112 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



The maximal effect of illmnination for a 30-minute period occurs 

 at a fairly low intensity (60 foot-candles or less). The magnitude 

 of the effect produced by 60 foot-candles of light increases with 

 the time of illumination up to an exposure period of about 20 min- 

 utes and remains constant with longer light periods. These results 

 are graphically illustrated in figure 1. 



In many of these studies it was observed that the rate of respira- 

 tion was not as constant as one would desire during the periods 

 prior to irradiation. It was thought that perhaps the metabolic 

 reactions of the seedling were affected in transferring them from 

 the germination conditions to those of the respiration chamber. A 



Figure 1. — Effect of illumination on respiration of etiolated barley seedlings. Percentage 

 increase in rate of respiration Is plotted against intensity of Illumination in upper 

 grapli and against duration of exposure in lower graph. 



number of changes were made in the germination conditions and 

 in the preliminary treatment of the experimental plants in the 

 respiration chamber. After many experiments of this nature it 

 appears that the rate of respiration either increases or decreases 

 continuously for a period of time following exposures of the seed- 

 lings to low or high carbon dioxide concentrations respectively. For 

 example, figure 2 shows the relative rates of respiration for succes- 

 sive half -hour periods following a conditioning period of 5 percent 

 carbon dioxide. 



From data of this type it would appear that conditions of car- 

 bon dioxide storage or depletion develop in the plant tissue depend- 

 ing upon the concentration of this gas surrounding the plants. In 

 subsequent periods, when the respiration is measured there is an 

 increase or decrease in the rate of CO2 excretion (i. e., in the apparent 



