FORM-ALTERATIONS AND GROWTH OF CACTI. 15 



intervals on only a few of the sahuaros under observation were measured 

 in the summer, and for two years the first measurements were not made 

 until after they had begun to expand subsequently to the first winter rain; 

 but from the observations that were made it is clearly seen that the ma- 

 jority of the intervals reaching their maximum contraction from June to 

 September were on the north, and of those which did not do so until just 

 before the winter rains the majority were on the south side. 



An accumulation of data of this sort, for the most part omitted here, 

 makes it certain that insolation exerts a marked influence on the expansion 

 and contraction of the trunk of the sahuaro and that, as regards contrac- 

 tion, the prolonged insolation of the north side for a period which includes 

 or is near the summer solstice results in reversing the relations observed 

 in the winter. The inference seems plain that strong insolation is followed 

 by a high rate of transpiration on the side presented to the sun, and this 

 is followed by more rapid contraction of the tissues on that side of the 

 plant. It is obvious that this fact must be borne in mind in any attempt 

 to express quantitatively the extent of mechanical adjustment to varying 

 quantities of stored water of which the sahuaro is capable. 



As regards expansion, the most important facts thus far established may 

 be seen at a glance by a reference to the records, or simply by an inspec- 

 tion of the corresponding curves. Referring again to sahuai'o No. 13, it 

 is noted that the first measurement after the rains of September 24 and 28, 

 1905, was made October 2 (fig. 7). All the intervals showed expansion, 

 those on the south side from 1 to 17 and those on the north from 3 to 18 

 units. On the date of the next measurement, October 10, only 3 intervals 

 on the south had continued to expand, while on the north 6 out of 7 showed 

 further expansion, which in one case was continued to October 16. Be- 

 tween June 27 and July 19, 1905, 4 intervals on the south side expanded 

 and 3 contracted, while on the north side all expanded, one as much as 12 

 units. A year later, between June 25 and July 30, 1906, the intervals on 

 the south side expanded variously from 1 to 12.5 units, and those on the 

 north side from 2 to 19 units. After the rain of November 18, 1906, the 

 intervals on the south side of this plant expanded more than those on the 

 north, but they also began to contract sooner, so that the northern fur- 

 rows remained at the maximum of expansion nearly two weeks longer 

 than the southern. 



The curves for intervals I (south) and II (north) (fig. 8) for sahuaro 

 No. 6 show that the interval on the south side responded more quickly 

 to the rain of February 6, 1904, but that on the north continued to expand 

 longer and was still expanding March 11, when the interval on the south 

 side had contracted to the same dimensions it had before the rain. From 

 a comparison of these two curves it would seem that the tissues where 

 interval II was measured are more elastic than those at I, and since it is 



