﻿BOTANICAL GAZETTE 



approach the problem of the mechanism of the light action by 

 studying the action of light upon organic substances. The study 

 of the action of light upon oxalic acid showed that by insolation 

 oxalic acid is decomposed into carbon dioxide, carbon monoxide, 

 and water. They found that oxygen is necessary for this decom- 

 position. Experiments were then conducted on the action of light 

 upon ferments. They showed that light, in the presence of oxygen, 

 destroys the power of yeast to ferment sugar. They had intended 

 to make an exhaustive study of the action of light upon organic 

 compounds, but Chastaing preceded them, showing that many 

 organic substances were oxidized by the light when in the presence 

 of free oxygen. 



Duclaux (15) in 1885 studied the action of light upon the 

 anthrax bacillus. He was the first to study the action of light upon 

 pure cultures of bacteria. He found that the ability of the organ- 

 isms to resist the action of the light varies with the species, with 

 individuals within the species, and with the nature of the culture 

 medium. 



In 1887 Roux (30) found that his culture media, when exposed 

 to sunlight, became toxic to the spores of the anthrax bacillus. 

 Duclaux had previously shown that carbohydrates are easily 

 oxidized in sunlight. Roux concluded, therefore, that his culture 

 media were rendered antiseptic by the oxidation of the carbohy- 

 drates which they contained. 



Ward (34) in 1892 separated the action of the light upon the 

 medium from the action of the light upon the organism by exposing 

 the spores in a thin film on glass and then adding agar. The 

 exposed spores were killed. In other experiments he first exposed 

 the agar to the light, and then added it to thin films of unexposed 

 spores on glass. The unexposed spores grew in the exposed agar. 

 He attributed the action of the light to the destruction, in the pres- 

 ence of oxygen, of fatty foods stored in the spores. Ward in later 

 papers (35, 36, 37) described experiments on the relative toxicity 

 of the various parts of the spectrum. Instead of exposing culture 

 tubes to various parts of the spectrum, as several observers had 

 done before, he exposed a culture evenly charged with organisms 

 (Bacillus anthrax and B. subtilis) to the spectrum, formed by a 



