AUTONOMIC MOVEMENTS IN SUCCULENTS 



335 



enough to get the whole tracing on the drum. These tracings have been reduced 

 and appear in figure 8. 



The measurement of water-intake and out-go was made by a method devised 

 by Briggs and Shantz 8 for the determination of the wilting point of succulent 

 plants. Briefly, the method consists in balancing the plant upon a knife edge 

 which is fastened at the juncture of pot and plant. The system is balanced in a 

 horizontal position by means of a movable screw; and then immediately returned 

 to a vertical position, where it remains until the close of the period considered. 

 At this time its point of equilibrium is again tested in the horizontal position. 

 The pot is sealed so that it can lose no water except to the plant. As the plant 

 loses water through transpiration, water moves from the soil and replaces that 

 lost by the plant and the pot side of the system becomes lighter and rises. This 

 process is repeated as often as the system is restored to equilibrium by means 



Plant No. 30. 



Jun. 20. Jun. 2a. 



Fig. 8. Correlation of movements of joints and turgidity changes in the 

 plant. Solid line, movements. Broken line, water-content changes. Two low- 

 est graphs indicate temperature and light conditions. See experiment 10. 



of the movable screw. If the plant does not obtain enough water to replace the 

 loss from transpiration the aerial side of the system will rise. The authors state 

 that by determining the loss in weight from the entire system on a pair of bal- 

 ances and comparing this with the weight which, when placed at the center of 

 pot or aerial part of the plant, will restore equilibrium, absorption and trans- 

 piration can be separated. This is true provided the conducting system is 

 able to pass up the water as fast as the roots take it in. 



The calculation is not quite so simple as might be supposed from the author's 

 statement. Furthermore, the accuracy of the results is limited by the fact that 

 an estimation of the center of transpiration loss must be made, and this involves 

 two sources of error, the first being that it must be assumed that all of the upper 

 part of the plant transpires at the same rate, and the second that there is difficulty 



8 Briggs, L. J. and Shantz, H. L., The wilting coefficient for different plants 

 and its indirect determination. Dept. Agr. Bur. Plant Ind. Bull. No. 230, pp. 

 47-51. 



