328 THE FOOD OF PLANTS 



Chemical decompositions of this nature as well as those metabolic 

 changes which are induced directly or indirectly by the action of light 

 (including therefore the formation of chlorophyll), are to be distinguished 

 from the synthetic processes for whose performance light supplies the 

 necessary energy. It is possible that in addition to the assimilation of 

 carbon dioxide other photosynthetic processes may be discovered, and it 

 has not even been tried whether the chlorophyll apparatus is perhaps able 

 to assimilate by photosynthesis such compounds of carbon as CO C1 2 , COS, 

 CO (NH 2 ), &c. 



Heyne 1 was the first to observe that the leaves of Bryophyllum calycinum 

 acquired an acid taste during the night, and Link found that the same occurred 

 in certain Crassulaceae. In 1875 Ad. Mayer studied these daily variations 

 more closely and showed that the diminution in acidity was accompanied by an 

 evolution of oxygen. G. Kraus proved the general nature of the phenomenon, 

 and Warburg showed that a marked production of free organic acids might 

 occur not only in plants belonging to the Crassulaceae, Cactaceae, Mesembry- 

 anthemum, &c., but also in many thin leaves possessing a strongly developed 

 cuticle, thus indicating the probable importance of the phenomenon. The rela- 

 tion between the gaseous exchanges and the variations in the acidity have recently 

 been reinvestigated in detail by Aubert. 



Plants of the Crassulaceae produce isomalic acid, of the Cactaceae malic 

 acid and Mesembryanthemum oxalic acid (Aubert). Since the acid is formed 

 by oxidation, a plant placed in darkness does not evolve any carbon dioxide 

 until a certain amount of organic acid has accumulated, when the amounts 

 of oxygen absorbed and of carbon dioxide exhaled assume their normal relative 

 proportions. This fact was known to de Saussure, but it is not due to any absorption 

 of carbonic acid, for a succulent plant in an atmosphere deprived of carbonic 

 acid gas still evolves oxygen in abundance when exposed to light 2 . The fact 

 that the accumulation of acid is definitely limited shows that it is not caused by 

 a deficiency of oxygen, but is due to the specific nature of the plant in question, 

 as may be shown by direct experiments*. 



External conditions influence the accumulation of acid, a high temperature 

 or illumination causing the acidity to diminish. Thus the percentage of acidity 

 decreases to about the same extent when the temperature of a darkened plant 

 is raised from 1 5 C to 45 C * (de Vries, Warburg) as when it is exposed to 

 daylight, the difference being that carbon dioxide is exhaled instead of oxygen ; 



perceptibly affected by light (Beitrage z. Kenntniss einheimischer Pilze, 1893, I, p. 48). Similarly 

 the acidity of dead plants or of expressed sap is not markedly altered by exposure to light (C. Kraus, 

 I.e., 1886, p. 40 ; A. Mayer, Versuchsst., 1887, Bd. xxxiv, p. 131). 



1 Heyne, Jahrb. d. Gewachskunde v. Sprengel, Schrader u. Link, 1819, Heft 2, p. 70. Cf. 

 G. Kraus, I.e., 1884, p. 14. 



* Saussure, Rech. chim., 1804, p. 64. Cf. Warburg, I.e., p. 99. See Ad. Mayer, 1878, I.e., 

 p. 284, &c. ; Aubert, Rev. ge"n., 1. c., p. 275. On the absorption of CO 2 , cf. Sect. 96. 



* See Warburg, I.e., p. 85, and Aubert, Ann. d. sci. nat., 1. c., p. 39. 



* [Gerber (Ann. d. sci. nat., iv, 1897, pp. 1-280) states that in fleshy fruits citric and tartaric 

 acids decompose at 30 C, malic at I5C.] 



