250 THE BIOLOGY OF FLOWERING PLANTS 



with a low salt content. As regards the fungus we do 

 not know the details of its food relations, nor do we know 

 how the fructification is suppressed. 



The Orchids. — More attention has been paid to the 

 mycorhiza of the orchids than to that of any other group. 

 In several cases they have been exhaustively studied from 

 the morphological standpoint, and their biological relations 

 have been the subject of a great deal of work during the last 

 twenty years. Our knowledge of this side is largely due 

 to the French botanist Noel Bernard (1909), and to the 

 amplification of his work by Burgeff (1909). A useful 

 account of recent work is given by Rayner (i9i6fl). The 

 general life-history of the association is fully described 

 by Magnus (1900) for Neottia. The fungus is sharply 

 limited to the three or four external cortical layers of the 

 root ; it is never found in the inner cortex, nor in the 

 central cylinder, and only sparingly in the epidermal layer 

 (2 to 3 cells thick). Very few hyphae find their way outwards 

 into the soil. In the rhizome as many as six cortical layers 

 may be infected, and the fungus even reaches a short 

 distance into the flowering axis. Infection of new roots 

 takes place, at a very early stage, from the rhizome. The 

 mycorhizal cells are further sharply separated into two 

 distinct classes — an outer and an inner layer of " digestive 

 cells," and between these a layer of " host cells." 



In the host cells a coil of rather thick-walled hyphae 

 clothes the inner surface of the walls, and from this thinner 

 hyphae traverse the protoplasm and vacuoles, and probably 

 are absorptive in function. Throughout the life of the root 

 these cells present the same aspect. In the digestive cells 

 the fungus at first forms coils of thin-walled hyphae. Soon 

 these die and are evidently digested by the plant cell ; the 

 remains of the wall substance collapse and, together with 

 some plasma, are separated as an indigestible " clump," 

 which is surrounded by cellulose material — a sort of internal 

 excretion of waste matter. Starch appears at the time of 

 infection in small grains, which soon disappear and are 

 reformed after digestion. The digestive process is accom- 



