FORM-ALTERATIONS AND GROWTH OF CACTI. 



25 



On March 2, 1909, after the winter rains were over, the three intervals 

 together measured 45 units, or about five-eighths of an incli less than on 

 March 4 of the preceding year, and this in spite of the fact that some 

 water had reached the roots of the plant by artificial means. After this, 

 at several times, the plant was watered (fig. 12), but not enough to take 

 the place of the thorough wetting of the ground from a rain. The plant 

 responded by an expansion of the furrows, but the increase only amounted 

 to 12 units, leaving them considerably smaller than in previous years. 



Table 5.— Greatest expansions reached by the intervals of sahuaro No. 1 for 5 years. 



It therefore seems plain that (l) the sahuaro has a limit of expansion, 

 and when it is reached the water taken up by the roots is limited by the 

 amount of transpiration; (2) that the maximum expansion is reached only 

 when the soil is saturated with water for some time. 



VARIATION IN WATER-CONTENT. 



For the determination of the water-content of the sahuaro trunk two 

 individuals were selected, one of which measured 211.5 cm. and the other 

 176 cm. in height. These were cut down May 22, 1907, after a long, dry 

 period, there having been only 0.5 inch of rain since February 1, and this 

 so scattering that relatively small amounts could have been available for 

 the plant. Under these circumstances the percentage of water would be 

 less than if the determination had been made after a rainy period. On the 

 other hand, the individuals were young and in normal condition, so the pre- 

 sumption is that the results are as nearly representative as could well be 

 obtained. 



Three sections, each measuring 3 cm. in thickness, were taken from 

 each individual, one 6 cm. above the ground, one just below the apex of 

 the trunk (just below where the marked curvature begins), and the third 

 halfway between these two. A quarter of each section was placed in a can 

 immediately after cutting, and the can then tightly closed to prevent loss 

 of water. In each case weighings were made of the can and its contents, 

 after which the piece of sahuaro was removed and thoroughly dried in a 



