482 



PHYSIOLOGY OF GROWTH. 



If we observe seedlings of Raphanus sativus grown in flower- 

 pots, it is seen that their cotyledons are expanded in the daytime, 

 but at night appear more or less -closed up. Young leaves of 

 Tropseolum majus illuminated from above assume in the daytime 

 a position such that the incident rays of light strike them at 

 right angles. At night they place themselves vertically. Accord- 

 ing, however, to the results of my investigations with pot plants, 

 this only takes place fully at a comparatively high temperature. 



Concerning the growth movements of flowers induced by change 

 of illumination, and using mostly plants growing in the open, I 

 made the following observations which we may repeat. 



FIG. 161. Flower scape of Leontodon FIG. 162. Flower scape of Leontodon hastilis; 

 hastilis; inflorescence closed. inflorescence expanded. 



I found that the flower heads of most specimens of Taraxacum 

 officinale, on sunny days at the beginning of May, open between 

 7 and 8 o'clock in the morning, and close again between 4 and 5 

 o'clock in the afternoon. The flowers of Tradescantia remain 

 open all day if the sky is overclouded ; under a cloudless sky, 



