THE GENERAL PROPERTIES OF POISONS 259 



inorganic nutrient substances act as poisons when concentrated. In the 

 case of the neutral salts of sodium and potassium a fairly high concentration 

 is usually necessary, but salts of iron as well as free acids and free alkalies 

 act as poisons when very dilute. Many other substances, including those 

 which the plant does not encounter in nature, exert a weaker or stronger 

 poisonous action. It is only natural that the minimal concentration required 

 to produce a poisonous effect should vary according to the plant examined, 

 and that the plant should be able to recover from weak or transitory doses 

 of poison, which when permanently applied may produce a gradual loss of 

 vitality, and when concentrated may produce immediate death. 



As an instance of adaptation to special conditions of life, it may be 

 mentioned that many obligate anaerobes are killed by a very low partial 

 pressure of oxygen, whereas most aerobes grow well when the percentage 

 of oxygen is much higher than it is in ordinary air. Further, many bacteria 

 are extremely sensitive to free acids, which do not affect fungi and acetic or 

 lactic bacteria even when moderately concentrated l . Hence when in open 

 competition, one organism by producing acid may kill its sensitive rivals. 

 The same occurs when a nutrient fluid contains yeast-cells which can with- 

 stand up to 14 per cent, of alcohol, and other plants which are killed by 

 from 2 to 10 per cent, of alcohol 2 . The sulphur bacteria again can exist in 

 the presence of large quantities of sulphuretted hydrogen 3 , while yeast-cells 

 and certain bacteria can live in solutions containing carbon dioxide under 

 pressure, in which most bacteria are unable to develop. Most terrestrial 

 plants indeed ultimately die in air containing between 4 and 20 per cent, of 

 carbon dioxide 4 . The development of nitrate and nitrite bacteria is 

 restricted by the presence of small quantities of peptone, sugar, and other 

 substances, which form most suitable foods for other bacteria and for fungi ; 

 but in this case death only ensues after prolonged functional depression, 

 a fact of considerable importance in nature. 



Plants, as well as animals, seem to have acquired an increased power 

 of resistance to poisonous products produced by themselves, although the 

 immunity is not always perfect 5 . Whether the protoplasm of plants is 

 unaffected by alkaloids and other vegetable poisons has not yet been 



d. Toxicologie, 1899, p. i. Injuries due to physical osmotic actions are not included under this 

 heading. 



1 Clark, Botanical Gazette, 1899, Vol. xxviu, p. 318. 



2 On alcohol as a poison cf. Bokorny, Pfliiger's Archiv, 1896, Bd. LXIV, p. 284; Clark, 1. c., 

 p. 384 ; Stevens, ibid., 1898, Vol. xxvi, p. 385. 



3 Kiihne (Zeitschr. f. Biol., 1898, N. F., Bd. xvm, p. 67) finds that sulphuretted hydrogen is 

 only feebly poisonous to Chara. 



* Frank, Krankheiten d. Pflanzen, 1894, 2. Aufl., Bd. I, p. 307; Lopriore, Jahrb. f. wiss. Bot., 

 1895, Bd. xxvm, p. 531; Frankel, Zeitschr. f. Hygiene, 1889, Bd. V, p. 322; Freudenreich, 

 Beiheft z. Botan. Centralbl., 1894, Bd. iv, p. 457. 



5 Lewin, Lehrb. d. Toxicologie, 1897, 2. Aufl., p. 423. 



S 2 



