372 



THE FOOD OF PLANTS 



absent (Sect. 64). No absolute proof of this has, however, as yet been 

 established, although, according to Frank, the growth of Finns and of 

 plants belonging to the Cupuliferae 1 is retarded when the association of 

 the roots with fungi is prevented by cultivation in a sterile soil. 



Future researches must determine whether in all cases and under all cultural 

 conditions this symbiotic association has the same importance. It may help 

 to provide saprophytes with the organic food they require, but in the case of 

 the Cupuliferae, &c., it may be of service only in the absorption of nitrogen com- 

 pounds or of mineral constituents. It has yet to be determined whether the 

 substances required are directly transferred by the fungus, or whether it is able to 

 elaborate certain nutrient substances for its host. (On the peptone-algae of certain 

 lichens, cf. Sect. 64.) The fungus may be of use in the assimilation of ammonium 



salts, for although these afford a much 



% A less suitable source of nitrogen for most 



terrestrial plants than do nitrates, the 

 latter are usually present only to a 

 slight extent in forest soil 2 . More- 

 over, the increased absorptive area 

 secured by the presence of fungal hy- 

 phae must be of importance, and in 

 addition the latter are usually able to 

 exercise a more energetic solvent action 

 than root-hairs can. Frank 3 supposed 

 that the fungi may be cultivated by 

 the roots in order to be devoured at 

 a later period, but this has not been 

 FIG. 55. Epiphytic mycorhiza of Fagus syivatica. proved, and is highly improbable. 



(A) twice magnified; (B) tip of root partially denuded , . . - 



of the investing mantle (x 3 o). Even when endophytic fungal hyphae, 



which die a natural death, are absorbed 



partially or entirely by the cytoplasm of the host-plant, this does not from any 

 proof of Frank's supposition, for dead fungi or bacteria may be reassimilated 

 in pure cultures by their own kind. 



Fungi and bacteria, even without entering into conjunctive symbiosis, are of 

 use in many ways in preparing media suitable for the development of other plants, 

 and a correct estimation of the mutual interdependence of different organisms in 

 the balance of nature shows that with regard to mycorhizas, whatever their use and 



1 Frank, I.e., 1888, p. 265; Figures in Pflanzenkrankh., 1895, p. 294. Pinus: Frank, Bot. 

 Centralbl., 1895, Bd. LXXII, p. 18. 



" Baumann, Versuchsst, 1887, Bd. xxxm, p. 302 ; Ebermayer, Ber. d. Bot. Ges., 1888, p. 217. 

 No conclusions can be drawn from the absence of nitrates in mycorhizas (Frank, I.e., 1888, p. 249), 

 for nitrates accumulate only when the conditions are suitable for passive secretion. 



3 Frank, I.e., 1891, p. 244; Lehrbuch, p. 561. On the fungus gardens of certain ants, see 

 A. Moller, in Bot. Mitth. a. d. Tropen v. Schimper, 1893, Heft 6. 



