308 THE FOOD OF PLANTS 



actually contain a compound chromophyll, which may be separated into chloro- 

 phyll and a pinkish-red pigment by treatment with alcohol 1 . The displacement 

 of the assimilatory maximum to the infra-red is therefore apparently due to the 

 red pigment acting as a sensitisor. 



According to Butschli the bacterio-purpurin is restricted to the outer layer 

 of the bacterial plasma 2 , and the only bacteria in which the pigment appears to be 

 definitely associated with the plasma are those green and red bacteria which contain 

 chlorophyll and have a distinct power of carbon dioxide assimilation. In all other 

 bacteria the pigment appears to be an excretion, and in very many cases it is easy 

 to see that the bacteria are colourless so long as they remain living, while it is only 

 in certain cases that the bacterial pigment has the peculiar property of being able 

 to occlude oxygen. In Spirillum rubrum, however, the pigment appears to form 

 part of the bacterial plasma, and Engelmann includes S. rubrum among the list of 

 the red assimilating bacteria, although Ewart was unable to detect any evolution 

 of oxygen from this form, nor indeed from any other bacterium than those already 

 mentioned. 



Historical. Priestley, the discoverer of oxygen, was the first to recognize 

 that green plants purify air rendered foul by the respiration of animals, and 

 Ingenhousz showed that this took place in light only, and that in darkness 

 plants, like animals, gave off carbonic acid gas (fixed air). Ingenhousz did not, 

 however, make it clear that the exhalation of oxygen (dephlogisticated air) was 

 accompanied by a corresponding decomposition of carbon dioxide. This discovery 

 was reserved for Senebier, who in his first work, and still more clearly in the later 

 ones, established the fact that organic substance is produced from carbonic acid gas 

 and water, while oxygen is excreted 3 . Senebier's experiments were by no means 

 perfect, and it was the masterly researches of Th. de Saussure 4 which first clearly 



1 Ewart, Annals of Botany, 1897, Vol. xi, p. 486. A green pigment exhibiting red fluorescence 

 and the characteristic bands of chlorophyll may be extracted by means of alcohol and benzene, not 

 only from pure cultures of Bacterium photomttricum, but also from Chromatium okenii and 

 C. vinosa. Cf. also Butschli, Ober den Bau d. Bacterien, 1890, p. 9. Even though Engelmann did not 

 work with pure cultures, the results observed could hardly have been produced by the presence of 

 green Bacteriff (cf. Winogradsky, Beitrage z. Morphol. u. Physiol. d. Bacter, 1888, Heft i, p. 56). 

 Engelmann's results have since been confirmed by Ewart (Joum. of Linn. Soc., xxxill, 1897, p. 151). 

 Elfving supposes that the red Saceharomyces glutinis can assimilate carbon dioxide (Stud. u. Ein- 

 wirkung d. Lichtes auf Pilze, 1890, p. 17), but Ewart was unable to detect any evolution of oxygen 

 from a red yeast isolated from the air. 



a [Fischer states (Unters. iiber Bact. u. Cyanophy., Jena, 1897, p. 120) that the pigment is 

 uniformly distributed in Chromatium.} 



8 Priestley, Phil. Trans., 1773, Vol. LXII, pp. 168, 193. Cf. Sachs, Gesch. d. Bot., 1875, p. 531. 

 Bonnet's observations (Unters. ii. d. Nutzen d. Blatter, Arnold, 1762, p. 14) are without value, for 

 he considered the evolution of bubbles from green plants exposed to light under water to be a purely 

 physical phenomenon. The older ideas as to the origin of plant-food are given by Sachs, 1. c., 

 P- 495J Ingenhousz, Experiments on Plants, 1779; Senebier, M^moires physicochimiques, 1782; 

 Recherches s. 1. lumiere solaire, 1783; Physiol. ve'get., 1800, T. in, p. 184 ; T. iv, pp. 37, 165. As 

 Pringsheim (Ober Chlorophyllfunction u. Lichtwirkung i. d. Pflanze, 1882, p. 26, &c.) has shown, 

 Hansen (Arb. d. Bot. Inst. in WUrzburg, 1882, Bd. II, p. 560) unjustly depreciates the value of 

 Senebier's researches. * 



* Th. de Saussure, Rech. chim. i. 1. ve"ge"tation, 1804. 



