EFFECT OF PARASITIC FUNGI ON THE CELL-WAIi. 



39 



acid the cellulose is dissolved out, and the primary wall remains 

 almost intact, while the secondary after swelling is converted 

 into sugar and gum. Certain fungi {e.g. Polyporus vaporarius, 

 P. Schweinitzii and P. sulphureus), act in the same manner, 

 first dissolving out and consuming the cellulose before attacking 

 the wood-gum. When wood is destroyed by fungi of this 



Fio. 14. 



Fig. 13. — Tracheid of Pinus sytvestris destroyed by Travutes pini. The primary 

 cell-WHll is completely dissolved from below upwards to a^ a; 6, secondary and 

 tertiary layers of the walls consisting in the under portion of cellulose only, in 

 which gi'anules of chalk are recognizable ; c, fungus-hyphae boring through the 

 walls, leaving holes d and e. (After R. Hartig.) 



Fic. 14. — Tracheid of Pinus destroyed by Poiyp&nis Schweinitzii. The cellulose 

 has been extracted, and the wall consists only of wood-gum. The fissures are a 

 result of drying-up, but they do not extend into the primary wall a, h. Crossing 

 of the fissures takes place at the bordered pits c, and at the bore-holes d and e ; 

 /, simple fissures. (After R. Hartig.) 



kind, the primary wall, containing but little cellulose, is hardly 

 affected, and the secondary membranes shrink together, so that 

 numerous fissures are produced running in a spiral direction, 

 corresponding with that of the stratification (Figs. 13 and 14.) 

 The tertiary membrane varies in its nature ; it may consist of 

 pure cellulose or be more or less lignified, or even cuticularized. 

 In the wood-fibres of some plants {Gytisus, ffumulus,) this 



