92 A3S1TUAL. REPORT SMITHSONTAN IJTSTITUTION, 1940 



paratus. The difficulty was traced to a minute amount of mercury 

 vapor entering the growth chamber from the mercury seal of the 

 air-circulating pump. 



The problem then resolved itself into one of obtaining a properly 

 designed circulating pump for the carbon dioxide measuring appa- 

 ratus. In a closed system of this type it is necessary to circulate the 

 air in a system absolutely leakproof to carbon dioxide. Even the 

 slightest amount, a few cubic millimeters, would introduce an error 

 that would invalidate the measurements made by this method. 



A metal bellows-type pump was constructed and installed. This 

 worked fairly well but carried with it certain disadvantages. A third 

 type of pump making use of a rotating magnetic field was next tried, 

 but was discarded because of its lack of power. A fourth pump was 

 constructed and, from the few preliminary experiments so far tried, 

 it is believed that it will meet the rigid requirements of this exacting 

 experimentation. 



A series of experiments on etiolated barley seedlings clearly shows 

 that there is enough chlorophyll formed in 1% hours' exposure to light 

 of about 100 foot-candles to be easily measured. 



The instrumental phases and the perfecting of experimental tech- 

 nique have now been completed to the point where work on the prob- 

 lems relating to the genesis of chlorophyll and the beginning of 

 photosynthesis may be carried on in greater detail. 



PLANT GROWTH UfVESTIGATIONS 

 PLANT HORMONES AND CHEMICAL FACTORS 



A standardized technique has been worked out for the extraction of 

 growth substances from the oat seedling and, in a comparative study 

 of the various methods employed by other investigators, has been found 

 to possess a number of advantages. It is becoming more generally 

 appreciated among the workers in this field that the problem of growth 

 substance assay is greatly complicated by the possible existence of 

 hormone precursors, of active and inactive forms of the growth sub- 

 stance itself, and of growth inhibitors. A complete understanding of 

 the behavior of the plant must take all these factors into account and 

 further work is now being done along these lines. 



In the study of the growth of excised oat shoots and leaves a number 

 of biochemical substances, several of which have been made available 

 through the generosity of Merck & Co., as well as various plant ex- 

 tracts, have been tested. As yet it has not been possible to develop 

 an artificial environment which will enable the excised organs to 

 develop in an entirely normal manner, but some interesting inter- 

 relationships among the various parts of the plant have come to light. 

 These studies are being continued. 



