Parr.—The Response of Pilobolus to Light . i8r 
that the maximum shifted in repeated experiments, and he concluded that 
the ‘ mechanical intensity ’ varied for one and the same colour. He believed 
that the blue-violet rays, because of their small intensity, were absorbed in 
the cells of the lighted side, hindering growth on that side. The longer 
waves, because of their greater energy, penetrated the tissue more deeply, 
having equal effect on the two sides, and no curvature resulted. He did not 
give further experimental proof of this theory. 
Haberlandt (1902) developed a very interesting theory of response. 
He considered the epidermal cells as small lenses which focus the light rays 
upon the sensitive protoplasmic membranes of the underlying cells. Max¬ 
well and Lebedev showed that light exerts a pressure approximately 
equivalent to 0-4 mg. to the square metre of absorbing surface, and Haber¬ 
landt suggests that the response is due to the pressure exerted by the light 
in a way somewhat similar to the response of a tendril when stimulated by a 
pressure equal to a weight of 0*0002 mg. 
Wager (1909) modified Haberlandt’s ‘ ocelli ’ theory in various ways. 
He believed that the effect of light is due to the absorbed rays in the 
colouring matter of the cells and not to a mechanical action upon the 
protoplasmic membranes. He found that the response/depends not upon 
the intensity of the light, but upon its quality, the more refrangible rays 
being the more active. 
Radi (1903) had previously proposed a theory of photic response 
in animals somewhat similar to that of Haberlandt in plants. He believed 
that orientation was a direct reaction to light pressure, which, as he says, 
may resemble the pressure of an air-current. 
Nathansohn and Pringsheim (1907) applied to reaction in plants the 
Talbot law which states that a definite quantity of light energy must 
be used to produce perceptible reaction. Pringsheim (1912, p. 157) found 
reaction to vary with phototropic attunement, or degree of adjustment 
in physiological condition according to the amount of light to which the 
plant was exposed previous to and during the period of one-sided illumina¬ 
tion. In an earlier article (1909, p. 274) he stated that exact measurements 
in his experiments were impossible, since there was no method of expression 
for the value of the light source in definite units. 
Blaauw (1909), by determining in photometric units the spectral 
regions, and calculating the energy values for each region from Langley’s 
curve, attempted to explain the lack of harmony in the results of preceding 
investigators on the basis of energy distribution and of photo-chemical pro¬ 
cesses. From his experimental data on response of Arena and of Phyco - 
myces in the spectral regions, he was able to construct curves consistent with 
those for visual sensitivity, but with the maxima located in the blue and 
indigo. 
Clark (1913) gave remarkable results in his paper on negative photo- 
