352 THE FOOD OF PLANTS 



its original assimilatory value. The solutions may either be placed in double- 

 walled bell-jars, or in glass troughs with parallel sides '. 



If a spectrum is projected upon a darkened leaf a formation of starch occurs in 

 the outermost layers opposite to B-C, but the more deeply penetrating yellow rays 

 are spread over a larger area, and have insufficient energy to induce a formation 

 of starch at any one point, although they may excite a greater total amount of 

 assimilation than the rapidly absorbed red rays do. Timiriazeff was able to 

 demonstrate the primary assimilatory curve by the formation of starch in the 

 red region of the spectrum thrown upon a leaf 8 . 



Artificial light. The assimilatory value of artificial light depends upon its 

 composition and concentration ; thus, exposure to ordinary gaslight causes percep- 

 tible assimilation in green plants, and electric-light is still more efficient *. In both 

 electric-light and gaslight the percentage of the more refrangible rays is relatively 

 less than in sunlight, and hence in such light the assimilatory curve is corre- 

 spondingly modified. This is especially the case when the red algae are examined, 

 for with these the assimilatory maximum is markedly displaced towards the green 4 . 

 It may incidentally be remarked that polarized light also enables the plant to 

 decompose carbon dioxide. 



Absorption of light in deep water. The daylight which falls upon a plant is not 

 always of the same composition, and under the shade of trees, as well as in deep 

 water, particular parts of the spectrum become especially prominent. Thus in 

 a clear sea with bright sunlight the different rays are reduced to about the 

 concentration of moonlight which is insufficient for growth, at the following depths : 

 red at 34 metres, yellow at 177 m., green at 322 m. B Hence arises the advantage 

 to red seaweeds of an assimilatory maximum in the yellow region of the spectrum 

 (Fig. 51). The distribution of red seaweeds is influenced by many other factors as 

 well, for although they may grow at considerable depths very many are found 

 between the tide-marks, or may grow in quite shallow water*. In turbid water 

 the light is absorbed much more rapidly, but even under the most favourable 

 conditions it is hardly possible that plants dependent upon the photosynthetic 

 assimilation can grow at a depth of more than 400 m. 7 



1 Pfeffer, Arb. d. Bot. Inst. in \Viirzbnrg, 1871, Bd. I, p. 53. A marked starch formation takes 

 place only behind solutions of potassium bichromate. Cf. Famintzin, Jahrb. f. wiss. Bot., 1867-8, 

 Bd. vi, p. 43; G. Kraus, ibid., 1869-70, Bd. vu, p. 518; Prillieux, Compt. rend., 1870, T. LXX, 

 p. 46; Kohl, Ber. d. Bot. Ges., 1897, p. in. Double-walled bell-jars were first used by Senebier 

 (Phys.-chem. Abhandlungen, 1785, Bd. I, p. 7), and later by Becquerel (La Lumiere, 1868, T. II, 

 p. 278) and Sachs. Kreusler gives a simple mode of preparing glass troughs (Landw. Jahrb., 1885, 

 Bd. xiv, p. 935, footnote). 



2 Timiriazeff, Compt. rend., 1890, T. ex, p. 1346. 



3 On growth and photosynthesis in electric light, cf. Bonnier, Rev. g&i. d. Bot., 1895, T. VII, 

 p. 241 ; Bailey, Report upon Electro-Horticulture, Ithaca, 1893 ; Siemens, Bot. Centralbl., 1880, 

 Bd. I, p. 815; Kreusler, Landw. Jahrb., 1885, Bd. xiv, p. 915. 



4 Engelmann, Bot. Zeitung, 1883, p. 8. 



4 Oltmanns, Jahrb. f. wiss. Bot., 1892, Bd. XXIII, p. 419. See also Walther, Bionomie des 

 Meeres, 1893, Bd. i, p. 35; Hiifner, Archiv f. Anat. u. Physiol., 1891, p. 68; C. Schroter u. 

 O. Kirchner, Vegetation d. Bodensees, 1896, p. 17. 



8 Certain green algae are able to grow in deep water (cf. Drude, Pflanzengeographie, 1890, p. 21). 



7 Cf. Oltmanns, I.e.; \Valther, I.e.; Engelmann, Bot. Zeitung, 1872, p. 396. 



