper 
and tin-foil were both used, but the only cases in which bending occurred 
seemed due rather to pressure of the cap on the root-tip, for the roots 
bent in all directions. 
B. The method used by Jost (18) was next tried, and 1-5 to 2 mm. 
of the tip were carefully removed with a razor. Of fifty-four roots of 
Ltipinus albus which were decapitated, 20 bent towards the filter-paper, 
11 bent away, and 23 remained straight or bent to one side. The 
greatest care was used to cut the roots squarely, and the experiment 
was not begun until some little efficiency had been attained. To the 
last, nevertheless, a large percentage (20-25 per cent.) of the controls bent 
in reaction to a wound stimulus. The reaction took place sometimes in 
two to three hours, and so had clearly no relation to hydrotropism. The 
results of the experiment were characterized by the greatest irregularity, 
so that this method was also abandoned. 
C. The tips of roots were killed by immersion for two minutes in 
boiling water, but the rate of growth was seriously diminished, so that 
no results could be obtained. 
D. A rectangular glass vessel was half filled with water. Roots 
suspended from two parallel cork strips, resting on the rims of the 
vessel, were immersed for a distance of 1 to 2 mm. from the apex. 
Between the two rows of roots, a paraffin trough was fixed at the water 
