179 
Parr .— The Response of Pilobolus to Light . 
placed with reference to the light source, he concluded that the directive 
stimulus is due to the direction of the impinging ray rather than to light 
intensity. Davenport and Canon (1897) repeated the experiments of 
Strasburger, using a wedge-shaped container filled with India ink solution, 
and found that the direction taken by Daphnia was in the path of the 
light rays. 
7^3. The relation between refrangibility and response was first attacked 
by Payer (1843), who used coloured glass screens spectroscopically tested. 
Pie found that cress seedlings behave in red, orange, yellow, and green as in 
total darkness, but respond positively in blue and violet, the blue being the 
more active. 
Dutrochet (1844), using similar screens, found that cress seedlings failed 
to respond, but that other seedlings curve towards the red rays. His further 
experiments lead to the conclusion that response is not due to refrangibility, 
but to the 4 brightness 5 of the light used. 
The Italian botanist, Zantedeschi (1842), showed that Oxalis multifloris 
responds to blue, violet, and green, but not to yellow, orange, and red rays. 
D. P. Gardner (1844) studied the effect of the various regions of 
the sun’s spectrum, and concluded that rays of all refrangibility are capable 
. of causing heliotropic response, but that the indigo rays had this property 
to the highest degree. He decided that the intensity of light had only 
a subordinate influence, since by increasing the intensity the tropic response 
increased only slightly. 
Guillemin (1858) exposed seedlings of cress and of mustard to the 
spectral regions obtained by passing the sun’s rays through prisms of flint 
glass, of rock salt, and of quartz. His records show that heliotropic curva¬ 
ture is produced by the invisible chemical and heat rays, as well as by every 
region of the visible spectrum, as had previously been stated by Dutrochet 
and Puillet. He further found that the seedlings showed two maxima 
of response—one in the region between the violet and ultra-violet and 
the other between the infra-red and green. The positions of these maxima, 
however, shifted with a change of prisms, or with the position of the sun in the 
heavens, or with the water vapour in the air. The lower the position of the 
sun and the greater the amount of water vapour present in the air, the more 
the second maximum advanced into the visible regions. The more ready 
response in the violet which Dutrochet obtained, he explained as due to the 
absorption of the ultra-violet by the lenses used before the prism. 
Sachs (1864), using coloured solutions, found heliotropic response only 
in the blue end of the spectrum. He made no attempt to secure pure 
colours or to measure the intensity of the light emitted. 
Wiesner (1879), by the use of solutions, determined that seedlings 
of Vicia curved in all regions of the visible spectrum excepting in the 
yellow, which he found to exert a retarding action upon the effect of orange 
