84 AA'NUAL REPORT SMITHSONIAN INSTITUTION, 1942 



Drs. Johnston and Weintraub have continued their experiments on 

 the factors that influence the change in rates of respiration. This, of 

 course, is basic work to our main project on photosynthesis. Our 

 results lead one to speculate on the possibility of the existence of a 

 carbon dioxide reservoir connected with the cell mechanism. It would 

 seem that when the plants are conditioned in air of high carbon dioxide 

 content a certain amount of excess carbon dioxide is stored in the tissue 

 so that subsequent measurements of "apparent respiration" would con- 

 sist of the carbon dioxide liberated by true respiration and that re- 

 leased from a well-filled reservoir. If, however, the plants are condi- 

 tioned in air with little or no carbon dioxide the hypothetical reservoir 

 is in a partially depleted condition and the first few periods of "ap- 

 parent respiration" show a gradual increase in rate. This would be 

 caused by the trapping in the reservoir of less and less of the respired 

 carbon dioxide, thereby liberating more and more carbon dioxide 

 which could be detected in the atmosphere surrounding the plant. 



Such a reservoir hypothesis, however, is not sufficient to account for 

 all the results obtained. There is evidence in much of our data that 

 the humidity of the air plays an important role in this gaseous ex- 

 change, perhaps in changing the size of stomatal openings. Just what 

 mechanism is involved in these plant responses is not yet clear. The 

 answer must await improvements of the humidity controls of the 

 apparatus. Such improvements are now being undertaken. 



The relationship previously found by Dr. Weintraub between light 

 intensity and inhibition of growth of the oats mesocotyl suggests 

 that two growth proceses are influenced by light; one alone at low 

 intensities and both together at higher intensities. It appears likely 

 that these two processes are cell elongation and cell division, respec- 

 tively. In order to determine the action spectra for the two processes, 

 and so to obtain an insight into their mechanisms, information on the 

 intensity relations in various spectral regions is required. Such 

 information has previously been obtained for a number of wave bands 

 at low intensities, using a double monochromator. In extending the 

 study to higher intensities, this method is not feasible and use must 

 be made of emission spectra furnishing lines of adequate intensity 

 and purity. Such lines as are available are now being studied over 

 a wide intensity range and the plant material is being preserved for 

 future histological study. 



The action spectrum (maximum activity in the red) found for the 

 mesocotyl inhibition at low intensities indicates the presence in the 

 etiolated plant of a photoreceptive pigment possessing a similar 

 absorption spectrum. An attempt to isolate such a pigment has been 

 begun. The evidence thus far obtained points to the occurrence in 

 the dark-grown oats seedling of traces of at least two pigments having 



