INFLUENCE OF EXTERNAL CONDITIONS ON TRANSPIRATION 247 



darkness causes it to diminish. The difference is not generally very great, 

 however, so long as the illumination is feeble, but in plants whose stomata 

 close in darkness it may in certain cases become considerable 1 . The 

 heating effect exercised by direct sunlight increases very greatly the rate 

 of transpiration so long as the plant is turgescent. Light also eventually 

 influences the shape and development of plants to a very pronounced 

 extent. It is to this action, and especially to the different degree of 

 development of the cuticle, that etiolated plants, and those growing in 

 the shade, are able under similar conditions to transpire more actively 

 than others which have been grown from the first in strong light. 



The researches of Daubeny and Miquel left little doubt that transpiration is 

 increased by exposure to diffuse light, and this fact has been made certain by 

 Baranetzky, Wiesner, Hellriegel, Kohl, v. Tieghem, Eberdt, &c. 2 The increase 

 has not only been observed in green and etiolated plants, but Bonnier and Mangin 

 found it occur also in fungi. It cannot be therefore due merely to the opening of 

 the stomata caused by light 3 . 



As examples, a few of Wiesner's * results are given below ; in these the 

 amount of transpiration was determined by weighing. 



The small maize plants used had their roots attached to soil, while the cut stems 

 of the inflorescences or flower-stalks were immersed in water covered by a layer of oil. 

 From what has already been said, it is not surprising to find that transpiration does 

 not immediately attain a constant rate, and that the variations produced by changes 

 of illumination are not always precisely similar, and do not correspond entirely 

 with the results obtained by other investigators with different plants 5 . In the 

 following table Wiesner gives the amounts of water exhaled per hour from 100 

 sq. cm. of surface. 



The more active transpiration observed in the green maize plants exposed to 

 sunlight is apparently due to the large amount of light they absorb exercising 

 a pronounced heating effect upon them. V. Tieghem 6 speaks of this increased 

 transpiration as chioro-vaporization, but from this term it must not be supposed 



1 Stahl, Bot. Zeitung, 1894, p. 125. Cf. Sect. 31. 



2 See the literature by Burgerstein, n, p. 34, and Eberdt, Transp. d. Pflanze, 1889, p. 4 ; 

 Daubeny, Phil. Trans., 1836, I, p. 159; Miquel, Ann. d. sci. nat., 1839, ii. ser., T. xi, p. 43. 



3 Bonnier et Mangin, Ann. d. sci. nat., 1884, vi. ser., T. xvn, p. 301. 



4 Wiesner, Uber d. Einfluss d. Lichtes u. d. strahl. Warme a. d. Transpiration, 1876, p. 21 

 (Sep.-abdr. aus Sitzungsb. d. Wien. Akad., Bd. LXXIV, Abth. i). Kohl (I.e., p. 15) finds, in 

 opposition to Wiesner, that the stomata of etiolated maize plants are entirely or partly open. 



5 For literature see Burgerstein, n, p. 431. 



* Van Tieghem, Bull. d. 1. Soc. Bot. de France, 1886, p. 88. On the importance of the red dye 

 in absorbing heat, see Stahl, Ann. d. Jard. bot. d. Buitenzorg, 1896, T. xni, p. 148. Cf. Sect. 88. 



