8 THE WATER-BALANCE OF SUCCULENT PLANTS. 



was a rain of less than half an inch, and by November 19 some of the 

 furrows showed a marked expansion. This was followed by other rams, 

 and afterwards most of the sahuaros expanded steadily for over a month 



(figr. 3). 



I 1906 

 Oct.3 



II 107 



IV 80 



V 75 



VI 63 



IX 130 

 XI 143 



N 



xn 110 



xni 115 



Nov. 

 17 £1 

 3 7 14 ,19, 27 



Dec. 



15 22 



Jan. 1907 

 5 10 21 



Feb. 



4 9 IB 27 469,15 20 29 



March 

 J', I 



April 

 I 5 10 ZO 2! 



May June 



6 13 26 



le eV ih 



Fig. 3. — Eight curves from measurements of four intervals in a north and four in a south 

 furrow of sahuaro No. 13, at corresponding heights on the two sides as follows: Inter- 

 vals II and IX, 12 inches; IV and XI, 3 feet; V and XII, 3 feet 11 inches; VI and 

 XIII, s feet 3 inches. October 31, 1906, to June 6, 1907. 



But it is not only these marked responses with great expansion that are 

 significant. The sligrht increase, determined only by exact measurements, 

 which follows light showers, and the barely perceptible contraction which 

 occurs when the earth has just begun to dry, afiford fully as conclusive 

 criteria. As little as 0.25 inch of rain is sufficient to produce a distinct 

 expansion, as is shown by the measurements following the rain of March 

 5, 1907 (fig. 4). Before this there had been a slight contraction, there 

 having been only 0.1 inch of rain since January 30, but after the fall of 

 0.25 inch on March 5, measurements of many intervals showed a distinct 

 expansion which could hardly be due to any other cause. 



