IRREGULAR NUTRITION 203 



in tlic cortex. All these indications show that the materials of llie digested 

 fungus have been absorbed into the host-plant. 



The saprophytic Orchid Gastrodia, which grows in Oak woods in Japan, 

 provides another example. The plant consists ol colourless and rootless 

 tubers, which may bear small daughter tubers as offsets. The inflorescences 

 are borne only by tubers of large size, and they are found to be invariably 

 infected by rhizomorphs of AriniUaria mellea, which preys on the roots of 

 the Oak trees. The Rhizomorphs ramify on the surface of the infected tuber, 

 and penetrate through its corky coating by haustoria curiously similar to 

 those of Cuscuia. The infection spreads laterally in the cortex, but does 

 not penetrate deeply, being checked by the active resistance of the inner 

 layers, which not only control, but also digest and absorb the substance of 

 its hyphae. It thus appears that while the Rhizomorph is dependent upon 

 the Oak, the Gastrodia is parasitic on the fungus, and depends upon that 

 parasitism for its successful flowering. It is a case of parasitism at second 

 hand. The attack of the Rhizomorph is tolerated, and kept under control 

 by the resistant powers of the Gastrodia, so that the would-be assailant, itself 

 a parasite, is compelled to disgorge. The success of the whole arrangement 

 depends upon the resistant powers of Gastrodia being greater than those of 

 the Oak. 



It seems probable that in the coalition called endotrophic mycorhiza 

 the initiative came from the fungus, by penetration from the soil into 

 the tissues of the host-plant. The plant at first defended itself by the 

 digestive process, by which the intrusive fungus was kept within 

 bounds, and headed off from tissues of vital importance. Incidentally, 

 however, the plant gains materials by the digestion which are of value 

 for its own nutrition. The nourishment acquired by the fungus is 

 first worked up into the substance of its own well nourished hyphae. 

 These are then digested by the higher plant into which it has pene- 

 trated, and converted to the uses of the host. Such host plants may 

 be styled " fungivorous," in a sense parallel to those plants which have 

 adopted a carnivorous habit. But a nearer parallel is probably to be 

 found in that intra-celltdar digestion, seen in the Amoeba : or in the 

 behaviour of the Leukocytes of the animal body, which is a leading 

 feature in "Phagocytosis." 



An association such as that of Neottia with the fungus Rhizoctouia, 

 or of Calluna with its fungus, should not be regarded as an individual. 

 It is rather to be compared with a Lichen, being like it an obligatory 

 product of two distinct organisms. Such symbiotic conditions as these 

 occupy a middle position between two extremes. The one is that 

 of mortal disease, where one organism of an association causes the 

 ultimate death of the other. The other extreme is that of ivimunity, 

 where though two organisms may be in relation, the one has no power 

 against the other. An intermediate state between these extremes 



