400 LECTURE XVI. 



The Stimulating Action of Light. The effect of variations 

 in the intensity of light, as such, upon the rate of growth 

 is a subject which has not as yet been adequately investi- 

 gated. The only facts bearing upon it are those ascertained 

 by Pfeffer in his investigation of the opening and closing of 

 flowers. He found, namely, that exposure to light of a 

 certain intensity caused the flowers to open, and to darkness 

 to close. Exposure to direct sunlight caused some flowers 

 (Oxalis valdiviana. Calendula, Leontodon, Venidium, and 

 other Composites) to close, so that possibly there may be 

 an optimum intensity for inducing opening. An increase of 

 intensity, within certain limits induces, then, opening, and a 

 diminution closing. It is probable, though he made no mea- 

 surements, that exposure to light or removal from light, 

 causes, like variations of temperature (see ante, p. 379), a tem- 

 porary acceleration of the mean rate of growth. We can- 

 not explain, here, any more than we could with regard to 

 variations of temperature, how it is that the induced vari- 

 ations in the rate of growth should be manifested by the 

 accelerated growth of the upper or the lower surface accord- 

 ing to circumstances. These remarks are probably also ap- 

 plicable to the rising and falling of the young growing leaves 

 of certain plants (Clunopodium album, Polygonum aviculare, 

 Stellaria, Linum, Impatiens, Polygonum Convolvulus},^ ob- 

 served by Batalin, and to that of cotyledons observed by 

 Darwin. 



The Daily Period of Growth in Length. The tonic influence 

 of light upon growth can in no case be more clearly observed 

 than by the study of the growth of plant-organs in its relation 

 to the normal alternation of exposure to light and darkness 

 which takes place in each day of twenty-four hours. The 

 alternation of day and night causes, as might be expected, 

 daily variations in the rate of growth in length ; these are 

 tolerably uniform for each period of twenty-four hours, and 

 constitute what is known as the daily period of growth in 

 length. 



