IX.] HEALING OF WOUNDS BY OCCLUSION. 219 



requires some time for its completion. For the 

 sake of illustration, I have numbered the various 

 phases in the diagram, with the years during which 

 the annual rings have been successively formed ; and 

 it will be seen at a glance that in the case selected, it 

 required seven years to cover up the surface of the 

 cut branch (cf. Figs. 27-32). During these seven 

 years more or less of the cut surface was exposed 

 (Fig. 30) for some time to all the exigencies of the 

 forest, and it will easily be understood that abundant 

 opportunities were afforded during this interval for the 

 spores of fungi to fall on the naked wood, and for 

 moisture to condense and penetrate into the interior ; 

 moreover, in the ledge formed at + in Figs. 29 and 30, 

 by the lower part of the callus, as it slowly creeps up, 

 there will always be water in wet weather ; and a sodden 

 condition of the wood at this part is thus insured. All 

 this is, of course,, peculiarly adapted for the germina- 

 tion of spores ; and since the water will soak out 

 nutritive materials, nothing could be more favourable 

 for the growth and development of the mycelium of a 

 fungus. These circumstances, favourable as they are 

 for the fungi, are usually rendered even more so in 

 practice, because the sawyers often allow such a 

 branch to fall, and tear and crush the cambium and 

 cortex at the lower edge of the wound. These and 



