200 Parr,—The Response of Pilobolus to Light , 
V. Conclusions and Theoretical Discussion. 
In considering the lack of conformity in the results of the investigations 
in the field of phototropism, it must be remembered that the plant is an 
organism existing in a physiological state continuously changing with the 
varied physical and chemical factors of its environment. A slight 
change in some one of the factors may markedly change the relation of the 
organism to every other factor (cf. Verworn, 1913). Attention has already 
been called to the disturbing influence of the slightest trace of illuminating 
gas in the laboratory air upon Pilobolus. The researches of Crocker and 
Knight (1908) have shown its vitiating influence upon the higher plants. 
A review of many articles upon phototropism shows that illuminating gas 
has furnished the source of light for the experimental work. Thus Wiesner 
(1879), Figdor (1893), Pringsheim (1909), in order to subject the plant to 
different intensities of light varied the distance from the gas-flame from a few 
centimetres to several metres. Even though the presence of the gas in the 
room was not destructive to the plants used by these investigators, one 
might see from the work of Richter (1906), and Crocker and Knight (1908), 
the inhibiting effect that mere traces of the gas, or the products of 
combustion, have upon the sensitivity. The presence of these deleterious 
agents unquestionably affects in degrees, according to the distance of removal 
from the burner, the physiological condition of the plants and thereby effect 
a change in their irritability. The above objections naturally do not affect 
those experiments of Wiesner, Figdor, and Blaauw, where the sun or arc 
lights were used. 
A noticeable difference between the results of the present series of 
experiments and those of earlier workers lies in the absence of the maximal 
and minimal points of response in the spectrum. Inquiry into the cause 
for this difference naturally leads first to the study of the nature of the light 
obtained from different sources. 
The theory of light as formulated by Maxwell and accepted by 
physicists of the present time states that light consists of short electro¬ 
magnetic waves which are produced by violent agitation of particles either 
from the electric current or other source of heat. Every source of light 
used for illumination has its own characteristic spectrum which differs from 
that of every other source, and the energy of radiation of each wave-length 
derived from one source differs from that of the corresponding wave-length 
from every other source (Nutting, pp. 12 and 197; Fery, 1908; Coblentz, 
1911). Moreover, the spectrum from any given source changes according as 
the absolute temperature of the source is increased. The maximal spectral 
energy is found to move towards the violet end with a rise in the absolute 
temperature (Ives, 1910 ; Drysdale, 1908 ; Nichols, 1903; Wien, 1893). Thus 
we find that bodies heated to 5oo°C. emit only red rays in the visible spectrum, 
