FORM-ALTERATIONS AND GROWTH OF CACTI. 



17 



and February, while most of the more slowly expanding- northern inter- 

 vals did not reach their maximum until March and April. 



Fig. 9. — Curves from measurements of two intervals, one in a north and the 

 other in a south furrow of sahuaro No. 6. Oct. 31, igo6, to May 2$, 1907. 



Table 2. — Periods in which the intervals attained their maximum expansion, by months. 



From the data here presented, and from measurements of other indi- 

 viduals which point in the same direction, it is evident that the simple 

 bellows-like movements of the ribs and furrows of the sahuaro, executed 

 in response to the absorption and loss of water, are appreciably modified 

 by the effect of insolation. There is no other apparent cause for the fact 

 that after a winter rain the ascending- curve of absorption of an interval on 

 the south side rises more promptly than does the corresponding curve of 

 an interval on the north side, and reaches a greater height, but falls more 

 quickly and to a lower point; while in summer, with more of northerly 

 insolation, the reverse proves to be true. It is equally clear that, so far as 

 mechanical adjustment is concerned, insolation acts as a secondary factor, 

 and that the curves of expansion and contraction, though distinctly modi- 

 fied, are not radically changed by its influence. But the effect of insola- 



