MYCORHIZA OF TREES 



245 



have their roots associated with fungi. Only in the Cruci- 

 fera5 and Cyperaceae did Stahl fail to find any mycorhizal 

 species. He found that mycorhiza was most frequent in 

 soils poor in nitrogen and mineral salts, and founded the 

 theory that the chief benefit derived by the flowering plant 

 from its fungus was an increased supply of salts. 



Ectotrophic mycorhiza is, as we have said, seen in its 

 most characteristic form in forest trees. It is particularly 

 well developed in the 

 pine, the larch, and other 

 conifers, in the beech, 

 oak, hazel, and other 

 cupuliferous trees. Young 

 side roots are infected by 

 fungi growing in the rich 

 humus soil. The roots 

 have their growth in 

 length curtailed and the 

 infected tips are club- 

 like. Infected pine roots 

 fork repeatedly and 

 thicken into a coral-like 

 mass. The mature my- 

 corhiza forms a felted or 

 hairy mantle round the 

 root, from which numer- 

 ous hyphse penetrate 

 between the walls of the 

 epidermal cells. Root- 

 hair formation is pre- 

 vented (Fig. 30). It has been recently shown by Melin 

 (1921), for the pine and the spruce, that infection takes place 

 through the root-hairs or epidermal layer, and that at first 

 the fungus exists inside the cortical cells, in which it is 

 ultimately digested ; later it passes between the cells of the 

 epiderm and forms the typical mantle. In trees growing 

 in wet bogs the fungus exists exclusively in the cortical 

 cells. Melin isolated three distinct fungi which can form 



Fig. 30. — Ectotrophic mycorhiza; i, 

 section of root of the hornbeam, the 

 fungus forming a mantle outside and 

 penetrating between the epidermal 

 cells ; 2, root system of seedling fir. 

 I, X 320 (after Frank). 2, Nat. 

 size. 



