DECAY 189 



which then has a perfectly free passage ; but in all highly 

 lignified tissues, and more particularly those in which secondary 

 growth of the wall is excessive, such contraction is always exhib- 

 ited. This action of the mycelium in perforating the cellulose 

 wall is by no means exceptional or peculiar to the fungi, since it 

 appears in a variety of forms throughout the plant world and 

 constitutes a well-known process in the liberation of spores from 

 the mother cell, as well as in the progress of the pollen tube 

 through the structure of the style. 



It now remains to inquire somewhat more particularly into 

 the specific action of fungi as expressed in 



4. Their effects upon the structure and their relation to forms 

 of preservation in fossil plants. While the general course of the 

 physical changes in decaying wood is fairly well known, there 

 are many features which demand more thorough and extended 

 study. This is particularly true of the changes which arise in 

 the cellulose and eventually resolve it into its proximate ele- 

 ments, whereby the chemistry of decay is recognized as one of 

 the most obscure problems with which the plant physiologist 

 has to deal. Nevertheless reference to the well-known reactions 

 for cellulose, as already given (p. 49), enables us to form some con- 

 ception of the nature of these changes, since those which pro- 

 ceed from the action of fungi are in many respects parallel with 

 those obtained through the action of reagents. The alterations 

 accomplished by the latter, either through the action of an acid 

 or of an oxidizing body, are brought about in the fungi by the 

 action of special ferment secretions included under the general 

 name of enzymes. But here it is to be noted that each fungus 

 behaves in a way peculiar to itself, and gives rise to specific 

 effects which cannot be associated with other fungi. These 

 differences may be reduced in the first instance to two groups, 

 in the first of which the action is primarily upon the intercellu- 

 lar substance, whereby the latter is destroyed with a separation 

 of the secondary walls from one another ; in the second place 

 the action is upon the secondary walls, which are largely removed 

 with the production of a skeleton composed of the primary 



