THE LEAF 



181 



198. Heliotropism — 



' ' turning with the sun' ' — is 

 the name given to the daily 

 movement of plants like the 

 cotton and sunflower in 

 turning their leaves or their 



2:35 236 



Figs. 235, 236. — Horse-chestnut leaves: 235, leaf rosette seen from above; 

 236, the same seen sidewise, showing the formation of rosettes by the lengthening 

 of the lower petioles. 



blossoms to face the sun. If you live where cotton is grown, 



notice the leaves in a field about ten o'clock on a bright 



sunny morning, and again from the same 



point of view at about four or five in the 



afternoon. Do you perceive any differ- 

 ence in their general dis- 

 position? Watch on a 

 cloudy day and see if 

 any change takes place. 

 Find out by observation 

 whether the " heliotrope " 

 of the hothouses is really 

 heliotropic. 



199. Adjustment 

 against too great intensity 

 of light. — Plants fre- 

 quently have to protect 

 themselves against excess 

 of light and heat. An 



Fig. 237.— Leaf 

 oyramid of mullein. 



compass plant, rosin- 

 weed (Silphium lacini- 

 atum) : 238, seen from 

 the east ; 239, soon 

 from the south. 



