22 THE WATER-BALANCE OF SUCCULENT PLANTS. 



period of high temperature occurred, the thermometer indicating as high as 

 96° F. The last rain before this was on March 5 , and amounted to only 0.25 

 inch, so that all conditions were favorable for contraction; but during this 

 time of high temperature the measurements made March 20 showed expan- 

 sion of 13 intervals. Similarly, the measurements of April 10, 1907, showed 

 that 8 intervals were expanding. As already stated, no rain had fallen 

 since March 5, but between April 8 and 15 high temperatures prevailed, 

 the thermometer reaching 97° F. Again, from May 15 to 28 of the same 

 year, no rain having yet fallen and the thermometer reaching as high as 

 99° F., 5 intervals on sahuaro No. 13 were found to have expanded when 

 the measurements of May 25 were made. 



These data appear to furnish sufficient evidence of a slight expansion of 

 parts of the sahuaro trunk in response to rise of temperature. It must be 

 remembered, however, that these changes are local, and in no case do they 

 represent more than a small percentage of all the intervals measured, the 

 larger number acting in such a way as to afford no indication of the inflix- 

 ence of temperature. But when all has been said, the consistency observ- 

 able in the cases reported leaves no room for doubt that there has been a 

 reaction to a definite stimulus, and no other cause than rise of temperature 

 is suggested by the observed facts. 



Cases in which contraction is observed to follow lowering of temperature 

 are less numerous and the evidence is less conclusive, but this would 

 naturally follow from the fact that cold periods in the winter usually come 

 just after a rain, so that whatever effect the fall of temperature might have 

 would be offset, wholly or in part, by increased absorption. There is, how- 

 ever, at least one place in the records where there are indications that a 

 fall of temperature induces contraction. From February 3 to 13, 1905, 

 was a period of continued cold weather, the mercury falling to 34° and 

 35° F. There was rain on February 4, 6, and 7, over 1.5 inches falling 

 on February 6. The intervals had not yet reached the limit of their ex- 

 pansion for the year, and under the conditions we should expect them to 

 expand. As a matter of fact, the larger number did so, but at least 14 

 intervals on different individuals were found to have contracted. 



Further evidence of the influence of temperature is obtained from the 

 data regarding the time and conditions under which the intervals reach 

 their greatest expansion. In the main this is controlled by the time of 

 greatest precipitation, but study of the records makes it plain that this is 

 not the only factor involved. The intervals expand rapidly after the first 

 winter rains, reach their maximum, and remain there, with slight varia- 

 tions, until the ground begins to dry. If the rains mostly come in the fall 

 and stop early, the period of greatest expansion is in the winter, but if 

 they continue even as light rains in the late winter and spring, intervals 

 that have remained almost stationary for two months during the heavy 

 rains will expand a little more. 



