THE INJURIOUS ACTION OF LIGHT 247 



at different stages of development. The resistance can be increased to 

 a certain extent by raising the concentration of the cell-sap and of the 

 imbibed water, so that the fact that cold acts as a stimulus inducing an 

 increased production of sugar in many plants is of great biological impor- 

 tance. In this way the freezing-point of the plant is appreciably lowered, 

 but whether those trees which form oil in winter are more resistant to cold 

 on this account is uncertain \ 



PART III 



THE POWER OF RESISTANCE TO LIGHT 



SECTION 69. The Injurious Action of Light. 



There is a definite maximal and ultra-maximal intensity of light for 

 each plant, and the maximum is high in the case of plants which grow in 

 fully exposed situations, but is lower in the case of typical shade-plants. 

 Many chlorophyllous Phanerogams and Cryptogams, including numerous 

 algae, cannot withstand exposure to the full intensity of direct sunlight 2 . 

 Among fungi all grades of resistance are shown, from plants which grow in 

 sunlight to others which are soon killed by diffuse daylight. Thus Elfving 3 

 found that the spores of AspergiUus glaucus, and Laurent 4 that those of 

 Ustilago carboj were killed only by prolonged insolation, whereas the 

 plasmodia of various Myxomycetes soon die in direct sunlight 5 . Some 

 bacteria are unable to grow in diffuse light, but others are only killed by 

 longer or shorter insolation 6 . Curiously enough the spores are often not 

 more resistant than the vegetative cells, and Arloing even found that the 

 spores of Bacillus anthracis were killed by two hours' insolation, but the 

 vegetative cells only after from twenty-six to thirty hours'. This action of 

 sunlight is therefore of the utmost importance in nature, and it appears that 

 all pathogenic bacteria can be killed by sufficiently prolonged insolation. 

 No organism is, however, known which is so sensitive to light that it can 

 only grow in complete darkness. 



Even plants which grow in sunlight can be killed more or less rapidly 

 when the illumination is sufficiently increased. This was first shown by 



1 Fischer, Jahrb. f. wiss. Bot., 1891, Bd. xxn, p. 155. Brittle frozen branches are more readily 

 damaged by storms. Cf. Frank, Krankheiten d. Pflanzen, 1894, 2. Aufl., p. 177. 



2 This applies more especially to many Florideae, but also to such plants as Spirogyra and 

 Nitella. 



3 Elfving, Einwirkungen d. Lichtes auf d. Pilze, 1890, p. 105. 



4 Laurent, Bull. d. 1. Soc. Bot. d. Belgique, 1889, T. xxvm, p. 162. 



5 Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, p. 723. 



6 Fliigge, Die Mikroorganismen, 1896, 3. Ann 1 ., Bd. I, p. 441 ; Migula, System d. Bact., 1897, 

 Bd. I, p. 361 ; Lafar, Technische Mykologie, 1897, Bd. I, p. 72 ; Dieudonne, Biol. Centralbl., 1895, 

 Bd. XV, p. 103 ; Tammes, Landw. Jahrb., 1900, Bd. xxix, p. 468. 



