DECAY 191 



arise in other conifers through the action of Polyporus vaporarius, 

 P. Schweinitzii, and P. sulphureus, one of their characteristic 

 results being expressed in a breaking up of the cell wall into 

 a series of spiral fibers corresponding to the original striation. 

 This feature of decay is very commonly expressed in macerated 

 woods among existing species, and it is exemplified in a very 

 striking manner by Pseudotsuga miocena from the Miocene of 

 Oregon (plate n). 



Whatever particular form the decay effected by fungi may 

 take, it is no doubt correct to say that the disorganization of 

 the cellulose is due in general terms, and in the first instance 

 at least, to a process of hydrolysis. From this point of view it 

 is possible to satisfactorily explain the varying degrees of resist- 

 ance offered by tissues of different degrees of modification, 

 since it will be observed that the decay acts inversely as the 

 amount of carbon present, wherefore the unmodified walls are 

 the first to be acted upon, the cork and cutin the last. 



The changes thus noted as arising through the action of fungi 

 have a more or less profound effect upon the degree of perfec- 

 tion with which structural details are retained when the plant 

 eventually becomes silicified or calcified and is converted into 

 the condition of a fossil. It has been shown that even when 

 present in the tissue, the destructive effects of the fungus may 

 be inhibited by the exclusion of oxygen, with the result that 

 the structure is preserved in a most perfect manner through 

 indefinite periods of time, and without the subsequent infiltra- 

 tion of mineral matter. Most commonly the wood exhibits the 

 effects of more or less extended decay, which, in the majority of 

 cases, shows a removal of the secondary wall and the formation 

 of a thin-walled skeleton, which readily collapses and thus gives 

 rise to extended distortion of the whole structure, whereby it 

 becomes exceedingly difficult to recognize even the genus. If 

 these changes proceed simultaneously with the infiltration of 

 mineral matter, the latter may eventually replace the former to 

 such an extent that while the lines of structure are preserved, 

 they are but faintly defined by thin lines of finely granulated 



