388 LECTURE XXIV. 



It has been established by the memorable labours of Robert Hartig ' that the 

 stalked Fungi growing out from diseased tree-trunks, such as Agaricus melleus and very 

 many species the fructification of which tends to grow out from tree-trunks in the 

 form of a sessile fan-shaped structure, such as Tramefes pitii, Polypoms fuhus, P. 

 vaporarius, P. mollis, and P. borealis, all of which occur on Conifers, as well as Hydtmm 

 diversidens, Telephora perdrix, Polyporus sulphureus, P. ignartus, P. dryadeus, and 

 Siereum hirsutum, all of which infest Oaks, and others, are by no means the harm- 

 less parasites they were formerly considered to be ; he shows, on the contrary, that 

 the delicate filamentous mycelium of these Fungi penetrates into the roots (Trametts 

 radiciperda and Agaricus melleus — the latter creeps in the soil and constitutes the 

 subterranean ' Rhizomorpha '), or often directly into the stems and branches, usually 

 through wounds previously produced, and vegetates for years in the wood and destroys 

 it. In consequence of this, even large trees may be killed by the mycelium dwelling in 

 them, or at least their wood be made useless. From the mycelium vegetating in the 

 interior of the wood, in all the species named, the fructifications already referred to 

 at length appear :: these are ofien very large, and in many cases themselves go on 

 growing further for years (e. g. Polyporus ignarius). 



Referring to Hartig's works ^ for the mode of life of these plants, I will here, 

 where we are concerned with questions of nutritiwi, subjoin only some of his 

 results with respect to the changes which the wood of Conifers undergoes by the 

 action of the mycelium. The changes of the cell-walls of the Avood, says Hartig, 

 which are produced by the Fungus, are of great interest. In the first place, it is 

 an important fact that the hyphae of the Fungus always vegetate in the interior of 

 the wood-cells, and only bore through the septa, whereby however it is not pre- 

 cluded that they occasionally grow forward also laterally or upwards or downwards 

 in the walls, and become branched in them. The mycelium of Agaricus melleus 

 lives in fact in the latter form, chiefly in the hard substance of the cell-walls of the 

 wood. The holes bored in the walls (which either have from the first the same 

 diameter as the fungus-filament or are often distinctly smaller, because the hyphae 

 inside the wall penetrated by them thin off) eventually become widened, as a rule 

 considerably, since the solution of the substance of the cell-wall appears to take place 

 more rapidly outwards, starting from the perforation. The activity of the fungus- 

 hyphse in the interior of the woody tissue varies. Sometimes they absorb directly 

 and unaltered the organic substances met with on the way, and produce fungus- 

 protoplasm and cellulose from them, with the separation of carbon dioxide : on 

 the other hand, they extract from the organic compounds at a greater distance 

 certain substances which they require for their nourishment, and thereby, according 

 to Hartig, cau-se a chemical alteration of the cell-contents or cell-walls. The fungus- 

 hyphae penetrating into the sound wood of the Oak absorb the tannin unaltered, 

 and it can be detected in them by means of salts of iron. The numerous 

 lateral hyphae, which bore through the walls like haustoria, entirely absorb the dis- 

 solved substance of the cell-walls of the wood, through their apices; they exert in 

 addition a very profound decomposing influence on the contents and walls of the 



^ Robert Hartig, ' Wichtige Krankheiten der Waldbätime'' (Berlin, 1874), and 'Die Zersetz 

 iingserscheitningen des Hohes der Nadelbäume und der Eiche' (Berlin, 1878). 



