380 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1932 



Here at the Smithsonian Institution we have a collection of numer- 

 ous unicellular green algae in pure culture. In the Division of Radia- 

 tion and Organisms we are using these algae for investigations relat- 

 ing to the ejffects of light of different intensities and different wave 

 lengths. All the experiments are being carried on with the under- 

 lying purpose of making the work strictly quantitative. Up to this 

 time, the lack of suiiicient physical data has made the results of simi- 

 lar research doubtful and often impossible to repeat or corroborate. 

 The use of delicate thermocouples makes possible exact measurement 

 of the intensity of the light as it falls on the organisms. All the 

 light filters used have been carefully measured for their specific trans- 

 missions and ingenious devices have been skillfully built for quantita- 

 tive measurement of the growth and development of the green cells 

 subjected to the various lights. 



An algal spectrogram (pi. 3) showing the lethal action of ultra- 

 violet light has been obtained by growing unicellular green algae in 

 pure culture on an agar-coated plate. Similar plates were exposed 

 to different regions of ultra-violet light in a large quartz spectrograph 

 for periods of time varying from 6 minutes to 18 hours. In the 

 regions of ultra-violet beyond 3022 A., the approximate limit of 

 ultra-violet irradiation in nature, the green algal cells were killed. 

 Decolorized lines appeared on the green algal plate for the lines 3022, 

 2967, 2894, 2804, 2753, 2699, 2652, and 2536 A. Wave lengths longer 

 than 3022 A., that is, wave lengths 3130, 3341, and 3650 A., had no 

 appreciable lethal effect on the algae. Yet by the thermocouple 

 measurements a greater intensity of light was directed on the cultures 

 at wave lengths 3130 and 3650 A. Experiments are now being car- 

 ried on with exposures of varying time and intensity to determine 

 more completely the lethal effectiveness of the different wave lengths, 

 also to investigate the possibility of a stimulative effect on the growth 

 of the algae for small doses of the different wave lengths. 



Another experiment has been designed for the simultaneous deter- 

 mination of the effect of four intensities of light on 18 different uni- 

 cellular green algae growing for a month or more in exactly similar 

 conditions of medium, temperature, and light. The different strains 

 of algae vary as radically in their reaction to these external condi- 

 tions as do other organisms under diversified environmental condi- 

 tions. It was found that some varieties grow better at a low light 

 intensity than at a high one; some like intermittent light better than 

 continuous light. One variety refused to grow in any of the light 

 intensities provided. 



In a similar manner, one variety of algae is submitted to light of 

 different wave lengths, in an effort to determine the optimum condi- 

 tion for the development and growth of the cells. 



