224 PARAHELIOTROPISM. [CH. VIII 



a copy of a tracing^ made by the main petiole of Mimosa 

 piidica from 4 p.m. Aug. 16, until noon of the following 

 day. The tracing was made by means of the hanging 

 writer described in experiment 203, which recorded the 

 position of the petiole at intervals of half-an-hour on the 

 revolving drum used for auxanometer experiments. The 

 tracing only records changes in the vertical position of 

 the free end of the petiole, and does not give the angle 

 which the petiole makes with the horizon, but if a few 

 readings of the angle are taken, the rest can be calculated 

 from the known length of the petiole and writer. It will 

 suffice for our present purpose to know that at 7 p.m. the 

 petiole was roughly 15° below, and at 4 a.m. 60° above 

 the horizon. 



The tracing shows that the leaf sank with increasing 

 and then decreasing rapidity from 4 p.m. to 7 p.m., when 

 it rose (at first slowly) until 3 a.m. It then remained 

 stationary until 4'30, when a fall again occurred, followed 

 by irregular movements continuing to noon. 



(257) Paraheliotropism : Averrhoa hilimhi. 



The leaves of many plants assume in bright sunshine 

 a more or less vertical position, which has been sometimes 

 called " diurnal sleep " but is now known as parahelio- 

 tropism. Oxalis acetosella, in which the leaves in bright sun 

 assume the same vertically dependent position that they 

 take at night, is a familiar example. Averrhoa hilimhi 



1 To diminish the horizontal extension of the diagram, the horizontal 

 lines drawn by the writing index are reduced. The engraving is more- 

 over reduced by \ from the drawing so prepared. 



