368 THE FOOD OF PLANTS 



substances of a plant were obtained from water, but even after the production 

 of organic material by the assimilation of carbon dioxide had been definitely proved, 

 the absorption of organic material was regarded as necessary for the existence of 

 all plants until Liebig gave the dogma its death-blow 1 . The upholders of this 

 theory vigorously attacked 2 Liebig's work, and even Th. de Saussure, the founder 

 of the auto-assimilation theory, in his seventy-fifth year reverted to the ancient 

 dogma. Boussingault, however, brought forward additional evidence in support of 

 Liebig's view, and, together with Salm-Horstmar, showed that green 'plants can grow 

 in soil free from humus, when supplied solely with inorganic salts (cf. Sect. 73). 

 Liebig went so far as to deny that plants could absorb any organic substances, but 

 Schleiden gave a fairly correct general account of plant-nutrition :l . 



Humus-saprophytes. Many non-chlorophyllous plants (Agaricus and other 

 fungi, Monotropa^ Lathraea, Epipogon, &c.), and plants with but little chlorophyll 

 (Neottia, &c. 4 ), can absorb the whole of their organic food from humus. The 

 variable composition of humus renders it difficult to determine with certainty 

 what substances are actually absorbed, while it is probable that the roots themselves 

 exert a direct solvent action, for water dissolves but little material from many 

 soils rich in humus ''. The fungal mycorhizas which form symbiotic unions with 

 the roots of most saprophytic Phanerogams (Johow, 1. c., p. 501) are probably 

 of considerable importance in rendering the humus available for use, and at the 

 same time they compensate for the poorly developed root-systems in such plants as 

 Neottia Epipogon, Corallorhiza, &c. Drude found that a young plant of Neottia 

 surrounded by only a small portion of humus gradually became starved as 

 development continued ". 



The specific peculiarities of different phanerogamic saprophytes and parasites are 

 mentioned in the literature given by Frank and Goebel 7 , while Zopf and de Bary 

 give full details concerning fungi. In these works numerous examples are given of 

 obligate, facultative, and temporary parasites. 



Phanerogams appear to have less marked accommodatory powers, but never- 

 theless various stages of parasitism seem to be exhibited among the Rhinantheae *. 



Details in Sachs, Geschichte d. Botanik, 1875, p. 481. 



* See for example Hlubeck, Beleuchtung d. organ. Chemie d. Herrn Liebig, 1842. 



3 Saussure, Ann. d. Chem. u. Pharm., 1842, Bd. XLII, p. 275; Boussingault, Ann. d. chim. 

 et d. phys., 1841, iii. sen, T. I, p. 208; Schleiden, Grund/iige, 1845, 2. Aufl., Bd. II, p. 469. 



* Wiesner, Pringsh. Jahrb., Bd. VIII, 1872, p. 575; also Johow, Jahrb. f. wiss. Bot., 1889, 

 Bd. XX, p. 479. 



5 See Sachsse, Agriculturchemie, 1888, p. 122. Attempts to nourish green plants with artificial 

 humus solutions are without value. Such experiments were performed by Hartig ( Liebig, Die 

 Chemie, &c., 1840, p. 192); Saussnre, 1842, I.e.; Unger, Flora, 1842, p. 241; VViegmann, Bot. 

 Zeitung, 1843, p. 801 ; Trinchinetti, ibid., 1845, p. 112. Nor is the slow diosmosts of humus 

 solutions of marked importance. Cf. Detmer, Versuchsst., 1871, Bd. XIV, p. 279; Simon, ibid., 

 1875, Bd. XVIII, p. 470; Grandeau, Compt. rend., 1872, T. LXXIV, p. 988. 



6 Drude, Biologic v. Monotropa, 1873, p. 26. 



7 Goebel, Entwickelungsgeschichte d. Pflanzenorgane, 1883, p. 361; Frank, Lehrbuch, 1892, 

 P- 559; and of the later literature, Heinricher, Cohn's Beitrage z. Biologic, 1895, Bd. vn, p. 364 

 (Lathraea). 



8 [See Heinricher, Jahrb. f. wiss. Bot., 1897, XXXI, p. 87. Contrary to the usually accepted 

 opinion, Monotropa is a saprophyte only (Johow, ibid., 1889, Bd. XX, p. 480).] 



