FORM-ALTERATIONS AND GROWTH OF CACTI. 



13 



narrower, so that the arc of contraction and expansion is shorter; and 

 furthermore the ribs arch and close together at the apex of the stem, thus 

 offering a mechanical obstruction to free movement. 



I 1905 

 June n 



n 



120 



no 



S 100 

 IV 



90 



140 



IX 



130 



N XI 



1+0 



XU 



120 



July 



Aag. 

 8 14 21 28 



Sept. 



II 18 25 



Oct. I 



10 16 23 30 



FlO. 7. — Cxirves constructed from measurements of six intervals, three in a northern and 

 three in a southern furrow of sahuaro No. 13, at the following heights: II and IX, 

 II inches; IV and XI, 3 feet; V and XII, 3 feet 11 inches. June 27 to October 30, 

 1905. (For rainfall data see fig. 5.) 



EFFECTS OF INSOLATION. 



After the first winter's measurements it was stated that " a comparison 

 of measurements from the north and south sides of the same plants shows 

 that the contraction is greater on the south than on the north side and 

 that, while the southern furrows may begin to expand earlier, the northern 

 ones expand longer." Further measurements have confirmed this state- 

 ment for the winter, the only part of the year during which, at that time, 

 observations had been made; but the summer records subsequently taken, 

 though less complete, indicate that for a period during which there is 

 greater insolation of the north side the reverse holds true. 



