248 THE BIOLOGY OF FLOWERING PLANTS 



the base of the hypocotyl, when, at a corresponding stage 

 in an infected seedling an extensive branched system of 

 fine transparent roots has been formed. The fungus-free 

 seedlings never get beyond this stage (Fig. 31). 



Here, then, we have an association much closer than that 

 exhibited by the mycorhiza of forest trees ; for in these, at 

 least in certain conditions, good development can take 

 place in absence of the fungus, while in the case of Calluna 

 — and, it is probable, in many other Ericaceae — normal 

 development, and in particular root formation, can take 

 placeonly if the fungus '^is present. Furthermore,* regular 



Fig. 31. — Seedlings of heather {Calluna vulgaris); i, in sterile cul- 

 ture ; 2, infected with mycorhizal fungus ; both five months after 

 sowing. (After Rayner.) 



infection by the fungus is ensured by the presence of its 

 hyphae on the seed. The manner in which development is 

 influenced by the fungus is at present quite obscure. We 

 now know, however, that the fungus definitely benefits the 

 general growth of the plant in a particular way. It has been 

 shown by Ternetz (1907) that fungi isolated from the roots 

 of the Ericacese are capable of assimilating free atmospheric 

 nitrogen. The identity of Ternetz 's fungi — species of the 

 genus Phoma — ^with the mycorhizal fungus, has been 

 confirmed by Rayner (1922), w^ho also showed that normally 

 infected seedlings could flourish in a medium quite free 

 from nitrogen compounds, and this the ordinary green 



