FORM-ALTERATIONS AND GROWTH OF CACTI. 7 



REVERSIBLE ALTERATIONS. 

 Direct Adjustment to Varying Amount of Soil- Water. 



If, after the earlier work to which reference has been made, any doubt 

 remained as to the direct relation of the contraction and expansion of the 

 trunk of the sahuaro to the amount of available water in the soil, it would 

 be removed at once by comparing the measurements of a single furrow 

 extending over a period of nearly five years with the rain record for that 

 time. On no occasion had this relation been more apparent than in the 

 winter and spring of 1903-04 (fig. 2) and the fall of 1906 (fig. 3). The 

 former period was a time of extreme drought, and out of 56 intervals* 



Fig. 2. — Three curves from measurements of three intervals on the north side of 

 sahuaro No. i, December lo, 1903, to May 12, 1904. 



measured, 19 reached the very lowest measure in that period. The rapid 

 response to a half inch of rain, February 6, 1904, is plainly seen in the 

 curves (fig. 2). In the fall of 1906 there had been but 0.29 inch of rain 

 from August 20 to November 18, and out of 84 intervals measured within 

 this period, 48 reached the lowest measurement for four years between 

 October 31 and November 19. On the night of November 17-18 there 



*The term "interval" is adopted for convenience to indicate the measured distance 

 between two points on opposite sides of a furrow or rib. In some cases several intervals were 

 located in the same furrow. 



