294 ^ STUDIES IN SYMBIOSIS, i., 



attendant of the facility of obtaining supplies of complex organic compounds. 

 As the dependence upon already synthesised organic matter becomes more pro- 

 nounced, the chlorophyll-bearing structures undergo modification either by the 

 loss of chlorophyll or by the elimination of the photosynthetic leaves, as in 

 Galeola cassythoides and Dipodium punctatum, or by reduction in the leaf 

 surface. 



When the roots of higher plants avail themselves of the organic matter in 

 the soil, certain changes must occur in the absorbing cells or hairs to permit of 

 the absorption of these substances — for example, the plasmatic membranes of 

 the cells must be modified to permit of the osmotic diffusion into the cell vacuole 

 — the osmotic pressure of the cells must increase temporarily or permanently to 

 cause an inward flow of larger molecules of organic substances. Such changes 

 are similar to those which are generally supposed to occur during the trans- 

 location of soluble foods from one cell to another, for example, from the endo- 

 sperm to the embryo, etc. 



Fungal hyphae which enter into the closest association with the humus 

 constituents of the soil have a much greater power of digestion and solution 

 of the organic matter than the root hairs of a higher flowering plant, so that, 

 when the mycorhizic fungnis becomes associated with the higher plant, it is able 

 to provide the latter with gTeater supplies of organised food than the root 

 hairs. In consequence root hairs cease to develop — their fimction and usefulness 

 are subserved by the fungal hyphae, which can and do supply, not only organic 

 matter, but water and ash constituents as well. 



Since the researches on mycorhiza by Frank (1892), the number of plants 

 with a similar condition of the root has proved very considerable. The number 

 of such plants is so gTeat, and they belong to such varied affinities and. phyla, 

 that it would appear that the host-plant must derive some considerable benefit 

 from its association with the fungus. Its occurrence has been demonstrated in 

 Dicotyledons, Monocotyledons, Conifers, Ferns, and Liverworts. 



Wahrlich (1885) found mycorhiza in 500 species of Orchids in cultivation 

 at Moscow; McDougall (1899) and Groom (1895a) and numerous other re- 

 searchers have described other symbiotic saprophytes. Lawson (1917) has found 

 an endophytic fungus in the prothalli of Tmesipteris and Psilotutn which do not 

 contain chlorophyll and are saprophytic and subterranean. Many of the cells 

 of the prothallus contain a mycorhiza! fungus which apparently provides all the 

 food necessary for its host. The loss of chlorophyll and the association with the 

 fungus, if not the result of the subterranean habitat, are at any rate in com- 

 plete harmony with it. In this instance it is stated that the fungus ultimately 

 causes the death and disintegi-ation of the host cells which are infected. The 

 nucleus undergoes a gradual change, and disintegrates with the other protoplasmic 

 contents of the cell. It is sig-nificant that the fungus remains within the proto- 

 plasm of the host-cells without causing immediate destruction; and it seems 

 to me that the subsequent destruction is partly associated with the naturally 

 transitory existence of the prothallus. Lycopodium, Botrychium and other 

 Ptieridtophytes have subterranean 'and sapropliytic prothalli. Lang (1899) 

 describes an endophytic fung-us in the prothallus of Lycopodium clavatum. It 

 is highly probable that in all these PteridojDhytes, but especially in Tmesipteris 

 and Psilotum, the prothallus is infected with the fungais early in its develop- 

 ment, indeed shortly after germination of the spore. The spores of these 

 Pteridophytes germinate in such shaded situations that it is not surprising 

 that chlorophyll is absent, that the ever-present mycorhizal fungais should be- 



