114 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



view that chlorophyll participates in the process of photosynthesis as 

 an individual molecule rather than by units of several hundred or 

 several thousand. 



The development of apparatus and method for the determination 

 of small amounts of total chlorophyll by Dr. Johnston and Dr. 

 Weintraub has been completed. With a 5-cm. absorption cell, the 

 sensitivity is 0.1 microgram (Vioooo milligram) of chlorophyll. 

 One square centimeter of leaf is sufficient for duplicate determinations 

 which check within 2 to 3 percent. 



The method is based on the transmission of light in the region of 

 the red absorption band of a solution of chlorophyll in acetone. The 

 transmitted energy is determined by means of a galvanometer and a 

 vacuum thermocouple of extremely high sensitivity that was designed 

 and constructed by Mr. Clark. The percentage transmission of the 

 acetone extracts of plant material is then compared with a calibration 

 curve constructed from data obtained with solutions of purified 

 chlorophyll. 



This method eliminates the constant use of standard chlorophyll 

 solutions and is not influenced by the presence of carotenoid pigments 

 in the extract. Furthermore, it is unaffected by minor fluctuations in 

 the light intensity, and errors involved in subjective intensity and 

 color comparisons are avoided. 



Experiments are in progress to determine the rate of chlorophyll 

 formation as influenced by age and condition of plant material, 

 temperature, and nutritional environment of the plant, the carbon 

 dioxide content of the atmosphere, and the role played by light in- 

 tensity and wave-length distribution. The relationship between 

 rate of chlorophyll formation and intensity of light appears to be 

 represented by a typical Blockman curve. In other words, in white 

 light, the rate of formation during the first few hours of illumina- 

 tion is substantially independent of intensity above 3 to 4 ergs/mm.^/ 

 sec. 



Our new method of chlorophyll determination at present is limited 

 to the estimation of total chlorophyll. It is very desirable to deter- 

 mine the components, a and 5, separately. Plans for modifying the 

 method with this end in view are now being formulated. 



PLANT GROWTH INVESTIGATIONS 

 GROWTH UNDER ABTTFICIAL ILLUMINATION 



Further progress has been made by Dr. Johnston in his investiga- 

 tions of suitable artificial illumination for the growth of plants under 

 controlled conditions. This year work has been done with two sizes 

 of fluorescent daylight lamps. Under the experimental conditions 

 used for the culture of tomato plants, it was found that the intensity 



