MYCORHIZA OF HEATHER 249 



plant cannot do. The presence of fungal hyphae outside 

 the plant, both in the air and in the soil, is probably of 

 importance in relation to the supply of nitrogen. The 

 heather thus receives nitrogen compounds from the fungus, 

 and the fungus in its turn must receive organic food from the 

 plant. Its main development is intracellular and its only 

 source of carbon compounds is the organic matter of the 

 cell. It does not seem to act harmfully, no degeneration 

 being visible in the cells of normally infected plants. But 

 the balance between plant and fungus is a very delicate one, 

 for, if the plant is enfeebled in any way, the fungus takes the 

 upper hand and overmasters its host. In normal conditions 

 the fungus never fructifies, but in a weakened plant the 

 fructifications of the fungus indicate its dominance. This 

 altered relation is beautifully seen in seedlings grown in 

 calcareous soils, Calluna is a well-known example of a 

 plant that lives habitually in acid humus, and when grown 

 in chalk or lime soils is stunted and chlorotic, as the result 

 of some difference in the soil chemistry or physics the 

 precise nature of which is at present not understood. In 

 seedlings grown in such soils, or even in watery extracts 

 from them, the fungus of the mycorhiza is seen to become 

 dominant ; so that even an environmental change which 

 may occur in nature, is sufficient to upset the balance 

 normally maintained between the two symbionts. An 

 account of the Calluna symbiosis by Christoph (1921) differs 

 from Rayner's in many points ; he finds that Calluna can 

 thrive without the fungus, which he regards as a parasite. 

 Rayner (19226) has shown that Christoph's conclusions 

 are not well founded. 



We have, then, a very complete though not exhaustive 

 account of the mycorhizal relation of Calluna. We do not 

 know the mechanism of the eflPect of the fungus on root 

 formation. It is not certain that the assimilation of nitrogen 

 is the only way in which the fungus serves its host. It is 

 quite possible that it afl^ects the supply of salts and of water. 

 We may remark on the prevalence of mycorhiza in the 

 heaths and their allies, typical inhabitants of peaty soils 



