TRANSACTIONS OF SECTION K. 847 



In tlie humus of forests we find the roots of beeches and other Cupulifera, 

 ■willows, pines, and so forth, clothed with a dense mantle of hyphas and swollen into 

 coral-like masses of mycorhiza ; in similar soils, and in moorlands which abound 

 in the slowly decomposing root-fibres and other vegetable remains so characteristic 

 of these soils, the roots of orchids, heaths, gentians, &c., are similarly provided 

 ■with fungi, the hyphae of which penetrate further into the tissues, and even send 

 haustoria into the living cells, but without injuring them. 



As observations multiplied it became clear that the mycorhiza, or fungus-root, 

 was not to be dismissed as a mere case of roots aS"ected by parasites, but that a 

 symbiotic union, comparable to that of the hchens, exists; and that we must 

 assume that both the tree and the fungus derive some benefit from the connection. 



Pfeifer, in 1877, suggested that the deficiency of root-hairs observed in orchids 

 might be explained by the fungus-hyphse playing the part of these organs, and 

 taking up materials i'rom the soil which they then handed on to the roots. He is 

 quite clear on the subject, and recognises the symbiosis definitely, comparing it 

 ■with other cases of symbiosis indicated by De Bary. 



Frank stated that, as the results of experiments, seedling forest-trees cannot be 

 grown in sterilised soil, where their roots are prevented from forming mycorhiza, 

 and concluded that the fungus conveys to the roots organic materials, which it 

 obtains by breaking down the leaf-mould and decaying plant-remains, together 

 with water and minerals from the soil, and plays especially the part of a nitrogen- 

 catching apparatus. In return for this important service the root pays a tax to 

 the fungus by sparing it certain of its tissue contents, and no doubt can well afl'ord 

 to do so. 



It appears that the mycorhiza is only formed where humus or vegetable-mould 

 abounds. _ In sandy soils the roots bear root-hairs, as usual, and it is now clear 

 that, while mycorhiza is a far more general phenomenon than was previously 

 supposed, it is not essential for all the roots, nor even under all circumstances for 

 any of them. 



Probably what really happens is this. Trees and other plants with normal 

 roots and root-hairs, when growing in ordinary soil, can adapt their roots to life in 

 a soil heavily charged with humus only by contracting the symbiotic association 

 with the fungus and paying the tax demanded by the latter in return for its 

 supplies and services. If this adaptation is impossible, and no other suitable 

 variation is evolved, such trees cannot grow in such soils. 



In certain cases— e.,!/., ground orchids, Monotropa, various Ericacece, &c. — it 

 would seem that the plant is unable to grow in other than humus soils, and always 

 forms mycorhiza. 



Much further we cannot at present go, but it is evident that various diSerent 

 grades of symbiosis exist in these mycorhizas. In the first place, there are several 

 different fungi concerned — those on cupulifer® and pines, apparently mostly 

 Tuberacea and Gasteromycetes, and allied forms, being diflerent from those in 

 orchids, some at least of which appear to be Nectrias or related genera. 



The physiological relations of the root to the fungus must be difierent in details 

 in the case of non-green, purely saprophytic plants, like Neottia, Monotropa, &c., 

 and in that of the green plants like Erica, Fagus, Pinus, &c. 



It is well known that ordinary green plants cannot utilise vegetable debris 

 directly, whereas trees in forests appear to do so ; this in appearance only, how- 

 ever, for the fungi, yeasts, and bacteria there abounding are actively decomposing 

 the leaves and other remains. 



Now it is possible that the mycorhiza theory is not applicable in all cases, and 

 that, sometimes, -what happens is this. The trees, once well established, make so 

 good a fight that in spite of the leaf-decomposing fungi attacking their roots para- 

 sitically, or merely ensconcing themselves in the dead primary cortex as it is 

 sloughed, they manage to keep going and to obtain such shares of the nitrates and 

 other products due to tlie fungus-action as satisfy their needs. But although 

 there may be something to be said for this view as regards a few forest-trees, it is 

 not easy to see how it would apply to the non-assimilating humus-plants like 

 Neottia, Monotrojja, &c., and we may probably regard the two sets of cases as 

 standing or falling together. 



