GENERAL 367 



form of mild parasitism, for the algal components can grow and develop 

 equally well when isolated, although in the form of a lichen they are 

 rendered more resistant to heat and to drought, and are protected against 

 excessive insolation l . It is possible that the misletoe transfers superfluous 

 organic material to its host-plant, but it is not known whether Viscum 

 absorbs only water and inorganic salts from its host, or organic nitrogenous 

 compounds as well. 



All stages of transition exist between purely autotrophic and purely 

 heterotrophic plants. Thus the saprophyte Neottia and the parasites 

 Orobanche and -Cuscuta contain a little chlorophyll and are able to produce 

 a small and perhaps unnecessary portion of their organic food by photo- 

 synthesis, while carnivorous plants and the Scrophulariaceae previously 

 mentioned obtain but little organic material from the external world. 

 Moreover, every autotrophic plant is able to assimilate nitrogenous and 

 non-nitrogenous organic substances to a certain extent, but it is not sur- 

 prising that it has not as yet been found possible to cultivate autotrophic 

 plants in a purely heterotrophic manner 2 . 



Most green plants do not require any supply of organic food from 

 without, and hence grow normally when supplied solely with the con- 

 stituents of the ash, with water and with carbon dioxide. Liebig showed 

 that the amount of humus might increase in cultivated ground in spite 

 of the yearly harvest removing large quantities of organic substance, and 

 hence correctly concluded that plants cannot obtain their organic food 

 from the soil ; and Pfeffer and others have shown that seedlings grown 

 in humus in a receiver containing no carbon dioxide cease to develop 

 when the reserve-materials of the seed are exhausted 3 . Hence humus- 

 saprophytes must have special absorptive powers, for even the easily culti- 

 vated moulds do not grow well upon humus. In correspondence with 

 other regulatory phenomena it is probable that any reduction in the photo- 

 synthetic production of organic substance by a humus-saprophyte will tend 

 to induce more pronounced heterotrophic nutrition. 



Historical. The humus theory, according to which all plants must obtain 

 their organic food from the external world, was a survival from the teaching of 

 Aristotle. Van Helmont in the seventeenth century asserted that all the component 



1 See Ewart, Journ. Linn. Soc. Bot., 1896, xxxi, pp. 376, 385. 



a Literature: Klebs, Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. II, p. 538; Laurent, 

 Bull. d. 1. Soc. Bot. d. Bruxelles, 1888, T. xxvi, p. 263; Nadson, Bot. Centralbl., 1890, Bd. 

 XLII, p. 50; Acton, ibid., 1890, Bd. XLIV, p. 224; Bokorny, Versuchsst., 1889, Bd. xxxvi, 

 p. 235 ; Bot. Centralbl., 1896, Bd. LXVi, p. 304; Biol. Centralbl., 1897, Bd. xvil, p. l ; Kriiger, 

 Zopfs Beitrage, 1894, IV, p. 114. 



3 Liebig, Die Chemie in Anwend. a. Agric., 1840, p. 14; Pfeffer, Monatsb. d. Berl. Akad., 1873, 

 p. 784; Godlevvski, Bot. Zeitung, 1879, p. 88. Cf. also Cailletet, Compt. rend., 1871, T. LXXIII, 

 p. 1476. 



