DEPARTMENT OF BOTANICAL RESEARCH. 79 



used. The non-conformity of the results with those obtained by Dr. 

 H. M. Richards on the same material (see Year Book 1911, p. 66, and 

 1912, p. 66) is probably due to a difference in the stage of metabolism 

 which existed at the time the plants were placed under controlled con- 

 ditions. Until this matter is given further investigation it is obviously 

 impossible to draw conclusions regarding the relations existing between 

 acidity changes and movements, or acidity changes and transpiration. 

 Data from the transpiration studies, the second main division 

 referred to above, are now being assembled and include the results of 

 many new experiments as well as those reported upon last year. The 

 following is a provisional summary of the results as far as they have 

 been elaborated: 



(1) Progressive desiccation of soil and tissues in Opuntia versicolor 

 is accompanied by a change in the relative transpiration (transpiring 

 power). This change is of such a nature that, while relative trans- 

 piration is greater by night than b}^ day when the plants are turgid 

 and supplied with plenty of water, by the time extreme desiccation is 

 reached the relative transpiration for the day is greater than for the 

 night. The change in relative transpiration is brought about largely 

 b}^ a decrease in the absolute night transpiration. 



(2) Experiments with severed joints show that it is the water-content 

 of the tissues which determines the transpiring power and that the 

 water-content of the soil is responsible only as it influences the condition 

 of the tissues. The characteristic day-to-night changes in transpiring 

 power take place for several days after the joints have been cut from 

 the roots; consequently, it can not be changes in the amount of water 

 absorbed which cause changes in the transpiring power. 



(3) Simultaneous measurements of water-intake at the roots and of 

 transpiration show that, under normal turgid conditions, transpiration- 

 rate is not the major factor governing the amount of intake. 



(4) Thus, from the results given under 2 and 3 above, it appears that 

 there exists some internal factor, or factors, controlling both transpira.- 

 tion and absorption. 



(5) The absorption capacity suggests itself as a possible internal 

 factor causing changes in water-intake and transpiration. Measure- 

 ments of the amount of distilled water absorbed by cylinders of tissue 

 cut from the stem show that water-absorbing capacity of the tissues is 

 correlated positively with the water-intake by the roots, and negatively 

 with transpiring power. So far these measurements have been made 

 for plants under normal conditions only. 



(6) Further experiments confirm the statement that, under normal 

 conditions of light and temperature which exist during the 24 hours, 

 cyhnders of tissue cut from joints show the greatest swelling capacity 

 when their acidity is lowest, and vice versa. 



