Chapter X 



191 



Loss and Retention 



have often been measured (Briggs and Shantz, 1916; Sayre, 1920; 

 Maximov, 1929a; Chung, 1935; Miller, 1938; and others). A more 

 critical analysis of the individual effects of various factors on the rate of 

 transpiration has been made through use of control cabinets where tem- 

 perature, humidity, light, and air flow are rigidly controlled (Davis and 

 Hoagland, 1928; Arthur and Stewart, 1933; Biale, 1941; Monter- 

 Moso and Davis, 1942; and Martin, 1943). 



2000 



/750 - 



23 



^■C 



25 



Days - u'une /9J2 



Fig. 53. — Diurnal fluctuations in DPD of the atmosphere at a station in Wash- 

 ington, D. C, during a 10-day period in June 1932. Data of Shull (1939). 



Rhythmic fluctuations in transpiration independent of environmental 

 conditions have been known for a long time (Kohl, 1886; Curtis, 1902; 

 Lloyd, 1908). Experimenting with Fouquieria splendens in a dark room 

 and as near constant temperature as possible, Lloyd found a variation in 

 transpiration rate. He felt that stomatal opening after midnight might 

 explain a rise observed in the transpiration at that time but that such 

 stomatal movement could not cause a lowering in transpiration rate after 

 the maximum was reached. The occurrence of an early morning maximum 

 in plants kept in total darkness was observed. The potometer method, us- 

 ing cut shoots, was employed, hence the fluctuations appear to be inde- 

 pendent of root pressure rhythm which Hofmeister (1862), and Grossen- 

 BACHER (1938) found. 



Using lemon cuttings and working in control cabinets with constant 

 darkness, temperature, and humidity, Biale (1941) verified the existence 

 of a rhythmic fluctuation in transpiration. Similarly, Montermoso and 

 Davis (1942) found such a rhythm in rooted Coleus leaves grown under 

 controlled conditions (Figure 54). The rigid control used in the cham- 

 bers permits no doubt as to the existence of this rhythm. By altering the 

 light and dark periods before placing the experimental material under con- 

 stant dark conditions, these authors were able to invert the maximum and 

 minimum points on the curve as shown in the figure. 



The work of Montermoso and Davis has not been carried to such a 

 point that a complete explanation of the phenomenon may be made. Un- 

 fortunately, no data on stomatal movement were presented. Two possible 



