MYCORHIZA OF TREES AND OF HEATHER 247 



not fix free nitrogen in isolation, it is possible that it may do 

 so when associated with the root. It is very likely that in 

 general the mycorhizal fungus has an important action in 

 making available for the plant the otherwise unavailable 

 inorganic nutrient substances present in the humus. W. B. 

 McDougal (1914) looks on ectotrophic mycorhizas of 

 broad-leaved trees as a chance association in which the 

 fungus is a parasite. The maples he regards as possessing 

 an endotropic mycorhiza which is symbiotic. 



Endotrophic mycorhiza : Calluna.— The best investi- 

 gated case is that of the Ericaceae and particularly of Calluna 

 vulgaris, the heather, which has been cleared up by the work 

 of Rayner (1913-1922). The fungus is not confined to the 

 root, though there lies the region of infection and of its 

 principal development. It extends in an attenuated form 

 through the stem and leaves, and into the ovary. From 

 the intercellular spaces of the leaves hyphas extend into the 

 air. From the ovarial wall it stretches over to the minute 

 seeds, and, when these are shed, they carry with them 

 fragments of mycelium in and on the seed coat. 



On the germination of the seed, as the radicle begins to 

 elongate, and even before it has left the seed coat, it is 

 infected by the fungus hyphae through its external cells, 

 no root-hairs being formed. The fungus passes rapidly 

 from cell to cell dissolving a passage through the cell walls, 

 and is presently found like a coiled skein of thread in almost 

 every cell of the root cortex and epiderm. It spreads to 

 the branch roots and to the shoot. Later the fungus 

 pushes hyphae between the epidermal cells, and forms a 

 fine network on the outside of the root. Indeed, although 

 its principal development is intracellular, and we may con- 

 veniently class it with the endotrophic forms, it has been 

 described as ectotrophic and is a good example of an 

 intermediate type. 



By suitable means the Calluna seeds may be sterilised 

 and uninfected seedlings raised, but these never develop 

 properly. The cotyledons and a few leaves unfold ; the 

 root system is represented only by a few minute stumps at 



