16 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



some 25 units wider it would naturally show more pronounced expansion 

 and contraction, but even with this advantag-e, in the summer of 1905, be- 

 tween June 27 and July 19, the interval marked in the southern furrow 

 showed more expansion. This, however, for whatever reason, was not 

 the case the following summer. In the fall of 1906 (fig. 9), after the rain 

 of November 18, the southern interval responded more decidedly at first, 

 but the northern one again outstripped it and remained longer at or near 

 its maximum. 



Similar data may bederived 

 from other individuals, and 

 some are given in the form of 

 curves in the paper already 

 referred to, but it hardly 

 seems necessary to review a 

 greater number. In general, 

 it may be said that in the fall 

 and winter the intervals on 

 the south side respond more 

 quickly after a rain than do 

 those on the north side, some- 

 times showing expansion sev- 

 eral days before the latter; in 

 the summer the observations 

 on this point were too few to 

 admit of a positive statement. 

 As to amount of expansion, 

 however, it is plain that in the 

 winter the intervals on the 

 south expand more than those 

 on the north side, but that the 

 latter remain expanded long- 

 er, and there is some evidence 

 that in midsummer the re- 

 verse is true. 



While the summer records 

 are incomplete, a tabulated 

 statement (table 2) of the 



months in which the northern and southern intervals attained their maxi- 

 mum expansion confirms what has been stated as to the more rapid response 

 of the southern furrows in the winter. The same intervals were chosen as 

 for the observation on contraction, and since the most favorable season for 

 expansion is in the winter, when most of the measurements were made, 

 the results are more conclusive. It is evident that the southern intervals 

 expanded more quickly, the majority attaining their maximum in January 



Fig. 8. — Curves from measurements of two inter- 

 vals, one in a south and the other in a north fur- 

 row of sahuaro No. 6. Jan. 29 to Apr. 20, 1904. 



