18 INTRODUCTION [CH. 



Kusano has shown that the rootless, saprophytic orchid, Gastrodea elata, 

 becomes dependent on the formation of mycorhiza only on the incidence of 

 its flowering period ; it is capable of vegetative growth without infection, and 

 the distribution of the fungus in its tissues is strictly limited; the fungus in 

 this case is Armillaria mellea, a well-known facultative parasite. 



This may be regarded as a relatively early stage in the development of 

 endotrophic symbiosis in which the angiosperm succeeds in utilizing as a 

 factor in its own nutrition the mycelium of the attacking parasite. It suggests 

 that in this relationship we are dealing not with an association of two or- 

 ganisms for the benefit of both, but with a struggle between the would-be 

 parasite and the host which controls, makes use of, and finally becomes 

 dependent upon it. The balance of power is often very delicate, and any 

 weakening on the part of the angiosperm gives the fungus an opportunity 

 to assume the parasitic habit ; thus the endotrophic fungus regularly becomes 

 parasitic on the old stamens and corolla of Arbutus and on the dying leaves. 

 Endotrophic mycorhiza occurs in the Ericaceae and in their allies showing, 

 so far as investigation has gone, the same extreme conditions as in Calluna; 

 in certain Gentianaceae, Solanaceae, Labiatae, Umbelliferae, Ranunculaceae 

 and Liliaceae, in most if not all orchids, in species of Viola and Arum, and, 

 according to the researches of von Tubeuf, in gymnospermous trees other 

 than the Abietineae. 



Endotrophic mycorhiza is also found in certain liverworts and mosses, 

 and in many of the Pteridophyta. It is well developed in the prothallus 

 of Lycopodium and in the leafy plant of species of both Lycopodium and 

 Selagiuella, in the roots of Cyathea and of several of the Marattiaceae, and 

 in both the prothallus and sporophyte of the Psilotaceae and Ophioglos- 

 saceae. In most of these cases it does not appear to be essential to the 

 nutrition of the sporophyte since the latter seldom shows a correlated reduc- 

 tion of the assimilatory apparatus, but it is often a principal factor in the 

 nutrition of the prothallus, which in the presence of mycorhiza may be 

 subterranean and lacking in chlorophyll. Mycorhiza has been recorded in a 

 number of fossil plants. 



Bernard has suggested a relationship between tuberization and the 

 presence of an endotrophic mycelium. It is significant that very many sym- 

 biotic plants, including notably the orchids and the sporophyte and gameto- 

 phyte of the lycopods, tend to assume a tuberous habit or to develop bulbs 

 or protocorms. Bernard's researches have disclosed mycorhiza in tuber- 

 forming species of Solanum, indicating a similar origin for the potato. 



Exotrophic Mycorhiza. The majority of our forest trees, including 

 Abietineae, Salicaceae, Fagaceae and Betulaceae, as well as some other plants, 

 possess an exotrophic mycelium forming a dense felt over the apical parts 

 of the infected roots; under this influence the development of root hairs is 



