126 GROWTH 



duration of light is an important factor in instituting the repro- 

 ductive phase ; by modifying the periods of exposure, bienniels 

 may be made to complete their life histories in a few months, 

 and, on the other hand, annuals may be converted into bienniels. 



In this connexion the results obtained by Blaauw * and of 

 Vogt f from their studies on the growth of the sporangiophore of 

 Phy corny ces and on the coleoptile of Avena respectively are 

 important. According to Blaauw, the effect of light on growth 

 is an acceleration followed by a retardation to a rate lesser than 

 the normal followed by a gradual increase to the normal rate. 

 The time of incidence of the initial acceleration varies with the 

 intensity of the illumination ; thus on exposure to a light of 

 intensity of one unit, the acceleration begins in about eight 

 minutes, but in an intensity of fifteen units, the acceleration 

 begins in about three and a half minutes. The amount and 

 duration of the reactions vary with the degree of illumination : 

 for the lower light intensities the total acceleration of growth 

 exceeds the total retardation ; and for the higher light intensities 

 the total retardation exceeds the total acceleration. For the 

 former, Blaauw finds that the increased growth is proportional 

 to the cube root of the amount of light. These results are 

 extended by Vogt, who not only finds the same acceleration 

 and retardation in the growth of the coleoptile of Avena sativa, 

 but also a considerable initial decrease in growth rate on tempor- 

 ary exposure to a sufficiently strong illumination. In this 

 instance, therefore, there is first a rapid decrease, immediately 

 followed by an acceleration, which is in turn followed, especially 

 under increased or more protracted illumination, by a second 

 inhibition phase which exceeds the previous acceleration ; hence 

 the total effect may be a considerable fall in the rate and in 

 the amount of growth. For a given reduction in growth, the 

 product of the light intensity and its duration is a constant. 

 Gregory J also found that under comparable conditions the 

 average leaf area of the cucumber is determined by the product 

 of the intensity and the duration of the light radiatioa 



The initial decrease in the growth rate is considered by 

 Vogt to be due to the action of light alone, not to the combined 



* Blaauw: "Zeitsch. Bot.," 1914, 6, 641. 



fVogt: Id., 1915,7, 193- 



I Gregory: "Ann. Bot.," 1914, 35, 97. 



