98 RELATION OF PLANTS TO TEMPERATURE 



brownish hue, with the margins inrolled, and that the petioles 

 are curved sufficiently to allow the laminae to depend in a posi- 

 tion almost vertical. Intense sunlight which does not raise the 

 temperature does not interfere with this action. If now a branch 

 is cut from the plant and taken into a warm laboratory room at 

 20 C. to 25 C. the leaves will begin to rise within two minutes 

 and will have assumed a position nearly horizontal in five or six 

 minutes. The base of the branch should be inserted in a vessel 

 of water as soon as brought into the room. After the leaves 

 have come to a state of rest remove to the open air at a temper- 



A B C 



FIG. 43. Branch of Rhododendrum maximum standing in vessel of water. A, 

 showing position of leaves one minute after removal to warm room. , same, one mi- 

 nute later ; upper movement of leaves has begun. C, branch with leaves of a normal 

 warm position about five minutes after removal to warm room. After Harshberger. 



ature below zero C. and note the reverse movements, which will 

 be much slower. Test the rigidity of the petioles when in a de- 

 pressed position. The lax condition of the leaf suggests that the 

 reaction is due to lowered turgor in the cells on the upper side 

 of the leaf. The general purpose of this reaction is doubtless the 

 same as that exhibited by leaves with pulvini, in which the leaf 

 is held in a rigid condition. The drooping position of the laminae 



