5 



iiutl luurj^ans of jj^niss bliules aiul loaves ot" low y^rowiiif^" plants 

 early iu the eveuiiijt?. Absorption by the root continues quite 

 vigorously after sunset by reason of the favorable tempera- 

 tures, and the au^^niented amount of fluid in the cortex of the 

 roots sets up a jjressuri" which ultimately f(jrces water into the 

 central cylinder and up throuj>;h the woody cells faster than it 

 may be used and transpired by the thin-walled cells of tin; leaves. 

 The vessels become filled with water which is forced out in 

 lic^uid form through the excretory openings iu the form of drops. 

 In some species the amount of water coming away from the 

 plant in this manner may reach quite an appreciable (piautity. 



On the other hand the forenoon witnesses the rapid acceler- 

 ation of transjjiration by all parts of the shoot at a time when 

 the soil temperature is decreasing to a minimum. The in- 

 crease of transijiration continues until mid-afternoon, while 

 the temi)erature of the soil reaches a minimum two or three 

 hours earlier and then begins to rise, but does not do so suffi- 

 ciently to favor absorjition to any great extent. It is true of 

 course that the needs of the leaves may be j^artially met by the 

 activity of the rootlets w^liich lie nearer the surface. 



The greatest amount of variation during twenty-four hours 

 that was recorded during thej'^ear amounted to 2° C. (3.6° F.). 

 This amplitude was shown on two occasions. June 28, 1902, 

 the temperature rose from 8° C. (46.4° F.) at 10 a. m. to 10° C. 

 (50° F.) at 10 p. m., in consequence of an increased tempera- 

 ture of the air, reaching a maximum of 27.8° C. (82° F.) at 1 

 p. m. The temperature of the soil fell from 12° C. (53.6° F.) 

 at 11 p. m., on July 16, 1902, to 10° C. (50° F. ) at noon on the 

 following day. Curiously enough this minimum coincided 

 exactly with that of the air a meter above the surface four 

 hours earlier. A reverse movement of any kind greater than 

 1° C. has not been recorded within the limits of twenty-four 

 hours. These slow changes in the temperature of the soil 

 would exert no stimulative iutiuence upon the action of the 

 roots. Thus, for instance, a sudden and great diminution of 

 the temperature of a root temporarily would increase the rate 

 of absorption which, however, soon begins to decrease again. 

 The least dailv variations occurred during the winter mouths, 

 when it did not exceed (1° C. (1.8° F.). 



It is interesting to recall iu this connection some of my OAvn 

 observations on the temperatures of the roots of small herbs 

 and grasses in Arizona Avhich were growing in volcanic sand. 

 The observations were made by mercurial thermometers in 

 1898, and it was found that these organs Avere carrying on 

 their functions under temperatures as high as 44° C. (Ill -f 

 °F.) to 45.5° C. (114 °F.). 



The general course of the temperature and the nature of 

 the variations is shown by the accompanying tracings (see 

 tigs. 2 to 7) and the following data: 



