VARIATIONS OF THE WATER-BALANCE. 



By D. T. MacDotjgal. 



PURPOSE AND SCOPE OF THE EXPERIMENTS. 



As soon as the measurements described in the previous section of this 

 paper were well under way, it became evident that some determinations of 

 the amount of the water- balance and its variations would be necessary for 

 the interpretation of the changes of form and volume revealed by these 

 measurements , 



A second lot of the plants was therefore selected in which the amount of 

 water present and its seasonal variations were determined by weig-hing-. 

 Attention was directed chiefly to obtaining data from succulents detached 

 from the substratum under conditions of natural exposure in the open air 

 and under the modified conditions furnished by the Desert Laboratory. 

 Some measurements were made of these detached specimens in order to 

 correlate changes in volume and form with variations in weight in the 

 same individual. 



The observations upon loss of water by plants separated from the sub- 

 stratum for extended periods secured facts bearing upon the effects of the 

 climatic factors of the various seasons, the relation of the water- balance 

 to growth and reproduction, and also justified some generalizations as to 

 the relation of these succulents to their habitats, especially concerning the 

 constructive efficiency of the water-balance as a factor in endurance and 

 survival. 



Chemical analyses of the sap of the plants used were made, and some of 

 the information secured has already been of great value in a consideration of 

 the conditions leading to parasitism, discussed in a previous paper (Mac- 

 Dougal and Cannon, Conditions of Parasitism, Publication No. 129, Car- 

 negie Institution of Washington, 1910). The fuller citation of the facts in 

 question has been made to demonstrate the relation between the state of the 

 water-balance and the varying composition of the juices. 



CARNEGIEA GIGANTEA. 



A sahuaro about 2 meters in length was taken from the bajada west of 

 Tumamoc Hill on May 8. 1908, the root-system was trimmed back to the 

 central cylinder of the stem, and set up on a base of loosely-piled stones, 

 being held in position by a rope girdle and guy- wires. On July 8 the body 

 was seen to be noticeably shrunken, the ridges being drawn together, thus 

 interlacing the spines, giving the plant a grayer aspect, by reason of the 

 covering of the green surfaces by them. The 61 days during which the 



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