PHOTOPERIODISM 197 



and other plants showed a greater average weight, a greater 

 average height, and earlier flowering when exposed to long 

 periods of illumination. In fact, the phenomenon of photo- 

 periodism * is bound up with and is influenced by other factors, 

 especially nutrition, the carbohydrate/protein ratio, tempera- 

 ture and humidity. f 



The photoperiodicity of plants taken in conjunction with 

 temperature* and humidity is clearly of importance in relation 

 to the geographical distribution of plants, and a knowledge 

 of these factors is of economic significance in that it will in- 

 dicate the varieties of crop plants suited for cultivation in 

 different latitudes. 



The results obtained by Blaauw J and of Vogt § from 

 their studies on the growth of the sporangiophore of Phy- 

 comyces 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 ac- 

 celeration varies with the intensity of the illumination ; thus 

 on exposure to a light 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 



* In passing, it may be mentioned that this duration of illumination is 

 reflected in the reaction of the plant juices, thus a short-day plant in response 

 to lengthened periods of illumination, which produce excessive vegetative 

 growth, shows an increase in acidity ; on the other hand, a short-day plant 

 induced to early flowering by shortened times of illumination has an acidity 

 much below that of the vegetative plant ; the same applies to long-day 

 plants. 



f See Gilbert : " Ann. Bot.," 1926, 40, 315. 



t Blaauw : " Zeitsch. Bot.," T914, 6, 641. 



§Vogt : id., 1915. 7» *93- 



