ii4 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



liarities J . The fact that the seeds of Orobanche and Lathraea 2 will only germinate 

 on the root of a host-plant, and certain pollen-grains 3 only in the stigmatic fluid, 

 seems to show that special chemical stimuli are required in these cases. The 

 numerous researches on the influence of chemical agents upon the germination 

 of other seeds 4 have brought to light no facts of special scientific value. 



SECTION 31. The Influence of Oxygen on Growth. 



Most aerobic plants cease to grow when the air around them contains 

 less than from 3 to o-i per cent, of oxygen. Certain aerobic organisms such as 

 the sulphur-bacteria are adapted to very small percentages of oxygen, and 

 cease to grow when the oxygen-pressure reaches that of the air at sea-level, 

 whereas the maximal pressure for ordinary plants is from 2 to 30 times that 

 of the atmosphere. Hence the curves showing the influence of oxygen 

 on growth vary in different cases 5 . Very commonly growth is accelerated 

 when the normal oxygen-pressure of the air is lowered, but frequently an 

 increased percentage of oxygen also accelerates growth, so that the curve 

 exhibits a primary and a secondary optimum. The position of the 

 cardinal points varies somewhat according to the stage of development 

 and to the external conditions. In fact certain anaerobes are capable under 

 special conditions of developing in ordinary air. Chudiakow 6 also found 

 that Bactridiinn butyricum, which normally does not develop when the 

 air-pressure rises above 15 mm. of mercury, can gradually be accommodated 

 to an air-pressure of 50 mm. of mercury (oxygen-pressure = 10 mm. Hg). 



The influence of oxygen on growth has been studied by Bert, Wieler, Jentys, 

 Jaccard, and Fr. Schaible 7 . 



Increased oxygen-pressure. Above a certain concentration oxygen acts as 

 a poison, and hence before this concentration is reached a certain retarding 

 influence is exercised upon growth. In such experiments the percentage of oxygen 

 may be varied while the total pressure is kept equal to that of the atmosphere. 

 A mere rise of gaseous pressure, if sufficiently great, will produce a retardation and 

 ultimate cessation of growth by antagonizing turgor just as an external mechanical 

 resistance would do. Jentys found that pure oxygen under a pressure of from 



1 See de Bary, Fungi, Bacteria, and Mycetozoa, 1886. 

 3 Heinricher, Jahrb. f. vviss. Bot., 1897, Bd. XXXI, p. 77. 



3 Molisch, Sitzgsb. d. Wien. Akad., 1893, Bd. Cll, I, p. 428. See also Lidforss, Jahrb. f. wiss. 

 Bot., 1899, Bd. xxxm, p. 240. 



4 Nobbe, Samenkunde, 1876, p. 269; Sigmund, Versuchsstat., 1896, Bd. XLVII, p. T. 



5 A higher oxygen-pressure is required for germination than for the subsequent growth of the 

 seedlings. Cf. Schaible, Beitr. z. wiss. Bot. von Fiinfstiick, 1900, Bd. iv, p. 93. 



6 Chudiakow, Centralbl. f. Bact., 1898, Bd. iv, p. 392. 



7 Bert, La pression barometrique, 1878 ; Wieler, Unters. a. d. Bot. Inst. in Tubingen, 1883, 

 Bd. I, p. 189. Methods are given there and in the following works: Jentys, Unters. a. d. Eot. Inst. 

 in Tubingen, 1888, Bd. II, p. 419 ; Jaccard, Rev. gen. d. Bot., 1893, T. v, p. 289; Schaible, Beitr. 

 z. wiss. Pot. v. Fiinfstiick, 1900, Bd. iv, p. 93. 



