302 MYCORRHIZAE AND MYCOTROPHY 



even the parenchyma cells of the central cylinder. One or more 

 distinct species may be involved in one and the same pseudo- 

 mycorrhiza. 



In the third kind the following features are noteworthy: (a) 

 presence of intercellular weft or Hartig net, (b) presence of a 

 fungal mantle, (c) hypertrophy of cortical cells, (d) occasional 

 intracellular hyphae in cortical cells, (e) profuse dichotomous 

 branching, and (f) continued elongation during one growing 

 season. 



Fungi involved in mycorrhizal formation. Numerous spe- 

 cies of funm have been found to be associated with mycorrhizae, 

 and it is not the present purpose to list all of them but merely to 

 indicate their number and taxonomic diversity. In short, mem- 

 bers of each of the three large classes, Phycomycetes, Ascomv- 

 cetes, and Basidiomycetes, enter into mycorrhizal formation. The 

 most extensive lists assembled appear to be those of the Italian 

 mycologist Peyronel [Rayner (1927)], who designates the fol- 

 lowing species: 



1. On Fagus sylvatica, Cortinarius proteus, C. bivelus, Boletus 

 cyanescens, B. chrysenteron, Hypochmis cyanescens, Scleroderma 

 vulgare, Amanita rubescens, Lactavius subdulcis, L. blennius, 

 Russula emetica, and R. nigricans. 



2. On Cory his avellana, Lactarins coryli, L. subdulcis, Boletus 

 cbrysenteron, Strobilomyces strobilaceus, Hypochmis cyanescens, 

 Amanita rube sc ens, Rhodopaxillus nudus, Cortinarius proteus, C. 

 multiformis, C. violaceus, and Hydmnn repandum. 



3. On Betula alba, Amanita muse aria, Amanitopsis vaginata, 

 Lactavius necator, L. torminosus, Boletus scaber, Scleroderma 

 vulgare, Russula rhodoxantha, and Trichoderma flavobrunneum. 



4. On Larix decidua, Amanita muscaria, Russula laricina, Hy- 

 grophorus bresadolae, H. lucorum, Scleroderma vulgare, Lacta- 

 rins rufus, Gomphidius gracilis, and Paxillus lateralis. 



5. On Popolus tremella, Cortinarius collinitus. 



6. On Quercus robur, Amanita citrina, Lactarius subdulcis, and 

 Russula cyanoxantha. 



7. On Cast an ea vesca, Amanita rube sc ens, Russula lepida, R. 

 rubra, and Scleroderma vulgare. 



.Mycologists have long known that certain Hymenomycetes are 

 restricted to the area beneath particular species of trees and are 

 never collected under other kinds of trees. In Sweden Boletus 



