250 THE MOVEMENTS OF WATER 



reaction of the transpiring organs may be modified. In correspondence with 

 the climatic conditions, a yearly periodicity can generally be recognized in 

 the transpiration even of evergreens, the maximum naturally occurring during 

 summer. Even in the coldest winter transpiration does not entirely cease, 

 though it becomes comparatively trifling in amount. According to Guettard 1 , 

 a cypress-tree lost more water in six summer days than during the whole 

 of the winter. The amount of water transpired by a plant during successive 

 days is subject to considerable variation ; for many reasons also much 

 more water is transpired during a bright day than during the night, while 

 a heavy fall of dew may cause transpiration to be reduced to a minimum 2 . 



Even although the external conditions were kept perfectly constant, 

 the progress of development would still cause marked changes in the 

 transpiratory powers of the plant. Under constant conditions, however, 

 transpiration varies but little in the course of a day, or during shorter 

 intervals. Apparently, therefore, the process exhibits no marked periodicity 

 independently of the external conditions ; indeed various authors have been 

 unable to detect any independent periodicity at all, although Eberdt has 

 observed the occurrence of slight variations in the rate of transpiration under 

 constant conditions 3 . As a matter of fact, even when the external con- 

 ditions are perfectly constant, movements may occur, as well as changes 

 of the tissue tensions and of root-pressure, &c., and all of these may 

 exert a more or less marked influence upon transpiration. It is possible 

 that in certain plants variations in the width of the stomata may be pro- 

 duced as the result of certain of the various automatic or induced changes 

 continually occurring in plants. 



The amount of water which many turgid plants can transpire under 

 favourable conditions is very great. On bright summer days very commonly 

 i to 10 c.c. of water may be given off per 24 hours from a single square 

 centimetre of leaf-surface. Some plants, on the other hand, under similar 

 conditions exhale only T x th of this amount 4 . When a very large surface- 

 area is exposed, the total amount of transpiration may be very great: 

 Hales found that a sunflower having a total area of leaf-surface of about 



1 Guettard, Hist. d. 1'Acad. roy., 1749, p. 291. On the transpiration of conifers in winter, see 

 Hartig, Bot. Zeitung, 1861, p. 20. 



* Observed by Hales and also Unger, Sitzungsb. d. Wien. Akad., 1861, Bd. XLIV, p. 214; 



F. Haberlandt, Wiss. prakt. Unters., 1877, Bd. n, p. 151 ; J. Boussingault, Agron., Chim. agric., &c., 

 1878, T. vi, p. 299, &c. 



3 Baranetzky, Bot. Zeitung, 1872, p. 107 ; Barthe'lemy, Compt. rend., 1873, T. LXXVII, p. 1081 ; 

 Eder, Sitzungsb. d. Wien. Akad., 1875, Bd. LXXII, Abth. i, p. 374; Eberdt, Transpiration, 1889, 

 p. 93. Cf. also Burgerstein, II, p. 53. 



4 Fr. Haberlandt, Wiss. prakt. Unters. a. d. Geb. d. Pflanzen banes, 1877, Bd. II, p. 146; 



G. Haberlandt, Sitzungsb. d. Wien. Akad., 1892, Bd. ci, Abth. i, p. 807; v. Hohnel, Transp. d. 

 forstl. Gewachse, 1879 (Sep.-abdr. a. Mitth. a. d. forstl. Verauchswesen Oesterreichs) , and Forsch. a. 

 d. Geb. d. Agriculturphysik, 1881, iv, p. 438 ; Aubert, Ann. d. sci. nat., 1892, vii. scr., T. XVI, 

 p. 80 ; Bnrgerstein, Material, zu einer Monog. d. Transpiration, 1889, n, p. 55, &c. 



