DESICCATION 249 



anaerobic organisms in strong light leads to the assumption that aerobic 

 organisms will also be killed, in the absence of oxygen, by light of sufficient 

 intensity. Whether the presence of oxygen unduly accelerates respiration, 

 or leads to the production of poisonous oxidatory products, has still to be 

 determined. The fact that moderate illumination does not appreciably affect 

 respiration affords no sure criterion as to the action exercised when the light 

 is intense. In the latter case indeed the pigment in the cell-sap of the 

 staminal hairs of Tradescantia is decolourized, although no such action is 

 exercised by ordinary light. This is shown by the absence of any power of 

 regeneration such as would be required to neutralize the effect of the daily 

 exposure to light 1 . The fact that the pigment may be oxidized and 

 bleached by means of hydroxyl without killing the cell suggests that the 

 intense light excites a production of ozone or hydroxyl, and that this carries 

 out the bleaching. Light is capable of inducing a variety of chemical 

 changes, involving analysis and synthesis as well as oxidation, so that a direct 

 photochemical action might well be exercised upon the protoplasm. 



In such media as light and air the fatal action is exercised within the 

 cells, but in certain media decomposed by light poisonous products of 

 external origin may act upon the living cells ' 2 . It is worthy of note that 

 dry bacteria, and fungal spores such as those of Ustilago carbo, are killed by 

 sunlight 3 . On the other hand, mosses and lichens which grow on rocks 

 are, when dry, uninjured by the strongest sunlight, nor is the vitality of dry 

 seeds affected by exposure to light 4 . 



PART IV 



THE EFFECTS OF THE WITHDRAWAL OF WATER 



SECTION 70. Desiccation. 



In adaptation to their surroundings different plants and organs have 

 acquired varied powers of withstanding the loss of water. The vegetative 

 organs of most flowering plants are killed by desiccation, which does not 

 affect the vitality of most seeds, but simply causes them to assume a dormant 



1 Cf. Pfeffer, Oxydationsvorgange in lebenden Zellen, 1889, p. 383. [A power of regenerating the 

 pigment might be present under normal conditions, but be destroyed by treatment drastic enough to 

 destroy all the pigment present in the cell. For instance, chloroplastids can be partially bleached and yet 

 remain capable of reproducing chlorophyll, but when completely bleached this power is always lost.] 



2 Cf. Richardson, Bot. Ztg., 1894, p. 304; Dieudonne, Arbeit, a. d. Kaiserl. Gesundheitsamte, 

 1894, Bd. ix, p. 537 ; Fliigge, 1. c., p. 443. s Cf. Fliigge, 1. c., p. 444. 



4 Tammes, Landw. Jahrb., 1900, Bd. XXIX, p. 467. [The tips of the moss-leaves are often 

 browned or injured by the prolonged exposure. The special resistance is due (i) to the opacity of 

 the dry plants, (2) to the rolling up of the leaves, (3) to the fact that the dry walls of living cells are 

 practically impermeable to oxygen, (4) chlorophyll is less readily decomposed when dry. It is in 

 fact rather a question of resistance to dry heat than to light.] 



