THE HISTORY OF PLANT LIFE. 23 



viz., the more delicate structures of the gametophyte, we 

 encounter one of the most serious difficulties in the prosecution 

 of such studies. In the case of delicate plants or the delicate 

 parts of woody plants, complete disappearance of structure 

 commonly follows, and if any evidence of organic structure 

 remains it generally appears in the form of highly altered 

 carbon, as coal or graphite, or else, as usual in the case of 

 leaves, only a mere impression of the external features is pre- 

 served in the surrounding material. It is altogether exceptional 

 that, as in Parka decipiens, we meet with structures of so 

 delicate an organization as the prothalli, and when such evidence 

 is obtainable it acquires an unusual value. 



2. Character of the Alterations. 



Plant remains are rarely found in their original condition 

 except in the most recent deposits, such as the Pleistocene. 

 A very superficial examination of the floor of any well-grown 

 forest, or of a forming peat bog, will serve to disclose the 

 general conditions under which plants pass into the fossil state. 

 Here we note that as the various forms of vegetation attain 

 maturity and fall to the ground, they at once pass into a state 

 of decay. If it then becomes possible for this decaying 

 material to be covered by a fresh deposit of silt, secondary, 

 effects follow ; the decay is either arrested or its character is 

 altered, and very possibly the structure becomes infiltrated 

 slowly with mineral matter which is held in solution by the 

 surrounding water. That such conditions have surrounded 

 the fossil plant is evident from the form in which it is 

 found. 



Decay, more or less pronounced, is an invariable antecedent 

 of the final preservation of plant remains. In many cases it 

 has been carried to an extremity, so that the structure is 

 unrecognizable, and then there remains nothing but a residue 

 of carbon, as so commonly occurs in the Eozoic formation. In 

 other cases it may be so limited as to permit a recognition of 

 all the original structural features preserved with great perfec- 

 tion. The fungi of decay are commonly met with, and are not 



