"petiole. " This eollapse of the whole leaf is in no way due to a mere re- 

 laxing of the leaf. The petiole is actually forced downward by the change 

 in size of the cells of the pulvinus. Many of these leaves have charac- 

 teristic positions with reference to the sun on bright days, and some 

 "follow the sun" from morning to e\'ening. These movements occur 

 readily with changes in light intensity provided the temperature is high; 

 if it is low, they are slow or do not occur at all. 



' Data from papers by G. VV. Blaydes, E. J. Salisbury, and L. E. Yocum. 



