SIGNIFICANCE OF MYCORHIZA 255 



fications. Special branches penetrate the tuber and spread 

 through the outer cortex ; in the outer zone the appearance 

 of the cells is that of typical host cells, in the next zone the 

 fungus absorbs the cytoplasm, in the innermost zone the 

 cells digest the fungus. Tubers can live separately and 

 attain their full size only if they are infected, and only in 

 this state can they flower. An account of this case, 

 investigated by Kusano (191 1), is given by Rayner (1916). 

 Relation to Fungus. — The systematic position of the 

 orchid fungi, except in the case of Gastrodia, is uncertain. 

 Many can be isolated and grown in pure culture, but in 

 some cases, e.g. Neottia and Corallorhiza, so intimate is 

 their association with the plant that isolation has not yet 

 been possible. The orchid cannot grow without the fungus. 

 What the physiological relation between the two is in the 

 mature plant we do not know. There is evidence that the 

 fungus cannot fix free nitrogen, and thus differs from that 

 of the Ericaceae. We are left with the assumption that it 

 helps in converting insoluble organic compounds in the 

 soil into forms available for the plant, or that it is active in 

 the transfer of water and salts. The latter alternative 

 would apply to the green orchids, and the former to the 

 saprophytes. Both possibilities may be realised in a single 

 species. Apart altogether from our lack of knowledge of 

 the way in which transformation of organic compounds may 

 occur, the case of some of the saprophytes is particularly 

 difficult to understand. As we have seen, in Neottia the 

 connections of the fungus with the soil are extremely sparing. 

 In such a case it seems almost necessary to believe that 

 absorption is carried on directly by the roots and that the 

 action of the fungus is purely supplementary to the meta- 

 boHsm within the cell. Corallorhiza grows in sandy and 

 Ophrys in chalky soil, where humus is scanty. The fungus 

 is presumed to benefit by obtaining food from the plant ; 

 but here again the case of the saprophyte is difficult to 

 understand if we are to assume that, in the first place, 

 the fungus supplies the plant with all its food, and in 

 the second withdraws some from it. The saprophytes 



