IRREGULAR NUTRITION 



199 



walls. They enter the epidermis, mesophyll, and cortex, and 

 accompany the vascular strands. The mechanism at least is 

 here present for the fungus to act as a physiological intermediary 

 between the Heather and its surroundings. Possibly the fungus is 

 able to fix atmospheric nitrogen in some degree. In the root it is 

 also in connection with the soil, which is as a rule heavily organic 

 where the Heather grows. It would thus be in a position to supply 

 organic material and combined nitrogen. The entry of the fungus 



FIG. 150. 



Section through the mycorhizic region of the tuber of Phalaenopsis. At the top 

 of the figure are normal cells of the host : at its lower limit are cells crowded with 

 fungal filaments, but still retaining their nuclei. Between these zones is the 

 digestive tract, consisting of cells with lobed nuclei. Various stages of digestion 

 of the fungal filaments are seen in these cells. (After Bernard.) ( x 98.) 



into the host may originally have been parasitic, but it is tolerated. 

 In the Heather it has even become indispensable. By means of the 

 symbiosis many Ericaceae " have solved the problem of growth on poor 

 and unpromising soils, but have solved it at the price of their indepen- 

 dence " (Rayner). 



A similar relation has also been demonstrated in a number of 

 Orchids, and it may be held as a general feature of that family. The 

 fungus in question is filamentous, and septate, and lives symbiotically 

 in the cells of the host. It is referred to the genus Rhizoctonia, and 

 three species have been recognised. In certain cells the fungus remains 

 permanently active. But other cells of the host, which may accordingly 



