106 NUTRITIVE CONJUNCTIVE SYMBIOSIS 



The internal structure of the mycorrhizas of dicotyledonous trees 

 is characteristically different from that of coniferous trees. (Fig. 51 .) 

 In the case of dicotyledonous trees the cortical cells are ordinarily 

 palisade-shaped on one side of the root and irregular in sha})e on the 

 other side while such a condition has never been reported on a 

 coniferous tree. No reason for this morphological phenomenon 

 is known. 



These mycorrhizas are caused by many kinds of summer and 

 autumn mushrooms. The mycelium of the mushroom penetrates 

 the outer cell wall of the rootlet and splits the wall by dissolving out 

 the middle lamella. It then continues to grow and branch until it 



Fig. 50. — Ectotrophic mycorrhizas of the hornbeam {Carpinus betulus) in leaf 

 mold. (Photograph by Somerville Hastings.) 



forms a fungus mantle which completely covers the rootlet, tip and 

 all. At the same time branches of the mycelium penetrate between 

 the outer cortical cells of the rootlet splitting the walls and pushing 

 the cells apart. Further growth of the rootlet is inhibited by the 

 fungus mantle which covers it and excessive branching is induced 

 just as when the tip of a root or shoot is cut off. The new branches, 

 however, are in turn transformed into mycorrhizas by the mycelium, 

 and the final result is the cluster of short stubby branches. 



The ectotrophic mycorrhizas of forest trees, in the northern and 

 northeastern parts of the United States, are produced during sum- 

 mer and autumn. They persist unchanged during the winter, so 



