246 THE BIOLOGY OF FLOWERING PLANTS 



mycorhiza with the pine, and showed that they belong to the 

 Hymenomycetes and to the genus Boletus. The endotrophic 

 mycorhiza found in bogs is due to a fourth form. 



It has long seemed probable that forest tree mycorhizas 

 are formed by the large toadstools characteristic of woods, 

 some of which are constantly associated with a particular 

 species of tree. This has been finally proved by Melin 

 who has shown, by infection of sterile seedlings with pure 

 cultures of the fungi, that the mycorhiza of the larch is 

 derived from Boletus elegans, a toadstool that is common 

 in larch woods. One of the fungi of the pine is Boletus 

 liiteus. Mycorhiza has thus been synthesised. 



Peyronel (1921) has demonstrated mycorhizal connection 

 between the roots of forest trees and a number of large 

 humus fungi belonging to the Tuberales and Basidiomycetes. 

 Thus the mycorhiza of the larch may be formed by Boletus 

 elegans, B. laricinus, B. cavipes ; of the aspen by Boletus 

 rtiftis ; of the beech by Cortinarhis proteiis , Boletus dysenterica 

 and B. cyanescens, Hypochnus cyanescens, and Scleroderma 

 vulgare. It may be taken that older work in which the 

 mycorhizal fungus was referred to species of Penicillium, 

 Mucor, etc., was vitiated by a faulty technique which easily 

 admits to cultures the omnipresent spores of these fungi. 



The ectotrophic mycorhiza of forest trees does not seem 

 to be essential to their existence. At least in some soils the 

 trees can exist without the fungus. It is not yet certain 

 whether under favourable conditions sterile trees grow as well 

 as infected. Nor is there a certain answer to the question 

 of the function performed by the mycorhiza when normally 

 developed. From the nature of the case it must replace the 

 root-hairs in the transference of water and salts to the root ; 

 but it does not follow that it supplies these more efficiently 

 than would the root-hairs themselves. Melin states that the 

 mycorhiza of pine and spmce is poorly developed in mild 

 humus and well developed in raw humus, and that it is 

 necessary to the success of the tree on drained peat. He 

 thinks that in one way or another the fungus transfers 

 nitrogen from the soil to the plant. Though the fungus does 



