38 



GENERAL BOTANY 



are expanded to the sun, while at night they fold up (Fig. 28), 

 assuming the so-called nyctitropic, or sleep, position. 



The mechanism of this curvature is supposed to consist in an 

 unequal absorption of water on two opposed sides of the pul- 

 vinus as a response to different intensities of light, as in the 



FIG. 21. Positions assumed by the leaves and roots of a mature caladium plant 

 in response to light and gravity. After Koerner 



leaves of bean plants, which may be seen to fall and assume a 

 vertical, hanging position at night, owing to the greater increase 

 in length of the upper side of the pulvinus as compared with the 

 lower side, thus inducing curvature and the downward bending 

 of the leaves (Fig. 16). This method of curvature is not unlike 

 that of roots and petioles already described, except that in these 



