SPECIAL ADAPTATIONS 373 



importance may be, the general principles of heterophic nutrition remain unaltered, 

 although Frank denies this. 



It has not yet been explained why mycorhizas are formed only on the roots 

 of certain plants, nor has it been determined to what extent the same fungus may 

 become associated with the roots of different plants. Epiphytic and endophytic 

 mycorhizas are probably formed by different fungi, and hence it is possible that 

 a variety of mycorhizal combinations with fungi may be possible. According to 

 Frank several of the fungi which grow on humus may form mycorhiza, and it 

 is possible that certain forms may be incapable of a separate existence. 



Mycorhizas seem only to be formed in a soil rich in humus, and hence they 

 isappear from roots which penetrate into sand, nor are any formed upon plants 

 grown in a water-culture ] . The development of a mycorhiza is accompanied by 

 partial or complete suppression of the root-hairs, which are indeed absent in most 

 phanerogamic saprophytes. This correlation affords an additional example of the 

 dependence of the formation of root-hairs upon external conditions (Sect. 26). 



Historical. Pfeffer was the first to suggest that the long known - endophytic 

 mycorhizas were examples of nutritive symbiosis, and the same view was put 

 forward by Kamienski with regard to the epiphytic mycorhiza of Monotropa 

 which he discovered 3 . The wide distribution of mycorhiza was, however, first 

 discovered by Frank, to whom this term and most of our knowledge on this 

 subject are due. 



The penetration of parasites. Miyoshi 4 has shown that the penetration 

 of a parasite is mainly due to the action of chemotropic stimuli, for the 

 penetration of non-parasitic fungal hyphae through stomata, or even through 

 the epidermis, may be induced by injecting a leaf with solutions of meat- 

 extract, sugar, plant-extracts, &c., which chemotropically attract the fungal 

 hyphae. The same result may be obtained with widely different fungi, 

 including normal saprophytes, and the hyphae of Penicillium glaucum and 

 Aspergillus niger may bore through a fragment of epidermis or an artificial 

 cellulose membrane lying between them and a nutrient solution, whereas if 

 floated on water this does not occur. 



The stimulus simply induces the fungal hypha to follow a certain 

 path, and thus may lead it to bore through the membranes which separate 

 it from the source of attraction. This is generally accomplished by an 

 excretion of solvent enzymes, which aid the boring action of the apex of 

 the elongating hypha, but although the solvent action is perhaps the most 

 marked, still the mechanical pressure exerted by the tip of a growing 

 hypha may suffice for the penetration of very thin gold-leaf. Radicles may 



1 Frank, 1. c., 1888, p. 253. According to Johow (1. c., p. 506), mycorhizas are formed only 

 upon the adherent surfaces of the aerial roots of tropical orchids. 



2 The literature is given by Wahrlich, Bot. Zeitung, 1886, p. 481 ; Johow, I.e., p. 503. 



3 Pfeffer, Landw. Jahrb., 1877, Bd. VI, p. 497 ; Kamienski, Bot. Zeitung, 1881, p. 461. 



4 Miyoshi, Jahrb. f. wiss. Bot, 1895, Bd. xxvm, p. 269; Bot. Zeitung, 1894, p. 23. 



