INTRODUCTION 23 



Portions of plants that are well separated from each other 

 by sand or mud during deposition give rise to fossils known 

 as mummifications or compressions. From these, it is often 

 possible to make preparations of the cuticle, by oxidizing 

 away the coally substance with perchloric acid. Examina- 

 tion under a microscope may then reveal the outlines of the 

 epidermal cells, stomata, hairs, papillae, etc. In this way, 

 a great deal can be discovered from mummified leaves. 

 Mummified stems and other plant organs, however, yield 

 less useful results. Even their shape needs careful interpreta- 

 tion, because of distortion during compression under the 

 weight of overlying rocks. 



By far the most useful fossils to the palaeobotanist are 

 those in which decay was prevented from starting, by the 

 infiltration of some toxic substance, followed by petrifaction 

 before any distortion of shape could occur. Such are, un- 

 fortunately, rare indeed. The most beautiful petrifactions 

 are those in silica, but carbonates of calcium and magnesium 

 are also important petrifying substances. Iron pyrites, while 

 common, is less satisfactory because the fine structure of the 

 plant is more difiicult to observe. While it has often been 

 said that during petrifaction the tissues are replaced molecule 

 by molecule, this cannot be correct, for the 'cell walls' in 

 such a fossil dissolve less rapidly in etching fluids than does 

 the surrounding matrix. This fact forms the basis of a rapid 

 technique for making thin sections of the plant material.*^ 

 A poUshed surface is etched for a brief period in the appro- 

 priate acid and the cell walls that remain projecting above 

 the surface are then embedded in a film of cellulose acetate. 

 This is stripped off and examined under the microscope 

 without further treatment, the whole process having taken 

 no longer than ten minutes. 



While it is frequently possible to discern the type of 

 thickening on the walls of xylem elements, it is, however, 

 rarely possible to make out much detail in the phloem of 

 fossil plants, for this is the region which decays most rapidly. 



