196 



PLANT LIFE. 



because usually illuminated from one side. Plants kept in 

 darkness have their leaves irregularly placed. 



286. (b~) Combined movements due to variations in the 



FIG. 192. A shoot of thorn-apple or " jimson " weed, showing imperfect leaf mosaics 

 of tall plants formed upon the same plan as in rosettes (lag. 193). One-seventh natural 

 size. After Kerner. 



amount of light or heat or both are especially exhibited by flow- 

 ers, whose opening and closing are frequently determined 



FIG. 193. A rosette of leaves of a bellflower (Campanula fusilla), showing length- 

 ening of petioles of lower leaves so as to carry blades from under upper leaves. 

 After Kerner. 



thereby. With some plants the predominant stimulus is heat; 

 with others, light. Closed flowers of the tulip or crocus may 

 be made to open in 2 to 4 minutes by raising the temperature 



