60 BOTANY 



preserved in three main ways. The first and best 

 known are impressions. These we see when we split 

 open a slab of shale or limestone, and a fragment of a 

 fern leaf, or a branch with its foliage, lies pressed between 

 the layers of the rock. Sometimes these impressions 

 look quite black against the stone, and this is due to 

 the carbonisation of the vegetable matter of the tissues. 

 In such an impression we have the external form of 

 the plant retained as if it were a pressed specimen, 

 but all its internal cells are decomposed. 



The second form of fossil is the cast. Here, as in 

 the previous kind, it is generally the external features 

 of the plant that are preserved. The cast is formed 

 by the enclosure of the parts in some generally fine- 

 grained, detrital matter. This retains the plant until 

 its characters are imprinted on it, so that when the 

 vegetable tissue decays the rock still holds its features, 

 as plaster of Paris holds the engraving of a medal. Both 

 casts and moulds of plants are formed, and sometimes, 

 too, we find casts of the internal features of hollow stems. 



The third and most useful form of fossil is the true 

 petrifaction. In this case there is often no sign of the 

 external features of the preserved plant. A mass of 

 silica, or of carbonate of lime, or of dolomite, entirely 

 encloses, permeates, and petrifies the inner tissue cells 

 and the wood of stems or leaves or seeds. Thin sections 

 of these stony masses can be cut in the same way as 

 sections are cut of minerals or fossil corals. Then, 

 through the microscope, we can see the cells just as 

 they can be studied in sections of living plants. From 

 series of such sections we can restore not only the 

 internal anatomy of plants that have been extinct, per- 

 haps, for millions of years, but even points in their 

 cytology are discoverable. Such fossils can sometimes 



