294 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



sunlight by the black letters. The portions of the plate not thus protected 

 were entirely free from living bacteria. 



When bacteria are exposed to sunlight the majority of them are killed in the 

 first few minutes of exposure. This was shown with twelve similar plate cul- 

 tures of Bacillus anthracis, one of which was kept in darkness throughout the 

 experiment, the other eleven being first exposed to sunlight for ten, twenty, 

 thirty, etc., minutes, respectively, and then exposed to darkness for the rest of 

 the culture period. When sufficient time had elapsed for the colonies to 

 develop, these were counted in each of the plates. The following table shows 

 the results of these counts. 



It is thus evident that light possesses very great disinfecting power, 1 and the 

 Italian proverb, "Where sunshine enters not, there enter the physician," has 

 a foundation in bacteriological science. Light is a potent factor in the auto- 

 matic purification of polluted rivers. As they issue from cities, streams contain 

 innumerable bacteria of many kinds, but before they have flowed far their 

 waters become practically free from these organisms, through the action of 

 sunlight. Water containing a hundred thousand cells of Bacterium coli com- 

 mune per cubic centimeter was found to be entirely free of living bacteria after 

 exposure to sunshine for a single hour. The ultra-violet rays {rayons abiotiques, 

 of Dastre 2 ) are especially injurious to colorless bacteria. 



The colored bacteria are not affected by light as are the colorless ones. The 

 purple bacteria studied by Engelmann are attracted toward brightly lighted 

 portions of the medium in which they are growing, and they develop best in 

 the presence of bright light. 



§6. Influence of Gravitation on Growth and Configuration. 3 — That stems 

 grow upward and that roots grow downward are such obvious facts that they 



1 E. W. Schmidt has attempted to utilize the sensitizing action of fluorescent substances upon micro- 

 organisms, enzymes, etc., as a means of disinfection. In this connection see: Tappeiner, Hermann, von, and 

 Jodlbauer, A., Die sensibilisierende Wirkung fluorescierender Substanzen. Leipzig. 1907- Schmidt, 

 Ernst, W., Enzymologische Mitteilungen. Zeitschr. physiol. Chem. 67: 314-323. 1010. 



- Cernovodeanu, P., and Henri, Victor, Etude de Taction des rayons ultraviolets sur les microbes. 

 Compt. rend. Paris 150: 52-54. 1010. Idem, Comparison des actions photochemiques et abiotiques des 

 rayons ultraviolets. Ibid. 150: 549-551- 1 9 10. Urbain, Ed., Seal, CI., and Feige, A., Sur la steriliza- 

 tion de l'eau par l'ultra violet. Ibid. 150: 548-549. 1910. 



3 Wiesner, 1881. P. 85-130. [See note 2, p 277.] Idem, Untersuchungen tiber die Wachsthumsbe- 

 wegungen der Wurzeln. (Darwin'sche und gectropische Wurzelkrummung.) Sitzungber. (math-naturw. 

 Kl.) K. Akad. Wiss. Wien. 89I: 223-302. 1884. Fitting, Hans, Untersuchungen uber den geotropischen 

 Reizvorgang. Teil. I. Die geotropische Empfindlichkeit der Pflanzen. Jahrb. wiss. Bot. 41: 221-330. 

 1905. Idem, same title. Teil II. Weitere Erfolge mit der intermittierenden Reizung. /6j'<j. 41 : 331-396. 

 1905. Bach, H., Ueber die Abhangigkeit der geotropischen Presentations- und Reaktionszeit von verschied- 

 enen Aussenbedingungen. Ibid. 44: 57-123. 1907- Nordhausen, M., Ueber Richtung und Wachstum 

 der Seitenwurzeln unter dem Einfluss ausserer und innerer Faktoren. Ibid. 44: 557-034- i907> 



