40 REACTION OF HOST TO PARASITIC ATTACK. 



layer becomes normally loosened from the other membranes, and 

 appears as a separate tube in the cavity of the fibre. 



Variations of this kind in the structure of the wood must of 

 course influence the action of the attacking fungus. The decay 

 may be a local one, as with Trametes pini, T. radiciperda, 

 Thelephora perdix, which cause destruction of isolated spots 

 only and produce holes here and there throughout the wood. 

 On the other hand, the wood may be uniformly converted into 

 a discoloured decayed mass. The walls may be simply pierced 

 by little holes corresponding to the perforating hypha, or large 

 portions of them may be more or less completely dissolved 

 away, and either the cellulose or lignin remain behind as a 

 skeleton. Hartig gives an interesting case which accompanies 

 dry-rot {Merulius lacrynwm£)\ the mycelium adherent to the 

 cell-walls dissolves out the lime granules included in the mem- 

 branes by the excretion of some fluid containing carbonic (or 

 other weak) acid, in much the same way as roots corrode 

 limestone. 



The dissolution of starch in wood has already been considered. 



In conclusion should be mentioned Hartig's observation 

 that normal spruce wood, on treatment with ferric chloride, 

 the reagent for tannin, gives no coloration, such as is given 

 by the same wood when destroyed by dry-rot. 



§ 8. EFFECTS OF PARASITIC FUNGI ON THE ANATOMICAL 

 STRUCTURE OF THEIR HOSTS. 



Effects of this kind can only be looked for where mor- 

 phological changes have resulted from the presence of parasitic 

 fungi, particularly in the case of hj^ertrophied organs. Wakker^ 

 was the first to collect recorded evidence of anatomical changes 

 due to hypertrophy ; he added to these by his own investi- 

 sations, and classified the results. We shall therefore in this 

 division depend chiefly on his publications. 



Enlargement of host-cells is one of the most frequent pheno- 

 mena accompanying attacks of parasitic fungi. It may take 

 place with both intracellular and extracellular parasites. 



A single cell hypertrophied in this way is the simplest 

 possible form of a "fungus-gall" (see p. 25). Examples of 

 'Wakker, Pringsheim's JahrJnich, 1892. 



