INJURIES INDUCED BY PLANTS 



cellulose, after which decomposition spreads generally through- 

 out the walls. Thus the walls constantly become thinner, till 

 finally only the corners remain where three tracheids join (Fig. 

 12). Several wood-parasites, e.g. Polyporus 

 Sckweinitziiaxid. P.sufy/t2ireus,mducG a form 

 of decomposition owing to which the walls, 

 with the exception of the middle lamella;, 

 shrink so much as to give rise to numerous 

 cracks which ascend from right to left. By 

 certain adjustments of the microscope we of 

 course see simultaneously the correspond- 

 ing cracks in that half of the wall 

 which belongs to the neighbouring fibre, 

 and this makes it appear as though the 

 cracks crossed each other. The walls, 

 which are very rich in carbon, assume a 

 brown colour (Fig. 13). We shall direct 

 attention in the special division dealing 

 with this subject to other forms of decom- 

 position, all of which are characteristic for 

 some species of fungus. Here it need only 

 be mentioned that the question whether all 

 the organic parts of the lignified cell-walls 

 require to be absorbed by the mycelium 

 of the fungus before being decomposed 

 into carbonic acid and water, or whether 

 to some extent they are directly oxidized 

 and converted into these substances, can- 

 not at present be finally decided. As a 

 large quantity of oxygen must be made 

 use of during decomposition, its rapidity 

 depends to a great extent on the facilities 

 that arc afforded for the entrance of au- 

 to the interior of the tree. A certain 

 amount of air is present in every woody 



fibre. In dicotyledonous trees the air is conducted to distant parts 

 by means of the vessels and intercellular spaces, and in resinous 

 conifers by the resin-ducts ; but in the case of the silver fir 

 and other conifers destitute of resin-ducts the mode by which 



FIG. io. Tracheid of 

 Finns sylvestris, de- 

 composed by Trametes 

 Pint. The primary 

 cell-wall has been com- 

 pletely dissolved as far 

 as a a. In the lower 

 part the secondary and 

 tertiary layers consist 

 only of cellulose, in 

 which lime - granules 

 are distinctly visible, 

 b ; filamentous my- 

 celia, c, penetrate the 

 walls and make holes 

 as at d and e. 



