124 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 3 



cal reactions which control the development of the various organs of 

 the plant. In the absence of light and in the presence of adequate 

 food reserves, higher plants fail to develop normal leaves and stems, 

 and in the dicotyledonous plants the hook that forms in the stem 

 of the germinating seed never completely disappears. 



Dr. W. H. Klein and V. Elstad have continued investigations of the 

 effect of light intensity and various growth regulatory chemicals on 

 the opening of the hypocotyl hook in Black Valentine bean. A new 

 set of subirrigated growth chambers has been constructed which 

 yield plant material of very great uniformity. By the use of a special 

 green fluorescent safe light employing a filter transmitting light be- 

 tween 520 and 610 millimicrons, it is possible to remove the hooks 

 from the plants and make measurements on them without producing 

 any detectable light effect. The hook sections are placed in petri 

 dishes containing a small amount of water and exposed to various 

 light and chemical treatments. A 24-hour exposure to very weak red 

 light in the region of 650 millimicrons at an intensity of 0.01 micro- 

 watt per square centimeter produces a 45° opening in a 24-hour period ; 

 in the dark there is no significant opening of the hook in this period. 

 The rate of opening of the hook is proportional to the logarithm of 

 the light intensity. It appears that this organ is a very useful tool 

 for the bioassay of photochemically synthesized growth factors. 



The auxin group of hormones such as indoleacetic acid opposes the 

 effect of the light. The effect is proportional to the logarithm of the 

 concentration of the auxin over a very wide range and the test appears 

 to have a sensitivity nearly equal to the classical Avena test, but is 

 a much simpler one to execute. 



Thus far no pigment system has been extracted from plants whose 

 absorption spectrum can account for the regulatory effect of the longer 

 wavelengths in the visible spectrum. In order to obtain information 

 as to the absorption spectrum of the pigment system, work has been 

 started by Dr. Withrow, Dr. Klein, and Mr. Elstad on determining 

 the effectiveness spectrum of the stem-hook response and the synthesis 

 of anthocyanin in bean stems. A system of 10 interference filter mono- 

 chromator units has been constructed, each of which employs two 

 interference filters in tandem for isolating a narrow band of wave- 

 lengths about 20 millimicrons wide. Each monochromator unit has 

 a separate source and cabinet, and the whole system is in a room 

 maintained at constant humidity and temperature. 



Dr. W. D. Bonner and L. Price have initiated a systematic bio- 

 chemical study of various fractions from dark-grown and far-red- 

 irradiated bean seedlings with the objective of finding those 

 biochemical systems that are associated with the light-initiated re- 

 sponses. Estimations of the activities of various enzyme systems have 

 shown no significant differences between the dark- and the light- 



