336 Robinson. —Experiments on the Effect of External Stimuli 
this experiment suggested that the direction was influenced by the shadow 
cast from the leaf fragment and not by the substance of the leaf. The 
effect of light on the direction taken by the germ tubes was therefore 
investigated. 
Several pairs of cultures were set up in which sowings of sporidia were 
made on the surface of a drop of gelatine as already described. One from 
each pair was placed on the laboratory bench in such a position that it was 
illuminated from one side by the light from the window. The control 
cultures were placed in the dark. After 16 hours all the sporidia in every 
cell had germinated equally well. Whilst the germ tubes of those in the 
dark were indifferent as to direction, the germ tubes of those which were 
illuminated from one side had all grown away from the light. Fig. 4, 
A B 
Fig. 4. a. Sowing of sporidia showing germ tubes- growing away from the light of the window. 
The arrow indicates the direction of the light, b. Control to the experiment figured in a carried 
out in the dark : the germ tubes are growing in all directions, x 45. 
A and B, represents one pair of these tests. In this figure and in Figs. 
5 and • 6 the position of the spores and the direction of the germ tubes are 
accurately indicated, the figures having been drawn with the aid of the 
camera lucida. 
In order further to test this effect of light on the direction of the germ 
tubes another double series of cultures was set up. One from each pair of 
these was placed on the bench as before in front of the window. The 
controls, instead of being placed in darkness, were rotated horizontally on 
a klinostat which was placed in front of the same window. As before, all 
the germ tubes in the cultures at rest were directed away from the light, 
while in the control cultures they had grown in all directions. The result- 
ing appearance was so similar to that shown in Fig. 4 that illustration is 
unnecessary. 
A still further confirmation of this effect of light was obtained by the 
following experiment. One of the cells was arranged as before, except that 
a minute partition of tin-foil, about 1 mm. high and 1 cm. long, was fixed on 
the surface of the gelatine. The culture was placed before the window with 
