THE INFLUENCE OF THE EXTERNAL CONDITIONS 563 



in the respiratory activity, and that this is mainly due to the less refrangible 

 rays. The same authors have also directly shown (I.e., 1886, vii. seV 

 T. in, p. 14), by rendering the chloroplastids inactive with ether that 

 green plants behave similarly to non-chlorophyllous plants or organs, and 

 evident that Pringsheim's hypothesis that light increases respiratory 

 activity arose in connexion with his theory of the action of chlorophyll and 

 was not founded on fact *. 



The prolonged absence of light must ultimately affect the respiratory 

 activity of all such plants as grow abnormally in darkness 2 , while in auto- 

 trophic plants owing to the cessation of carbon dioxide assimilation the plant 

 is gradually starved, and hence its respiratory activity will as a general 

 rule decrease, as was observed by Borodin 3 when twigs were kept in con- 

 tinuous darkness. Hence the respiratory activity of the roots and rhizomes 

 of green plants may decrease slightly during the night when photosynthetic 

 assimilation is feeble, whereas Areboe 4 could detect no such diminution 

 when assimilation was active. Whenever light injuriously affects the plant 

 and depresses its vital activity, respiration must also diminish, and many 

 bacteria are killed by direct sunlight, or even by bright diffuse daylight, in 

 a comparatively short time. On the other hand, by inducing a more rapid 

 decomposition of organic acids, light may temporarily accelerate the evolution 

 of carbon dioxide in the case of fleshy plants and of fungi (Sects. 56 and 86). 



Nutritive and poisonous substances. When food is supplied to a starved 

 plant the general activity is increased and hence that of respiration also, 

 while an excessive supply will ultimately retard both. Respiration is, 

 however, by no means directly dependent upon the food supply, and the 

 awakening of growth and respiratory activity occurs in an onion only when 

 the winter rest is over, in spite of the constant presence of a rich store 

 of glucose. It is however possible that the increased respiratory activity 

 exhibited when frozen potatoes which have become sweet are returned 

 to an ordinary temperature, may be mainly due to the accumulation of 

 sugar 5 , although the latter is not always the cause of the more active 

 respiration which usually follows a sojourn under abnormal conditions 6 , 



p. 130; Areboe, Forsch. a. d. Geb. d. Agr.-Phys., 1893, Bd. xvi, p. 450. On intramolecular 

 respiration, Wilson, Flora, 1882, p. 96. 



1 [Kolkwitz has recently found (Jahrb. f. Wiss. Bot. 1898, Bd. XXXIII, p. 128) that light dis- 

 tinctly accelerates respiration in the case of certain fungi, but that this increase does not amount to 

 more than ten per cent, of the previous values. It is however possible that the apparent increase 

 is due to the direct action of light upon the organic acids which these fungi produce. See infra.'} 



2 On the respiration of green and etiolated leaves, cf. Palladin, Rev. ge"n. d. Bot., 1893, T. V, 

 p. 369. On that of mosses grown under different conditions, Jonsson, Compt. rend., 1890, T. CIX, 

 p. 442. On the influence of light on potatoes, Ziegenbein, Jahrb. f. wiss. Bot., 1893, Bd. XXV, p. 592. 



3 Borodin, Bot. Jahresb., 1876, p. 920. 



* Areboe, 1. c., p. 459. This daily periodicity was first observed by Saikewicz, Bot. Jahresb., 



1877, P- 723. 



5 Miiller-Thurgau, Landw. Jahrb., 1893, Bd. XI, p. 794; 1885, Bd. Xiv, p. F6o. 



6 Johannsen, Unters. a. d. Bot. Inst. z. Tubingen, 1885, Bd. i, pp. 707, 716. 



002 



