THE INFLUENCE OF THE EXTERNAL CONDITIONS 561 



present and thus prevent any accumulation even if such products are 



still formed. For an account of the processes of putrefaction, cf. Flugge, 



1. c., p. 254 ; Lafar, 1. c., p. 260 ; Herfeldt, Centralbl. f. Bact., 1895, Abth. ii, 

 Bd. i, p. 74. 



SECTION 104. The Influence of the External Conditions. 



Any change in the external conditions will as a general rule exercise 

 some effect or other upon respiratory metabolism, and hence any induced 

 modification of growth or movement will be accompanied by a certain 

 alteration in the respiratory activity. This has not been proved in all 

 cases, but all the experimental evidence seems to show that at a constant 

 temperature growth and movement are accompanied by increased respi- 

 ration, although the reverse does not necessarily follow. Respiration 

 rapidly accommodates itself to changed conditions, but when any injurious 

 effect is produced a return to the original activity may be gradual, 

 and may only be completed some time after the original conditions 

 have been restored. A change in the respiratory activity may not directly 

 result from the altered conditions, but may be due to a regulatory reaction 

 leading to the assumption of a new condition of equilibrium. Many 

 mechanical and chemical stimuli which are not sufficiently powerful to 

 depress vital activity may induce a transitory or permanent increase in 

 the respiratory activity, and this may in some cases be accompanied by 

 more rapid growth. 



Temperature. Respiration ceases in living turgid plants only at com- 

 paratively low temperatures, being still feebly active at -2 to -4C, and 

 perceptible even at ioC. in the case of conifers and lichens 1 . It con- 

 tinually increases as the temperature rises, until the injurious action of 

 the high temperature becomes manifested by a decrease in the respiratory 

 activity at once or after the lapse of a certain latent period. Hence 

 respiration exhibits no distinct optimum lying within limits of temperature 

 which can be endured for prolonged periods (cf. Fig. 50), whereas growth 

 and assimilation exhibit optimal points beyond which the activity decreases 

 without any permanent injury being necessarily produced. It is possible 

 that plants are unable to regulate their respiratory activity, and hence at 

 high temperatures necessarily consume a larger amount of food-substances 2 , 

 while at the same time the increased production of heat tends to lower the 

 maximal temperature which the plant can withstand. 



1 Jumelle, Rev. gen. d. Bot., 1893, T. xxv, p. 599. Cf. also Kreusler, Landw. Jahrb., 1888, 

 Bd. xvi I r, p. 172; Clausen, ibid., 1890, p. 897; Ziegenbein, Jahrb. f. wiss. Bot., 1893, Bd. xxv, 

 p. 599. The older literature is here quoted (Ad. Mayer, Rischavi, Pedersen). 



2 On the alteration of the economic coefficient according to temperature, cf. Sects. 66 and 95. 

 On the relationship between CO 2 -assimilation and respiration, Sect. 58. 



PFEFFER O O 



