740 BIOLOGICAL EFFECTS OF RADIATION 



eflfects of very short daily exposures to light, which frequently are carried 

 on in conjunction with experiments on photoperiodism, have revealed 

 many of the same characteristics exhibited by plants grown with insuffi- 

 cient intensity. Before proceeding with a discussion of the effects of 

 radiation intensity per se on the growth of plants, a consideration of the 

 effects of definite quantities of light on plants will be given, as well as 

 the effects of sudden illumination or darkening of plant organs. 



THE "light-growth REACTION" 



Every plant and plant organ goes through a definite period of growth 

 during which the growth rate gradually increases to a maximum, and then 

 diminishes to zero. The descending limb of the growth curve is usually 

 steeper than the ascending limb. This is spoken of as the grand period 

 of growth. In the discussion which follows, the different investigators 

 are referring to disturbances in the grand period of growth induced by the 

 action of light. 



In 1914, Blaauw (11), working with sporangiophores of Phycomyces 

 nitens found that exposures to light caused a profound influence upon the 

 rate of elongation. After exposure there was an initial stimulation in 

 growth, followed by a depression. The time interval between exposure 

 and reaction decreased, while the depth of the depression and height 

 of the maximum increased with increasing amounts of light from 1 up 

 to 210 meter-candle-seconds. For 240,000 to 1,920,000 meter-candle- 

 seconds, secondary minima and maxima appeared, which tended to 

 cause the rate of growth alternately to increase and decrease in a wave- 

 like manner. He concluded that a given amount of light calls forth 

 a typical growth reaction in the cell and that this reaction runs a definite 

 course. 



Similar experiments were carried out by Blaauw (12) with seedlings 

 of Helianthus globosus. With these plants, exposure to light caused a 

 depression in the rate of growth, with a minimum after 41 to 51 min. for 

 low amounts of light — 14 to 26 min. with large amounts — followed by a 

 return to the original rate of growth. In all other respects, the response 

 was similar to that with Phycomyces. When exposed to continuous light 

 of 1 to 4096 meter-candles, a similar response occurred. The response 

 started after 2 to 8 min., the time interval being shorter, the higher the 

 intensity; the minimum was attained at 25 to 30 min. The rising limb 

 of the curve showed secondary minima at the higher intensities. From 

 these facts he deduced the theory that phototropic bending depended 

 upon the differences in the light-growth reactions caused by different 

 intensities of light. 



Vogt (118) carried out similar studies on the coleoptile of Avena 

 saliva. When exposed to continuous light of 5, 25, 100, and 1000 meter- 

 candles, the coleoptiles completed their growth progressively earlier and 



