VARIATIONS OF THE WATER-BALANCE. 



55 



root-system and regained its original volume in a period under 100 days 

 in length. How much less than this can not be told, since no examina- 

 tion was made in the interval. 



On October 16, 1909, the plant had been duly watered and cared for in 

 the glass-house, so that growth ensued much as in plants under the usual 

 habitat conditions. The apex had expanded, showing a clear central area 

 surrounded by the reddish, newly-formed spines, which however, had 

 hardly reached their full length. The whole preparation was now brought 

 into the shaded laboratory, and the spaces between the body of the plant 

 so wedged with cotton wool that the evaporation from the soil would be 

 reduced to a negligible quantity and the roots left undisturbed in the soil, 

 from which they might continue to draw water for some time. The total 

 weight was now found to be exactly 26 kg. The vessel containing the 

 soil was of tin and no water could be lost through its walls. 



On October 21, 1909, the weight was 25.895 kg. The entire prepara- 

 tion was now set out on the stone wall in the courtyard of the laboratory 

 with No. 9. Both were covered b\' a sash of glass to shield them from 

 the direct action of the rain. 



The records on various dates from November 8, 1909, to May 13, 1910, 

 are given in table 16. 



Table 16. 



It is to be seen that this preparation displayed a rate of loss of 60 grams 

 daily in October and November, 30.5 grams daily during November, 

 December, and early January, 32.5 grams daily during the greater part of 

 January, which increased to 40.5 grams daily during February; then, with 

 conditions of the arid foresummer becoming increasingly favorable for 

 transpiration, the rate fell to 30.3 grams daily during March, 23 grams 

 daily during April, and 19.9 grams daily during May. The lessened rate 

 may be attributed to the increasing concentration of the sap, as the plant 

 had now apparently been depleted of over half of its original store. 



