196 GROWTH 



and quality. Sierp * found that the general effect of an 

 increase in light intensity is to accelerate the rate of growth of 

 the coleoptile of Avena saliva and to shorten the time within 

 which the maximum rate occurs and that the incidence of 

 this maximum is earlier as the light intensity increases. The 

 net result is that the total growth is reduced as the intensity 

 of light increases. 



Of the effect of duration of light on plant growth, Garner 

 and others f conclude from their observations on the tobacco, 

 soy and other plants that the amount of vegetative growth 

 is proportional to the duration of exposure to daylight, short 

 exposures resulting in small slender plants exhibiting a slower 

 rate of growth. They also find that the duration of light is 

 an important factor in instituting the reproductive phase ; 

 by modifying the periods of exposure, biennials may be made 

 to complete their life histories in a few months, and, on the 

 other hand, annuals may be converted into biennials. To take 

 a few examples. The soy bean illuminated for twelve hours 

 daily blossomed in no days, but with a five-hour illumination 

 daily, the flowering began in 27 days ; a variety of tobacco 

 plant subjected to the same periods of daily illumination 

 flowered after the lapse of 152 to 162 days and 55 to 61 days 

 respectively. Thus it is seen that shortened periods of daily 

 lighting results in earlier flowering in these plants. But this 

 is not necessarily so for all plants : Lespedeza, a leguminous 

 plant useful as fodder, does not thrive with shortened periods 

 of light ; red clover, many grasses and the radish when given 

 shortened days show a retardation in flowering, and the 

 groundsel, foxglove and tomato are more or less indifferent 

 to either long or to short days. J 



Results contrary to some of these are recorded ; thus 

 Adams § found that mustard, soy, flax, tomato, sunflower 



* Sierp : " Ber. deut. bot. Ges.," 1918, 35, 8 ; " Bot. Zentrlbl.," 1920, 



40, 433- 



f Garner and Allard : " Journ. Agric. Res.," 1920, 18, 553 ; *' Science," 

 1927, 66, 140. Garner, Bacon and Allard : " Journ. Agric. Res.," 1924, 

 27, 119. 



X Adams: " Amer. Journ. Bot.," 1924, II, 229. Tincker : "Ann. 

 Bot.," 1925, 39, 721 ; 1928,42,101. 



§ Adams : " Ann. Bot.," 1923, 37, 75 ; 1924. 38, 5°9- 



