178 Parr .— The Response of Piloholus to Light. 
I. Theories of Response. 
A number of theories of response based on the interpretation of data 
obtained by experimental methods have been formulated, and these shall 
be referred to as (1) Intensity difference, (2) Ray direction, (3) Wave¬ 
length, (4) Energy. 
1. De Candolle (1832), the author of the s intensity ’ theory, believed 
that, due to a difference in the light intensity upon the sides of the plant 
turned toward and away from the source of light, there results an increased 
carbon dioxide liberation and also an increased transpiration on the lighted 
side of the organism which brings about an earlier maturation of its cells and 
hardening of its tissues. Among the adherents to this theory, or to a more 
modern modification and interpretation of the same, may be mentioned 
Wiesner (1879), Darwin (1880), Engelmann (1883), Oltmanns (1892), 
Yerkes (1903), Loeb (1906), and Davenport (1907). 
Wiesner attributes the response to the difference in the lighting of the 
two sides coupled with the inhibiting action of light. He argues that if 
the organ were entirely transparent and no light were lost by reflection, 
heliotropic response would be impossible. He offers no experimental proof 
in support of this view because, as he says, of the difficulty in measuring the 
difference in light intensity of the two sides. 
Charles and Francis Darwin (1880) concluded that the difference 
in the intensity of the light on opposite sides of the plant modifies the 
nutation and results in the tropic movement. 
Engelmann (1883) found that a very gradual increase, or decrease, of 
light intensity produced no response in Paramecium bursaria. The same 
difference produced a response if the change took place rapidly. He con¬ 
cluded that response follows a time rate of change of intensity. 
Loeb (1906) believes that because of the more intense light on one side 
of the organism there is produced a difference in chemical constitution of 
the cells on the two sides which results in heliotropic action. 
Oltmanns (1892), by the use of India ink solutions in prismatic wedges, 
attempted to show that the response of the organism is due to a difference 
of intensity rather than to direction of ray. A later paper (1897) maintains 
that the intensity of the stimulus determines the direction, positive or 
negative, of the response of the organism. 
2. Sachs (1876) advanced the theory that the direction and degree of 
curvature is determined by the direction of the ray passing through the 
organism. The stimulus is perceived when the long axis of the organ forms 
an angle with the incident ray. 
Strasburger (1878) added substantial proof to the ray-direction theory. 
By the response of the swarm-spores of Botrydium and Bryopsis in a trough 
behind a prismatic wedge filled with a solution of humic acid variously 
