BY JOHN MCLUCKIE. 295 



come associated with the pi'othalli, and that the prothalli should become sapro- 

 phytic. 



While numerous hypotheses have been formulated to explain the relation 

 between the fungus and the higher plant, the physiological association of the 

 two organisms is far from being clearly understood. A brief statement of these 

 hypotheses may not be out of place here — 



(1) Frank (1892) considers endophytic myeorhiza as a case of the higher 

 plant trapping the fungus and living parasitically upon it. 



(2) Fixation of nitrogen hypothesis. — The tubercules of Podocarpus are 

 inhabited by the hyphae of a fungus, and it is stated that the Conifer cannot 

 be cultivated without the fungus. Nobbe and Hiltner (1899) claim to have 

 grown Podocarpus for five years in quartz sand from which nitrogen was absent. 

 From this experiment it is concluded that the fungus of the tubercles of Podo- 

 carpus enables the plant to fix atmospheric nitrogen. Spratt (1912) has proved 

 that the tubercular bacteria fix free nitrogen. 



The investigations of Hiltner, Vogi and Nestler on Lolium temulentum with 

 which a mycorhizie fungTis is associated suggest the probable fixation of Nitrogen 

 hei-e also. 



Hiltner claims that the luxuriant growth of many plants affected by a 

 mycorhizie fungus supports the "Fixation of Nitrogen" hypothesis. 



(3) Froteid hypothesis. — Magnus (1901) in his investigation of Neottia 

 observed that the fungus within certain host-eeUs was partly digested, and 

 suggests that the abundant proteid constituents of the hyphae furnished nourish- 

 ment for the host. 



Shibata (1902) observed similar digestion of the fungal hyphae in the cells 

 of Psilotum rhizomes. 



This process of digestion of the hyphae of the mycorhizie fungus is similar 

 to the digestion of the baeteroids in Leguminous tubercles. The inference drawn 

 from these observation^.^ is that the host-plant obtains its proteid from the 

 fungal hyphae by their digestion, while the hyphae assimilate free atmospheric 

 nitrogen, or poorly oxygenated nitrogen compounds of humus — the higher plant, 

 in other words, obtains its supplies of nitrogen from a soil relatively poor in 

 combined nitrogen through the agency of the fungus. 



(4) In green plants possessing myeorhiza, the fungus probably supplies 

 the nitrogen and the higher plant the carbon, but the relationship of two non- 

 ehlorophyll-bearing organisms may be essentially different. The higher plant 

 would appear to be a parasite upon the fungus. After all, the symbiosis of a 

 green plant and a fungus is little removed from the true parasitism of a fungus 

 on a higher green plant. Certain cases of holosaprophytism in Angiosperms, 

 e.g., Thismia, have all the appearance of parasitism of the higher plant upon the 

 fungus, since, as far as present investigation shows, both the supplies of C. and 

 N. are obtained by the fungus. 



(5) Peptone- Asparagin hypothe.sis. — That the higher plants growing in 

 the humus soils of the forest are peptone or asparagin organisms, and that the 

 fungus is indispensable in manufacturing these nitrogenous compounds from 

 the materials of the humus. 



(6) Materials of the Ash hypothesis.— In 1900 Stahl put forward the 

 theory that the fungus does not supply the higher plant with nitrogen at all, 

 but its function is to provide it with the materials of the ash. Fungi collect ash 

 constituents with great avidity and therefore are vigorous competitors with 

 phanerogams in the humus soil which contains little nutritive salts. He claims 

 that higher plants gxow more vigorously in humus which has been deprived of 



