FORM-ALTERATIONS AND GROWTH OF CACTI. 



Individual and Local PECtfLiARiTiES. 



Long--continued measurements have brougrht to light several minor facts 

 which, while they do not affect the main question, are of sufficient impor- 

 tance to record. As might be expected, each plant has a marked individ- 

 uality, and it was possible before the measurements were made to predict 

 with considerable accuracy what their deportment with regard to present 

 conditions would be. One plant responded to rain more quickly than 

 another, or in a more marked degree, and some expanded for a longer 

 period and were slower to contract when the weather became dry. The 

 latter class usually included the more vigorous individuals, with larger 

 trunks and wider furrows. There was no external indication of a cause 

 for this condition of affairs, but it seems likely that in such cases the 

 water was in some way held longer in the soil 

 or pockets of rock to which their roots had 

 access. 



As already noted elsewhere, not only the in- 

 dividual plant, but parts of the same plant, re- 

 spond differently to variations in amount of 

 water in the soil, according to their location 

 as to height and to points of the compass . But 

 aside from this there are sometimes marked 

 differences in the action of two intervals so 

 close together that neither height nor ex- 

 posure to the sun could account for the dif- 

 ference. Perhaps the most marked case of 

 this kind is that of intervals Nos. I and VI on 

 sahuaro No. 4 (fig. 5). Both are on the 

 southwest side of the stem, little more than 

 10° apart, and No. I is but 3 inches higher than 

 No. VI, the latter being located just below the 

 insertion of a rib where the furrow forks. As 

 would be expected from their proximity and 

 general similarity of position, both exhibit the 

 same general response to changes of water- 

 supply, the curves rising and falling together with much > regularity; but 

 VI plainly shows more sensitiveness and mobility, and its changes of width 

 are much greater than those recorded by No. I. Thus on May 20, 1905, 

 No. I measured 118.5 units* and VI 163.5 units; but between May 20 and 

 May 30 No. I contracted 1.5 and VI 15.5 units; and by June 27, No. I had 

 contracted 7 and VI 42 units more. By August 21 No. I was actually wider 

 than VI, but conditions were reversed again as soon as the rain came. 



Fig. 4. — Curves from four in- 

 tervals on sahuaro No. 4, and 

 one from No. 7, showing re- 

 sponse to one-fourth inch of 

 rain on March 5, 1907. (See 

 also fig. 3.) 



*A unit equals one sixty-fourth of an inch. 



