Chap. V] 



THE SOIL 



109 



granulatus, and Noack also in species of Geaster, Agaricus, Lactarius and 

 Cortinarius, and in the well-known Agaricus muscarius, the mycorhizal fungi 

 of our forest trees. 



The relations between fungus and root are symbiotic, that is to say, 

 useful to both organisms, at least in the case of endophytic fungi, for 

 P. Groom's observations on Thismia lead to the conclusion that the pre- 

 sence of the fungus promotes the elaboration of proteids in the root-cells, 

 and that between both organisms an exchange of nutritive matter occurs, 

 although the nature of this is unknown. 



The relations between fungus and root are very simple in epitrophic mycorhizae ; 

 in those that are endotrophic, on the contrary, they are often very complicated. As 

 an instance of the latter, the mycorhiza of Thismia Aseroe, which has been studied in 

 detail by P. Groom, may be described somewhat more minutely (Fig. 59). The 



Fig. 56. Monotropa Hypopitys. Portion of a 

 young plant. After Kamienski. 



Fig. 57. Monotropa Hypopitys. Epi- 

 dermis and mycorhiza-fungus. Magnified 

 450. After Kamienski. 



coral-like branching root-system has a fine papillose surface. The thin-walled 

 peripheral tissues free from starch, which the author termed the sheath (s/i.), are 

 traversed longitudinally by a few fine mycelial hyphae. Within the sheath next 

 occurs a sharply differentiated layer of cells (e.c), all of which contain swollen 

 hyphae, wound round in a kind of coil. These hyphae are externally coated with 

 cytoplasm. Inside the exocortex (e.c.) comes the limiting layer (/./.), in the cells 

 of which fine, delicate hyphae here and there dilate into thick vesicles filled with 

 proteids. An inner part of the cortex (mediocortex m.c.) is composed of two or 

 three layers rich in starch and characterized by possessing dead, yellow masses of 

 mycelium in all its cells, except those containing raphides. The endodermis and 

 central cylinder are free from the fungus. 



On entering from the sheath into the deeper lying cells the terminal point of the 

 mycelial thread grows directly towards the nucleus. In the medullary layer of 

 the cortex (mediocortex), where the relations are clearest, the starch of the infected 

 cells is at once dissolved, but reappears on the death of the fungus. The latter 



