DEPARTMENT OF BOTANICAL RESEARCH. 99 



while during the day the intake is greater than or at least equal to 

 the outgo. When the movements of a plant were measured simul- 

 taneously with the water intake and outgo, it was found that the 

 upward motion in the day is accompanied by an increase in the amount 

 of water in the plant and that a downward motion at night is accom- 

 panied by a decrease in the amount of water in the plant. If the 

 plants were placed under excessive evaporation conditions in the day 

 time, a slight upward motion occurred, but was followed by a large 

 downward motion, while at the same time a decrease in the water left 

 in the plant occurred. The night following such a day always showed 

 an increase in water-content and a rise of the stem. So that, without 

 exception, when intake is greater than outgo the stems rise or straighten 

 and when intake is less than outgo the stems fall or curve. 



An examination of the water-content of stems from plants in the 

 open and from the green-house showed that the highest water-content 

 is at 5 p. m. after the close of a bright day, and the lowest just before 

 daylight the next morning, with an intermediate amount at noon. 



Small cylinders of equal dimensions were cut from the center and 

 from the outer cortex of stems, and their water-absorbing capacity 

 was tested by immersion in distilled water, the gain in weight per gram 

 of dry weight being determined by weighing at definite intervals. The 

 original water-content and the dry weight of these same pieces were 

 also determined. Acidity determinations were made of similar pieces 

 taken at the same time of day, the amount of acid being expressed in 

 terms of the equivalent cubic centimeters of N/50 NaOH required to 

 neutralize the acid in one gram of water in one gram of dry weight. 

 The results show that, at all times of day and night, both water-content 

 and absorbing capacity are greater in the outer than in the central 

 part, and that the acidity of the center is always greater than that of 

 the outer cortex. The absorbing capacity and water-content of the 

 entire stem are greater at 5 p. m. than at 5 a. m. and the acidity is 

 higher at the morning hour. The absorbing capacity and water- 

 content increase more during the day for the outer than for the inner 

 region. This constitutes a second line of evidence that the greater the 

 acidity^ the less the absorbing capacity of the material. 



Thus at least a partial explanation of the cause of the movement is 

 found in the varying water-content of the stems, which is caused by 

 a varying absorbing capacity of the plant material. The greater trans- 

 piration rate at night is also explained by the same phenomenon. 

 The chain of events is somewhat as follows: At night the absorbing 

 capacity of the tissues becomes less and they therefore more easily 

 give up for transpiration the water which they contain; at the same 

 time they have a lessened ability to take up the water, to replace that 

 given off by transpiration, and thus the water-content of the plant 

 becomes less at night. During the day the absorbing capacity increases, 



