12 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



in volume may be estimated at about 744 c.c; but when weig-hed its loss 

 was only 70 grams. This discrepancy can only mean that in the part 

 remaining- 674 c.c. must be accounted for as air-space, and that the soft 

 tissues are in a state of tension or neg^ative pressure, making them very 

 responsive to any external or internal change of condition; and a response 

 to such change might easily be expressed by a local shifting of pressure, 

 the outer shell of the sahuaro being sufficiently flexible to permit this to 

 show itself to a measurable degree. 



DIFFERENCES IN AMPLITUDE OF VARIATION FROM BASE TO APEX 



OF TRUNK. 



But, aside from these irregular variations, the measurements have made 

 it possible to establish certain facts in reg-ard to the difference of response 

 of different parts of the same sahuaro to changes of condition, thus con- 

 firming what was inferred from the work of the first year, though not fully 

 established at that time. 



It is hardly necessary to repeat that both contraction and expansion are 

 less in the lower, woody parts of the plant, but it is of interest to compare 

 a series of curves made from measurements at different heig-hts along the 

 same furrow, as in Nos. 12 and 13. On the latter the lowest point was 5 

 inches and the highest 6 feet from the ground, the plant itself being 12 feet 

 or more in height, the variations of the intervals increasing' upward (fig. 

 7). On No. 12, however, a plant about 6 feet high, the variations were 

 less at the top and bottom and greatest in the middle. In table 1 the 

 numbers grouped are for the same furrow and show that in this case the 

 greatest variation occurs in each furrow about halfway from the base to 

 the apex of the trunk. The variation is ascertained by subtracting the 

 width of the furrow at its time of greatest contraction from that of its 

 greatest expansion for the entire period during which records have been 

 kept. The variation for any one year would be somewhat less than this. 



Table 1. — Variations at different heights in three furrows of one plant {No. 12). 



In such a case as the one just described the solidity of the tissues at the 

 base of the trunk prevents wide variation, while in the upper part of the 

 plant the tissues are still growing, and the turgidity of growth necessarily 

 interferes with the processes of contraction and expansion; the furrows are 



