SIGNIFICANCE OF MYCORRHIZAS. 51 



on. In some instances the tips of the growing hyphse are 

 attracted to the plastids or to the centers of carbohydrate for- 

 mation. The presence of the fungus with the organs of inter- 

 change generally sets up disturbances in the cells occupied, of 

 which enlargement of volume, hyperchromatism, and sometimes 

 fragmentation of the nucleus are the most noticeable. It is 

 most notable, however, that the cells occupied by the vegetative 

 mycelium undergo almost no variation from the normal charac- 

 ters of such organs. The cells of the middle portion of the 

 cortex are generally very much enlarged, but this has been 

 shown to be due to the character of the food material received 

 rather than to the presence of the fungus. Branches of the 

 vegetative mycelium pass outwardly through the epidermis, 

 which may or may not traverse the root hairs, and after emer- 

 gence into the soil they are numerously divided and extend to 

 unknown distances in the soil. The mycorrhizal fungi of the 

 gametophytes of the lower forms usually occupy the external 

 or the lower layers of the thallus. 



On account of the great elasticity of the various factors to 

 be considered in the determination of the nature of the inter- 

 change which takes place in mycorrhizas, the final solution of 

 the matter has been attended with great difficulty. The chief 

 features of the metabolism of the uniting cells of the two organ- 

 isms, the composition of the substratum, the anatomy of the 

 higher plant, with special attention to degenerations, and the 

 behavior of the two symbionts when grown alone or in pure 

 culture must all be taken into account, and the consideration of 

 all these points entails an enormous amount of observation, 

 and no single mycorrhiza has yet been carried through all stages 

 of the work. Our conclusions, which are fairly final as far as 

 they extend, are therefore derived from evidence more or less 

 fragmentary, so far as any single formation is concerned, yet is 

 to be taken as fairly and justly supplementary. 



So far as the various theories concerning mycorrhizal plants 

 are concerned, it is to be said that Unger concluded that 

 Hypopitys (Monotropa hypopitys] was parasitic on the roots of 

 trees (1840). Pfeffer recognized the symbiotic relations of the 

 members of a mycorrhiza in 1877, anc ^ this was demonstrated in 



