198 GROWTH 



finds the same acceleration and retardation in the growth of 

 the coleoptile of Avena saliva, but also a considerable initial 

 decrease in growth rate on temporary exposure to a suf- 

 ficiently strong illumination. There is first a rapid de- 

 crease, immediately followed by an acceleration, which is 

 in turn followed, especially in increased or more protracted 

 illumination, by a second inhibition phase which exceeds the 

 previous acceleration ; hence the total effect may be a con- 

 siderable 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 * also found that in 

 comparable conditions the average leaf area of the cucumber 

 is determined by the product of the intensity and the duration 

 of the light radiation. 



The initial decrease in the growth rate is considered by 

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

 effect of alternating light and darkness nor to increased trans- 

 piration. 



The following observations of Vogt illustrate the phototonic 

 reaction : when the coleoptile of the oat was alternately 

 illuminated by light of the same intensity, and darkened for 

 periods of fifteen, thirty, and sixty minutes, lesser growth 

 was found only in the two latter periods of illumination. In 

 these instances the greater growth in darkness is considered 

 to be due to the stimulation of the previous exposures to light. 

 The slower growth in periods of illumination is merely a part of 

 the complete reaction, an acceleration being followed by a 

 depression. 



Mention has been made of the morphological appearance 

 of seedlings, such as the pea, when grown in the dark. The 

 work of Trumpf f and Priestley % shows that if such etiolated 

 plants are exposed to light for a short period daily, § normal 



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



f Trumpf : " Bot. Archiv.," 1924, 5, 381, 410. 



+ Priestley: " New Phyt.," 1925, 24, 271 : 1926, 25, 213. See also 

 Priestley and Ewing : id., 1923, 22, 30. 



§ The period varies according to the sensitivity of the plant to light ; 

 with the pea and broad bean Priestley found a response after a period of 

 two minutes exposure. 



