52 THE WATER-BALANCE OF SUCCULENT PLANTS. 



lations of the rate for extended periods. Furthermore, the rate is not 

 given except when changes ensued. It is to be noted that this plant was 

 kept in a room devoted to other uses and that the opening of doors and 

 windows and the occasional heating would account for some of the erratic 

 features of the curve of water-loss. Allowing for these, however, it is to 

 be seen that the rate of transpiration was much higher during March-June 

 of the first year than the corresponding period in the second. The maxi- 

 mum (29 grams daily) was reached early in July, 1908, at the close of the 

 dry foresummer, the rate for the previous month being between 19 and 20 

 grams daily. The average high rate of 14.4 grams daily persisted until 

 July 6, 1908, and it then fell to 11.6 grams daily for the period of 31 days 

 ending September 2, rising to 15.7 grams during September, and averaging 

 5.5 grams for the month ending November 4. The rate was 1 gram dur-. 

 ing November, 2.7 grams dviring December, 4.4 grams during January, 

 and about 3 grams daily during February and the first half of March. 

 During the last 14 days of March the rate was 3.5 grams daily. This is to 

 be compared with a rate of 14 grams daily during the corresponding period 

 of the previous year. During 42 days ending May 12, 1908, the rate was 

 11 grams daily, and during 43 days ending May 12, 1909, the loss was but 

 7 grams daily. The loss was 12.2 grams daily during the 31 days ending 

 June 11, 1908, while it fell to 8.2 grams daily during the 31 days ending 

 June 12, 1909. 



This plant weighed approximately 42 kg. when the experiment was be- 

 gun in March, 1908, and about 38 kg. of this amount may be estimated as 

 water; 3.666 kg. was lost during the first year. 



Some slight manifestations of growth were seen at the apex of the trunk 

 in August of 1908, during the first season of deprivation of a water-supply, 

 but none ensued in the following year. The rate of transpiration during 

 the second year was found to vary between one-fourth and two-thirds of 

 that of the previous year, although scarcely more than a tenth of the total 

 water-balance originally present had been lost. A comparison with the 

 data obtained from other specimens shows that the loss from this plant was 

 much less than would have occcurred had the plant been exposed to the 

 evaporating action of sunlight and air-currents. (See plate 8.) 



ECHINOCACTUS No. 2. 



On November 5, 1908, a small individual growing near the Desert Lab- 

 oratory was taken up and the roots cut away cleanly, after which it was 

 found to weigh 5.136 kg. A mounting of wire netting was provided and 

 the plant was set up in the same room with No. 1. 



On December 8, 1908, the weight was found to be 5.065 kg., indicating 

 a loss of 71 grams in 33 days, a rate much in excess of No. 1 for the same 

 period when comparison of weights or surfaces is made. The actual loss 

 from No. 1 during the corresponding period was 32 grams, although its 



