118 Hydration and Growth. 



of the author by Dr. J. M. McGee at the Desert Laboratory in 1916. 

 Mature jomts of Opuntia were arranged vertically in two sets, one with 

 all in a meridional position and the other in an east-and-west position, 

 so that the edges only were exposed to the rising and setting sun. Sets 

 of two each were observed for short periods, in which the joints were 

 kept moving at a rate which kept the edges or the full surface exposed 

 to the direct rays of the sun during the entire day.^ 



The course of the temperature on two days is shown in figure 22. 

 The joints which were in a fixed north-and-south position showed a 

 dry weight slightly in excels of those which were in an east-and-west 

 position. The temperature of the plants facing north and south rose 

 steadily until about 2 p. m,, then began to decline, falling to a point 

 below that of the air soon after sunset. Joints facing east and west 

 rose in temperature until about 11 a. m., then a shght drop took 

 place until the sun shone on the western face of the joint, at which 

 time it began to rise, reaching a maximum at 4 p. m. It is evident 

 that the position of these members may through the temperature- 

 relation modify the rates of growth or of hydration. During midsum- 

 mer the mature joints may attain daily temperatures of 53° to 55° C. 

 The data obtained by the author show that such delicate members as 

 etiolated shoots, as well as green joints, may endure temperature of 

 51.5° C. and to show enlargement a degree or two below that point. 

 (See page 135.) 



The newly developed method for determining temperatures of leaves 

 by a thermo-electrical method developed by Mrs. E. B. Shreve at the 

 Desert Laboratory promises to be of great use in the accurate deter- 

 mination of this important condition in plants.^ 



A series of measurements were made with joints of Opuntia grown 

 at the Coastal Laboratory, in which the degree of hydration was high, 

 so that the total possible swelhng in fresh material was small, although, 

 as may be seen by reference to page 132, the increase of dried sections 

 was indicative of the presence of a carbohydrate-protein complex of a 

 high hydration coefficient. 



Two recently matured joints grown in the open, and subjected to 

 air-temperatures of 16° to 22° C, were taken for each test. The sections 

 at the highest temperature had an average thickness of 9.4 mm. Those 

 used in the tests at 24° to 25° C. were about 12 mm. in thickness, and 

 the third lot had an average thickness of about the same. 



Trios were placed in glass dishes containing about 25 c.c. of Hquid, 

 the total volume of the sections being about 3 c.c. Temperatures were 

 taken by mercurial thermometers, the bulbs of which were thrust into 

 the dishes from time to time. The testing-chamber was not illumi- 



^ McGee, J. M. The effect of position on the temperature and dry weight of joints oi Opuntia. 

 Rep. Dept. Bot. Research, Year Book No. 15 for 1916, Carnegie Inst. Wash. 1917. 



* Shreve, E. B. A thermo-electrical method for the determination of leaf temperature. 

 Plant World, 22: 100. 1919. 



