SIGNIFICANCE OF MYCORRHIZAS. 53 



in general, that we are justified in the following conclusions as 

 to the relations of the symbionts ; the higher plant furnishes a 

 habitat for the vegetative mycelium of the fungus, and yields 

 to it certain carbohydrate foods, principally starch and sugar. 

 The fungus takes up humous compounds from the soil which 

 are poor in oxygen, conducts them to the branches inside the 

 body of the higher plant, and manufactures proteids which it 

 in turn yields to the higher plant. The actual food substance 

 may all come through the body of the fungus, yet it undergoes 

 elaboration in the higher plant to forms which are very advan- 

 tageous foods for the fungus, such as starch and sugar. 



Ectotropic mycorrhizas are generally unmistakable, but in 

 many instances it is difficult to determine whether the fungus 

 inhabiting a root is parasitic or sustains symbiotic relations to 

 the higher plant. Indeed it may do both at different stages of 

 development perhaps. The chemical exchange between the 

 two plants may balance at one time, while at another, one may 

 receive a preponderance of benefit. Then again a mycorrhizal 

 formation may enable the individual to obtain its food more 

 economically, yet such easy nutrition may set up degenerations 

 which will unfit the seed plant for living outside of the nar- 

 rowest conditions, and which may result in the extinction of 

 the species. Pterospora appears to be an example of such 

 degeneration. 



As a result of the investigations of the last decade, it is found 

 that the habit of forming mycorrhizas is of very wide prevalence 

 among the seed plants, ferns, lycopods, liverworts, and equise- 

 tums. This and the determination of the fact that all plants 

 are capable of absorbing and using organic substances of some 

 complexity of structure must be accounted among the most 

 important results of research upon the nutrition of plants 

 during the closing decade of the century. 



So far as relationships are concerned, it may be said that the 

 conifers, the orchids, the heaths, oaks, poplars, beeches are 

 most thoroughly given over to the formation of mycorrhizas, 

 and a few of the orchids have developed this adaptation to such 

 extent that they have lost their chlorophyl and depend entirely 

 upon the fungus for carbon compounds. The pine saps {Mono- 



