1171 
The quantity of light-energy which at various temperatures was 
necessary to cause a definite degree of curvature was determined. 
As a standard a curvature of 2 mm. was always taken, that is to 
say, the apex of the coleoptile was bent 2 mm. out of the vertical. 
To begin with, experiments were made at 20°U., since a quantity 
of light energy of 20 M. C. S. (metre-candle seconds) gave a cur- 
vature of 2 m.m. In order to find the quantity necessary for a 
curvature of 2 mm. a few boxes of seedlings were stimulated for a 
varying number of seconds, and it was ascertained after about 1'/, 
hours how many seedling had curved. Boxes in which 50°/, of the 
seedlings showed a curvature of 2 mm. served as a standard. The 
product of duration of stimulus and intensity of light then gave the 
required quantity of luminar energy in M. C. S. 
The experiments were performed at temperatures ranging from 
0° to 40° C. No experiments were made above 40° C.; after one 
hour’s preliminary warming at 40°, so prolonged an illumination 
was necessary and the curvatures which finally occurred, were so indis- 
tinct, that there was no question of determination after more pro- 
longed: warming. At 48° the seedlings died. . 
From 0° to 25° the observations were made at intervals of 5°; above 
25° more frequent determinations were found to be necessary. 
Ai each of the temperatures to be investigated there was first a 
warming of one hour’s duration, afterwards of 2 hours, 4 hours, 6 
hours ete., in order to see whether increased duration, of preliminary 
warming had any effect. 
The results of the experiments are collected in the table given 
below, in which in successive columns is given in M. C. S. the 
luminar energy necessary for a curvature of 2 mm., after 1 hour, 
2 hours’, 4 hours’ warming, etc., corresponding to the temperature 
given in the first column. 
It is clear from the table that the phototropic stimulationprocess is 
dependent on temperature and that at higher temperatures the time- 
factor is of a great influence. 
From 0° to 25° the length of preliminary warming has no influence 
on the quantity of luminar energy. At 27.5° and 30° longer preli- 
minary warming has a favourable influence; that is to say after a 
longer exposure to a higher temperature a smaller quantity of lumi- 
nar energy causes the same curvature as a greater quantity after 
a shorter preliminary warming. The harmful influence of longer 
preliminary warming is first observable at 32.5° and this is the case 
also at 35°, 37° and at higher temperatures, in always increasing 
amount. 
