THE MODE IN WHICH LIGHT ACTS 



99 



attain their normal size only when in a phototonic condition remain small 

 in darkness, even when abundantly supplied with food-materials 1 , while 

 the internodes become elongated even when the supply of food is scanty. 

 Hence leaves and stems when exposed to light in an atmosphere deprived 

 of carbon dioxide 2 assume their normal shape and size if adequately 

 supplied with food, although no photosynthetic production of food is 

 possible 3 . This applies to shoots nourished by the assimilating parts 

 exposed to air 4 , or by reserves stored up in rhizomes, tubers, bulbs, or 

 seeds. In the latter case the shoot or seedling dies of starvation as soon 

 as the supply of food is exhausted 5 . The best supply of food will not 

 cause phototonic leaves to grow, or fern and moss spores to germinate 

 in darkness, but the latter will germinate when exposed to light in an 

 atmosphere deprived of carbon dioxide 6 . The fact that fern spores will 

 germinate at 32 C. shows that they are sufficiently supplied with food, 

 and when sugar causes the spores of Hepaticae to germinate in darkness, 

 it probably acts primarily as an exciting stimulus. 



Correlative influences must always play an important part in pro- 

 ducing the phenomena of etiolation. Thus in Phaseolus and Mimosa it 

 is evidently the active growth of the parts exposed to light which retards 

 or inhibits that of the leaf-primordia on shoots kept in darkness, for 

 these begin to grow when the growing apices are removed from the parts 

 exposed to light, and probably the same would occur if the growth of 

 these parts was mechanically prevented. 



Sachs erroneously concluded that the small size of etiolated leaves was 

 merely due to the deficiency of food, while G. Kraus supposed that the leaves 

 were only able to grow by means of the food they themselves assimilated 7 , 

 and others have assumed that etiolated leaves remain small in correlation to 

 the increased growth of the stem 8 . A generally correct view of etiolation was 

 first put forward by Godlewski D , and also in the first edition of Pfeffer's Physiology. 



1 Sachs, Bot. Ztg., 1863, p. 28; Kraus, Jahrb. f. wiss. Bot., 1869-70, Bd. vu, p. 212 ; Batalin, 

 Bot. Ztg., 1871, p. 672. 



2 The apparatus shown in Fig. 48, p. 318, Vol. I, may serve for this purpose. A better 

 arrangement, which may be adapted for single shoots, is shown in Fig. 62, p. 523, Vol. r. 



3 [The inner chlorophyllous cells may obtain supplies of carbon dioxide from the inner living 

 non-chlorophyllous cells, and also in solution in the water derived from the roots.] 



4 De Vries, Arb. d. Bot. Inst. in Wiirzburg, 1878, Bd. n, p. 120; Vochting, Bot. Ztg., 1891, 

 p. 113; Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 178; Jost, Ber. d. Bot. Ges., 1894, p. 191. 



5 Godlewski, Bot. Ztg., 1879, p. 89. The same follows from the behaviour in red and 

 blue light. 



6 Heald, Gametophytic Regeneration, Leipz. Diss., 1897, p. 47. 



7 Sachs, Lectures on Physiology, 1887, 2nd ed., p. 541 ; Kraus, Jahrb. f. wiss. Bot., 1869-70. 

 Bd. vir, p. 212. Batalin's supposition (Bot. Ztg., 1871, p. 674) that the small size of etiolated 

 leaves is due to the suppression of cell-division in darkness requires no discussion. 



8 Kraus, Flora, 1878, p. 145; Mer, Bull. d. 1. Soc. Bot. d. France, 1875, T. xxn, p. 190; 

 Rzentowsky, Bot. Jahresb., 1876, p. 745. 



9 Godlewski, Bot. Ztg., 1879, p. 113. 



H 2 



