26 



THE WATER-BALANCE OF SUCCULENT PLANTS. 



ventilated attic. As this was done in summer, when there was a very low 

 percentage of atmospheric humidity, it is safe to assume that very little 

 more loss of weight would have resulted from any other method of artifi- 

 cial drying. Accordingly the weights before and after drying and the 

 water-content in per cent of the original fresh weight, as given in table 

 6, may be taken as correct within a narrow limit of error. 



Table 6. — Amount of water in sahuaro trunk obtained by air-drying at ordinary temperatures. 



From the data thus obtained it is seen that for the sahuaro 211.5 cm. 

 high the total amount of water in an average section 9 cm. thick is 5-048 

 liters, and the amount in the whole plant (not allowing for narrowing at 

 the apex) 118.618 liters. For the sahuaro 176 cm. high the total amount 

 in an average section 9 cm. thick is 3.302 liters, and in the whole plant 

 (not allowing for narrowing at the apex) 64.576 liters. 



A more accurate and complete analysis was made of one plant growing on 

 the slopes west of Tumamoc Hill on October 17, 1909. This plant con- 

 sisted of a single cylindrical trunk 5.6 meters high, with a maximum diam- 

 eter at the largest part of 60 cm. The total weight was found to be 

 767.8 kg. A sample including the normal proportion of spines, epidermis, 

 cortex, wood, and medulla, weighing 3.024 kg., was taken for water deter- 

 minations, being dried in an oven. From the data thus obtained it was 

 found that the body of the plant contained 91 per cent of water, which may 

 be taken as being near the maximum, since the plant was very turgid. 

 The highest water-content is found near the apex of the trunk and the low- 

 est near the base, in which the woody cylinder occupies a large part of 

 the cross-section. 



By quite a different method the attempt was made to calculate the 

 amount of water which might accumulate in a trunk within a stated period. 

 Sahuaro No. 11, a plant some 6 meters in height, had the spines removed 

 in two circles about the trunk, one about 2 meters, the other about 0.65 

 meter from the ground. The circumference was accurately measured at 

 these places by a metal tape. By this means the extent of shrinkage dur- 

 ing drought and of expansion after rain was determined for the entire 

 circumference. 



The extreme differences in the circumference at the upper place of 

 measurement between the time of the greatest contraction (November 14, 



