197 
Parr .— The Response of Pilobohis to Light . 
Attention must be called to the fact that the graphs representing the 
time and frequency relation (Fig. 4) of Pilobolus for the two sources of light 
(Nernst and tungsten) are not nearly identical, as would be the case were 
wave-frequencies alone responsible for the differences in the induction 
periods. Furthermore, the two graphs diverge somewhat from the violet N 
towards the red, giving additional evidence that energy is a factor in the 
relative time required for phototropic excitation. The fact that energy 
does play a part in response is further manifest from the results given 
in Table VIII. This table shows that while the square roots of the 
frequencies constitute an ascending series, the square roots of the 
presentation times form a slightly descending series. It should also be 
noted that the constant for the Nernst is considerably larger than that 
for the tungsten. 
Table VIII. 
Nernst. 
Tungsten . 
Colour. 
Wave¬ 
length. 
Frequency. 
v/ 
Presentation 
time in min. 
Vf* t. 
Presentation 
time in min. 
V/xt. 
Red 
±708 
±424 
20*6 
78 
1606 
6S 
1400 
±667 
± 45 ° 
21*2 
76 
1611 
66-5 
1409 
Orange 
±631 
±472 
21-7 
75 
1627 
^ 4'5 
1399 
+ 612 
±494 
22.2 
73 
1620 
64 
1420 
Yellow 
± 5 8 9 
± 5°9 
22-5 
72 
1620 
63 
1417 
±58 5 
± 5 T 2 
22*6 
72 
1627 
63 
1423 
Green 
± 54 ° 
±556 
23-6 
69 
1628 
60 
1416 
±523 
±574 
23*9 
6 7’5 
1603 
59 
1410 
Blue 
±496 
+ 607 
24.6 
65 
L 599 
57 
1402 
± 47 o 
±638 
25'2 
63 
1587 
55 
1386 
Indigo 
±464 
+ 648 
25*4 
62 
1574 
55 
1397 
„ 
±437 
+ 686 
26-3 
60 
1578 
54 
1393 
Violet 
± 4 H 
±738 
27-1 
56 
1517 
51 
1382 
>> 
± 39 8 
±753 
27*4 
55 
1507 
5 ° 
1370 
Since the time of response of Pilobolus to the light of the tungsten and 
Nernst lamps, respectively) is by no means in direct proportion to their 
spectral energy values, and since there is found to exist some relation of the 
energy to the time of response, the question arises as to the possibility of 
expressing this relation in simple mathematical terms. 
Wiesner (1879) has shown that the excitation of the plant increases 
less rapidly than the photic stimulus which produces it. 
Pfeffer (1883), through the study of the reaction of swarm spores and 
bacteria to chemical stimuli, established the application of the Weber- 
Fechner law to plants. This law states that whereas the intensity of the 
stimulus increases in geometrical progression, the intensity of the reaction 
increases in arithmetical progression, or that response varies with the 
logarithm of the stimulus. This is expressed in the formula: 
vSj — S 2 = A log y, where S 1 and S 2 are the sensations, I x and / 2 the 
J 2 
respective intensities of the stimuli, and A is a constant which varies with 
the quality of the stimulus. 
