420 Plant Physiology 



clear solution may be readily prevented by exposure to 

 light. The chlorophyll of shoots protects in a measure the 

 diastase from the injurious action of light during the day. 

 Nevertheless, from this and other causes starch conversion 

 in the leaf is reduced to a minimum during days of bright 

 sunshine. 



Bacteria and other hyaline microscopic organisms are 

 killed by direct sunlight, and this fact is important in sani- 

 tation. The convincing demonstration of the effect of sun- 

 light upon bacteria was made by Ward. He prepared 

 cultures of the bacteria upon clear agar in Petri dishes, and 

 then exposed the dishes to sunlight. It was determined 

 that the organism causing anthrax, Bacillus anthracis, 

 may be killed in such cultures in direct sunlight by an ex- 

 posure of from a few minutes to several hours, depending 

 upon the intensity of the light. Striking results were 

 obtained by covering the dish to be exposed with a black 

 paper or metal stencil so that the contrast between exposed 

 and unexposed parts of the plate may be sharp. A spec- 

 trum was also thrown upon prepared cultures and it was 

 determined that the blue- violet rays constitute the effective 

 killing portion of the spectrum. With the use of glass 

 covers or globes the injurious rays are to a considerable 

 extent excluded. 



256. Artificial light. - - Interesting studies have been 

 made upon the use of artificial light in greenhouse culture, 

 as in forcing lettuce, endive, radish, and certain flowers. 

 In such experiments the artificial light has been employed 

 usually at night, or supplementary to daylight. Eco- 

 nomically, artificial light is probably a failure, owing to the 

 expensiveness of it; but the results of the experimental 

 work bring out some points of interest. 



