34 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



ment, May 21, was only 2 units smaller than on January 2, when it was 

 turned toward the wall. Moreover, between March 30 and April 10 it ex- 

 panded 6 units after having- been contracting- for nearly two months. The 

 rapid contraction in other parts of the plant must have caused a mechan- 

 ical pull which held the ribs on the opposite side apart while they were 

 drying up, thoug-h less rapidly, than the other parts exposed to the sun 

 (fig. 15). 



Fig. 15.— Three curves from measurements of intervals on bisnaga No. lo. 

 November 17, 1908, to May 21, 1909. 



This evidence is from plants which were not under normal conditions, 

 but taken in connection with the morphology of the bisnaga it confirms 

 the conclusion that the effects of insolation in the bisnaga are practically 

 the same as in the sahuaro. 



COMPARISON OF WEIGHTS AND MEASUREMENTS. 



In addition to the observations on normal growing plants, some of the 

 bisnagg,s described on pages 54 and 58 which had been uprooted and 

 brought to the laboratory were marked and measurements made in order 

 to correlate, if possible, the loss by weight with that by volume, as indi- 

 cated by the measurement of parts of the circumference. At the same 



