1923] Fruit-body of New Parasitic Rhizopogon 105 



between the host and the would be parasite." In support of this last 

 view he calls attention to the fact that E. Bruns (2) had shown that 

 Polysaccum causes an ectotrophic mycorhiza of pine roots, and that 

 "some of the hyphae actually dip deep into the tissue of the root, at 

 the same time absorbing so vigorously as to play havoc with the in- 

 fected tissues." 



The earliest observers of mycorhizas thought that Elapliomijces 

 and tubers or truffles caused all the ectotrophic mycorhiza. Gradually 

 the list of fungi known to cause mycorhiza has been enlarged. Mac- 

 Dougal (9) in 1899 gave a list of the fungi known at that time to 

 produce mycorhiza. The list included species of Fusiporium, Euro- 

 tium, Pythium, Nectria, Celtidia, Elaphomyces, Polysaccum, Geaster, 

 Boletus, Lactarius, Cortinarius, Tricholoma, and Agaricus. Of these 

 Elaphomyces, Polysaccum, Geaster, Boletus, Tricholoma, and Agari- 

 cus formed mycorhizas with the conifers, some of these with other 

 plants too. Kauffman (8) has shown that Cortinarius rubipes forms 

 mycorhizas on red oak, sugar maple, and Celastrus scandens. Penning- 

 ton (13) in 1910 reported that a species of Cortinarius and "probably 

 a form of Russulu emetica^^ produced mycorhizas upon the red oak, 

 and that Boletus speciosus and Tricholoma transmutans produced 

 them upon the black oak. McDougall (10) added four species to 

 the known list of ectotrophic mycorhiza-forming fungi: Russula sp. 

 on Tilia' americana. Boletus scaher fuscus on Betula alba var. papyri- 

 fera, Cortinarius sp. on Betula alba var. papyrifera, and Scleroderma 

 vulgare on Quercus alba. He show^ed that at least four, and probably 

 more, different species of mushrooms may form mycorhizas on the 

 same tree. He also showed that the endotrophic mycorhizas of the 

 maples are sometimes symbiotic associations, and sometimes associa- 

 tions in which the fungus is surely a parasite of the roots ; but that the 

 ectotrophic mycorhizas of forest trees are not symbiotic associations, 

 but instances of parasitism of fungi on the roots of trees. This opin- 

 ion is reiterated in a later paper (12). 



There has probably been more research and publications on the 

 endotrophic mycorhizas than for the ectotrophic forms. Many of 

 them seem to show real symbiosis, or what McDougall (11) terms 

 "reciprocal parasitism." The Orchidaceae and Ericaceae show gen- 

 erally a mycorhizal condition. MacDougal (9) says that Warhlich 

 examined 500 species of Orchids in cultivation at Moscow and found 



