SECRETARY'S REPORT 63 



far-red energy interferes with the developmental process by acting 

 on a product of the photochemical reaction initiated by the red energy. 

 These studies have all been done on seedlings of the bean plant, using 

 the changes in growth of the bean hypocotyl hook as an assay of the 

 growth regulating effects of the radiant energy. 



2. — One of the first evidences of damage to living organisms by any 

 form of ionizing radiation, including X-rays, is the breaking of the 

 threadlike hereditary structures of the cell known as chromosomes. 

 It has been found that if X-rays are preceded by exposure to red visi- 

 ble light, the incidence of X-ray-induced chromosome damage is mark- 

 edly reduced on the order of 30 to 50 percent. If, on the other hand, 

 the X-rays are preceded by exposure to radiant energy from the far- 

 red or near infrared, the damage is potentiated by these wavelengths 

 by as much as 30 to 40 percent at the energy levels used. The plant 

 material employed for these studies was the root-tip cells of broad 

 bean (Vicia faha)^ the chromosomes of which are extremely sensitive 

 to X-ray damage. 



Work is now in progress to study the mechanism of these reactions 

 of inhibition and potentiation of X-ray damage and how the results 

 may be applied to the mediation of the damaging effects of ionizing 

 radiations. The medical implications of these findings are extremely 

 important in the control of damage by ionizing radiation and in 

 cancer therapy. 



3. — Auxin, the only plant hormone isolated to date, controls certain 

 phases of the growth process such as cell elongation. The effects of 

 auxin may be produced or modified by a number of growth-regulating 

 chemicals, including the common weed killers and other materials 

 used for controlling rooting of plants, fruit set, and sprouting. 



Although auxin has been identified for many years as indole acetic 

 acid, it has not been possible to quantitatively measure its activity on 

 any metabolic function and biological assay methods have been used 

 in the past to determine auxin activity. During the past year, how- 

 ever, using differential centrifugation methods to fractionate plant 

 tissues, it has been found that the fraction remaining after sediment- 

 ing cell walls, nuclear material, mitochondria, plastids, and other par- 

 ticles within this size range, contains an enzyme system which, on the 

 addition of auxin, brings about a marked oxygen consumption. The 

 rate of auxin activity can be measured quantitatively on this cell-free 

 fraction. Avocado fruit tissue is being used as the experimental plant 

 material. Studies are now in progress to determine the initial bio- 

 chemical steps of the process. 



Respectfully submitted. 



L. B. Aldrich, Director. 



Dr. Leonard Carmiciiael, 



Secretary, Smithsonian Institution, 



