334 EDITH BELLAMY SHREVE 



sponding temperature changes. The twenty readings showed a maximum differ- 

 ence of 4% in the value of the change of volume per degree. A correction fac- 

 tor of 0.157 cc. per degree was found by averaging twenty readings. At each read- 

 ing for the water-intake of the plant, the temperature of the water was read and 

 thus the total amount of temperature change for the period found. The change 

 in volume per degree, as determined above, was then multiplied by the number 

 of degrees of change the water had undergone and this correction subtracted 

 from the total change in level. The tables contain the corrected amount of water 

 absorbed. 



The difference between absorption and transpiration gives the gain or loss in 

 water-content of the plant. These changes in water-content have been plotted 

 and appear in figure 7. A convenient point on the paper was selected to repre- 

 sent the original water-content and, using this as a starting point for plotting 

 the losses or gains, each new loss or gain was measured from the point immediately 

 preceding it. Measurements of the positions were made, as formerly, with a meter- 

 stick. 



Results: From table 7, it is seen that under average evaporative intensity 

 and with the normal day to night changes in light intensity and air temperature 

 the plants lose more water at night than they take up at the roots, and in the day 

 they take up more than they lose by transpiration. This means nothing else 

 than that the turgidityof the plant increases during the day and decreases during 

 the night. Simultaneous measurements of the position of joints show that in- 

 creased turgidity is accompanied by a rise and decreased turgidity by a drop of 

 the joint. 



The measurements taken on July 5-6, under increased evaporative conditions 

 show, as in experiment 7, that the movements for day and night have been reversed 

 and, what is more important, the results of these days show that the turgidity 

 changes still parallel the movements. 



Thus the evidence is that the short period movements are the direct result of 

 turgidity changes. In order to gain further proof of this fact the same kind of 

 measurements were taken in the next experiment by different methods. 



Experiment 10. 



Object: The simultaneous measurement of water-content changes and the 

 movement of joints. 



Material: Plants Nos. 30, 31, 32, and 34. They varied in age from about three 

 to eight years. All were transplanted from the open about eight weeks before 

 the beginning of the experiment. They became well established in their new 

 environment, as was shown by the considerable growth they made. During the 

 experiment their pots were kept saturated with water. 



Method: The movements were measured by means of automatic pen tracings 

 on a drum. Figure 9 shows the details of the arrangement. The apparatus has 

 been described in a slightly different form by MacDougal 7 and has recently been 

 used by him for the growth measurements of Platyopuntias, with the lever arm 

 resting directly upon the joints. In order to adopt the apparatus to the measure- 

 ment of the movements it was necessary to make the lever which touches the joints 

 longer than the pen lever, in order to reduce the magnitude of the movement 



7 MacDougal, D. T., Practical text-book of plant physiology. (Longmans 

 Green and Co.), p. 291. 



