VARIATIONS OF THE WATER-BALANCE. 49 



habitat are such that a whole year might pass without sufficient precipita- 

 tion to yield a supply which would be equal to the transpiration loss. 

 The strata in which the anchorage of this plant is made are loosely placed 

 and the shiftings which would disconnect the absorbing organs are fre- 

 quent. The large balance would allow the extension of the root-system 

 in a way that would remedy the defect, although during the interval in 

 which the plant was drawing upon its accumulated balance but little 

 growth would ensue. 



Another form of survival is to be seen when the trunk dies and large 

 branches remain alive. The death of the trunk often results from infec- 

 tion of wounds in the trunk, however slight, in the rainy season, and the 

 soft outer tissues decay quickly, leaving the bare woody skeleton, upon 

 which two or more branches are often held in place, apparently untouched 

 by the disintegration of the remainder of the living tissues. Many exam- 

 ples of this have been observed. 



One illustration occurred on Tumamoc Hill, near the Desert Laboratory, 

 and was photographed on March 4, 1908. In this instance one large branch 

 and one small secondary branch remained alive . The large branch was care- 

 fully freed from the dead skeleton and its woody base set in a box of soil in 

 the greenhouse on July 8, 1908, but its tissues soon began to turn yellow and 

 decay began by July 30, partly as a result of the heightened temperatures. 



A second example in the same locality, bearing two large branches, was 

 followed through the autumn and winter wet season of 1909-10. (See 

 MacDougal, Across Papagueria, Plant World, xi, page 93, 1908.) 



The most notable demonstrations of this kind, however, are to be seen at 

 a point 22 km. east of Tucson, on the bajadas that come down from the 

 southern base of the Santa Catalina Mountains. A dead skeleton was seen 

 on May 25, 1909, bearing three living branches, the trunk having decayed 

 during the previous year. Despite this fact, the living branches bore a 

 profusion of flowers. On September 21, 1909, one large branch remained 

 alive and a small globular secondary branch which had arisen on an isolated 

 green branch was alive, but would probably soon undergo desiccation. The 

 large water-balance seems to have a highly localized value and the woody 

 bases of the branches offer some resistance to the spreading of infection 

 and decay processes. Isolated members of many of the cacti are capable 

 of regeneration and thus effect vegetative propagation, but in no instance 

 has this been observed in the sahuaro, nor has it been accomplished 

 experimentally. It does not seem probable that it occurs in any part of 

 the range of the plant, since no root-formation seems to ensue, except 

 about the bases of the trunks. (Plate 7.) 



The bases of the trunks of the sahuaro, especially in small trees, are 

 girdled, probably by rabbits and mountain sheep, leaving the central cyl- 

 inder intact, in which condition they survive for extended periods and may 

 attain great age. 



