340 THE FOOD OF PLANTS 



is unable to detect an extremely feeble assimilation of carbon dioxide, 

 such as occurs in moonlight, which is about ^^ the intensity of sunlight \ 

 whereas in twilight an evolution of oxygen may readily be detected from 

 a chlorophyllous cell by this method. 



Respiration and assimilation about balance one another when highly 

 chlorophyllous organs are exposed to light of T Vth to ^th the intensity of 

 bright diffuse daylight (Sect. 55), but even then an evolution of oxygen may 

 be detected by means of the bacterium method from the more strongly 

 illuminated cells or chloroplastids. Similarly an actively assimilating leaf 

 or green alga may evolve traces of carbon dioxide even when exposed to 

 bright light 2 , nor is it surprising that plants may turn green or react 

 heliotropically when exposed to an intensity of light insufficient to cause 

 any actual evolution of oxygen. 



It is well known that green plants are unable to assimilate sufficiently 

 actively in deep shade, and the same is the case with plants kept in a dark 

 room. Even when placed at an exposed window they receive only the 

 light from one-half of the sky, instead of the whole as they do when growing 

 in the open. The light rapidly decreases towards the interior of the room, 

 so that if the window is 2 m. high and 1-5 m. broad, the plant receives at 

 a distance of 0-5 m. only 0-3, and at a distance of 2 m. only 0-08 of the diffuse 

 daylight it would receive in the open (Detlefsen). Sachs found that a seedling 

 of Tropaeolum inajus exposed to seven hours' morning light in a west window 

 grew badly, but ultimately increased slightly in dry weight 3 . 



The photosynthetic activity increases proportionately to the intensity 

 of the light, as has been repeatedly shown since the first experiments 

 by Wolkoff. There is, however, a limit to the increase, and according 

 to Reinke this is about reached in direct sunlight ; above this intensity 

 the number of gas bubbles evolved by Elodea remained about the same 

 even in sixty times concentrated sunlight, and an injurious effect was not 

 exerted until a much higher intensity had been reached, when a diminution 



1 Boussingault, Ann. d. sci. nat., 1869, v. se"r., T. x, p. 335. On the northern nights, cf. 

 Curtel.Rev.g^n. d. Bot., 1890, T. II, p. 12. [Although no compensation for respiration is possible 

 by the bacterium method, still the latter forms a more certain test for the absence or presence of 

 assimilation than do the methods of gas analysis, for the most highly chlorophyllous phanerogamic 

 leaf contains a large number of non-green cells which respire without cessation. The bacterium 

 method will detect the minutest evolution of oxygen from each individual cell (see Ewart, I.e., pp. 

 371, 380). Moreover oxygen may be evolved at one part of a cell although it is absorbed at another.] 



* Cf. Garreau, Ann. d. sci. nat., 1851, T. xvi, p. 280; Blackman, Phil. Trans., 1895, 

 Vol. CLXXXVI, pp. 540, 557. [A microscopical method for demonstrating the simultaneous evolution 

 of traces of oxygen and carbon dioxide from green algal filaments exposed to light, by means of 

 hanging drops, one of an alkaline solution of phenophthalein, the other of water containing not 

 more than a few score Bacterium termo, in an atmosphere of hydrogen or of nitrogen, is given by 

 Ewart, Journ. Linn. Soc., xxxni, 1897, pp. 126, 138, &c.] 



3 Sachs, Experimentalphysiol., 1865, p. 2 1 ; Detlefsen , Arb. d. Bot. Inst. in Wiirzburg, 1884, Bd. 

 Ill, p. 88. 



