14 THE WATER-BALANCE OF SUCCULENT PLANTS. 



Sahuaro No. 13 was chosen for observation with direct reference to as- 

 certaining the differences in behavior of the north and south sides of the 

 same plant. On each of these sides a furrow was selected in which seven 

 intervals were marked at corresponding- heights, and measurements of 

 these gave the data for comparison. The curves for this plant show little 

 difference as to promptness of response to change in amount of soil-water, 

 except that in March, 1907, some of those on the south side began to fall 

 earlier than the others (fig. 3); but there is a marked difference in the 

 amount of contraction and expansion on the two sides and a plain reversal 

 in the summer of what had been observed in the winter. This is shown 

 most plainly in the descending curves indicating contraction, which will 

 be considered first. 



In the fall of 1906, between September 7 and November 19, the inter- 

 vals on the north side contracted severally from 2 to 18 units, while those 

 on the south side contracted from 8 to 31 units in the same period. But 

 in the summer of 1905 the measurements taken June 27 showed that since 

 May 30 the intervals on the south had contracted from 1 to 8.5 units, 

 while those on the north had contracted from 3 to 18 units in the same 

 time. The same condition of affairs was shown, though in a less marked 

 degree, in the summer of 1906, when, between May 19 and June 25, the 

 intervals on the south side contracted from 2 to 13 units, and those on the 

 north from 2.5 to 21 units. 



Without attempting an exact statement as to the modifying nature of inso- 

 lation on expansion and contraction, the results of the observations may 

 be summed up in the following: 



The precipitation records in connection with the curves of expansion and 

 contraction show very clearly that there are two principal seasons of con- 

 traction, i. e., the dry times succeeding the summer and winter rains, and 

 that the maximum contraction is reached at the end of these periods. 



April, May, and June are usually months in which slight precipitation 

 occurs, and the rapid drying of the soil is followed by contraction of the 

 intervals. After the summer rains are over there is another long, dry 

 season lasting until the winter rains begin, which may be in November, 

 or, in some years, not until January, and contraction of the intervals again 

 takes place. It is, as a rule, the early summer and the late fall in which 

 the sahuaros reach their maximum contraction, and this is manifestly due 

 primarily to the drying of the soil. 



It is equally plain, however, that insolation is a potent auxiliary factor. 

 To demonstrate this, an equal number of intervals on the north and the 

 south sides of different sahuaros, 52 intervals in all, were located within 

 50 degrees or less of the meridian, and the times of their maximum con- 

 traction noted. It is impossible to give a complete tabulated statement of 

 the times when the point of maximum contraction is reached, since the 



