THE INFLUENCE OF LIGHT 34I 



in the rate at which the bubbles escaped became perceptible l . It does not, 

 however, follow that the chloroplastids had reached their maximal activity,' 

 for the limited supply of carbon dioxide might have prevented any further 

 increase in the intensity of the light from accelerating the photosynthetic 

 activity beyond a certain limit. On the other hand it is possible that in 

 Elodea and other plants the curve of assimilation falls again if the illumination 

 is increased beyond a certain point, even when a full and adequate supply 

 of carbon dioxide is assured, and this result will certainly be produced by 

 prolonged exposure to intense light, owing to the inactive condition which 

 may be induced in the chloroplastids (Sect. 58). In any case the intensity 

 of the light which actually reaches the chloroplastids is very much less 

 than that falling upon the outer surface of the plant, and it is very 

 doubtful whether the optimal intensity of illumination for the chloroplastid 

 ever approaches anywhere near to that of strong direct sunlight. Every 

 plant has its own specific requirements and powers of resistance, and it is well 

 known that many shade-plants are unable to withstand continual exposure 

 even to only moderately strong sunlight 2 . 



Wolkoff experimented at various distances from a strongly illuminated sheet 

 of ground glass, whereas Reinke exposed the plants he was observing to sunlight 

 concentrated by means of a lens. Both these authors and V. Tieghem also 

 used the bubble-counting method, which is apparently more convenient for such 

 comparative researches than direct gas analysis, and Reinke (1. c., p. 698) criticizes 

 the results obtained in this manner by N. J. C. Miiller and Famintzin. Kreusler 

 also found a proportionate relationship to exist between the activity of assimilation 

 and the intensity of the illumination. This applies not only to white but to coloured 

 light, as is shown by bubble-counting experiments conducted behind coloured 

 media, and also by Engelmann's researches (Sect. 60) 3 . 



Pringsheim's protective theory. No mathematically exact relation can be 

 expected between the photosynthetic activity and the intensity of light, for as the 

 light increases other influences may be exerted, which directly or indirectly modify 



1 [This result may have been due to the continuous rise of temperature in the water exposed to 

 intense sunlight causing the gases of the intercellular spaces to expand, and hence inducing a stream 

 of bubbles. If plants of Elodea are exposed to concentrated sunlight in water containing ether or 

 chloroform, a stream of bubbles may still continue to escape after the plants have been fatally injured, 

 and it is obvious that no accurate results can be obtained by the bubble-counting method unless the 

 surrounding medium is kept at a perfectly constant temperature. Ewart (Annals of Botany, 1898, 

 XII, p. 384) has shown that living chloroplastids of Elodea and Chara are completely bleached by 

 five to ten minutes' exposure in cold water to sunlight of eight- to ten-times-concentrated photo- 

 chemical intensity, and that at this intensity of illumination the assimilation of carbon dioxide 

 immediately or almost immediately ceases (p. 393).] 



a See Ewart, Annals of Botany, 1897, p. 440 (Effects of Tropical Insolation). 



3 Wolkoff, Jahrb. f. wiss. Bot., 1866-7, Bd- v, p. i; Reinke, Bot. Zeitung, 1883, p. 713; 

 v. Tieghem, Compt. rend., 1869, T. LXIX, p. 492 ; N. C. Miiller, Bot. Unters., 1873, Bd.'l, pp. 3, 

 .374; Famintzin, Ann. d. sci. nat., 1880, vi. seY., T. X, p. 67; Kreusler, Landw. Jahrb., 1885, 

 Bd. xiv, p. 952. 



