48 THE WATER-BALANCE OF SUCCULENT PLANTS. 



an analysis of the portion of the trunk corresponding to the samples pre- 

 viously treated. The data obtained are given opposite No. 2 and No. 3 in 

 table 14. 



Another specimen taken on August 10 yielded the data shown opposite 

 No. 4 in table 14. 



In order to obtain data upon which to base an estimate of the amount of 

 water in the bodies of sahuaros an individual consisting of a single columnar 

 trunk growing near the southwestern corner of the domain of the Desert 

 Laboratory was cut down on October 17, 1909. It was found to be in a 

 condition of extreme turgidity and measured 5.6 meters in length, repre- 

 senting the height above the surface of the soil , while the diameter at the 

 swollen middle portion was 60 cm. The trunk was cut into sections of 

 about 60 kg., which were weighed on a balance having a draft of 100 kg., 

 the whole plant having a draft of 767.8 kg. A sample, including one 

 ridge with spines and a section of the cortex and wood extending to the 

 center of the medulla, was taken for a water determination. Its weight 

 was 3.024 kg., and the residue after drying was 276 grams, indicative of 

 an original water-content of 2.748 kg., or 91 per cent of the sample. The 

 entire plant therefore held 698 liters of water. The age of this plant may 

 be estimated as about 60 years. 



Plants of maximum size, 12 to 20 meters in height, would contain from 

 2,000 to 3,000 liters of water. Such individuals might lose from 1,000 to 

 1,600 liters of water during a protracted arid period and still survive. 

 The results of the observations described on page 54 suggest that the total 

 loss might be regained within a few days if the drought were followed by 

 sufficient precipitation. No noticeable growth would ensue, however, in 

 the desiccated condition, and the lessened rigidity of the trunks and the 

 branches would render them especially liable to be broken by the action of 

 the wind. 



The acidity of the sap seemed to bear no direct relation to its concen- 

 tration, the lowest being found in a plant growing in the open at the end 

 of the dry foresummer, while it was slightly greater in a plant that had 

 lost 63 per cent of its water-balance and was highest in a plant examined 

 at the end of the dry foresummer. The proportion of total solids carried in 

 solution by the sap increases with desiccation, but not at the same rate as 

 the dissolved salts. The latter may increase from 1 to 3 per cent, while 

 the total solids vary between 3.4 per cent in the most turgid condition to 

 5.9 per cent in the most desiccated state, which indicates an organic content 

 of 2.34 per cent in a plant containing a maximum amount of water, 2.8 and 

 3.6 per cent in the sap of plants at the end of the dry foresummer, and 3.4 

 per cent in the plant which had lost 63 per cent of its water-balance. 



The water-balance of the sahuaro doubtless constitutes a very important 

 factor of safety, since the vicissitudes of the climate of a large part of its 



