364 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1932 



It is easy to realize that the chances of preservation of plants is 

 remote; we can picture low-lying ground covered with trees and 

 smaller plants being flooded with water and ultimately buried under 

 sand and mud; we can see rivers transporting logs of wood and 

 broken foliage shoots, but conditions essential for preservation would 

 seem to be fortunate accidents and of comparatively rare occurrence. 

 We can not hope to recover from the rocks more than a few and usu- 

 ally fragmentary samples of plants which happened to be growing 

 where it was possible for them to be saved from complete destruction. 

 The term " fossil " is often used in too narrow a sense ; many people 

 associate the expression " fossil plant " with wood that has been 

 rendered indestructible by petrifaction, a common though unfortu- 

 nately by no means the commonest method of preservation. A few 

 examples will serve to illustrate the true nature of fossils, that is, 

 plants or pieces of plants buried in the earth by natural causes. 



At several places on the English coast, as on other coasts, there 

 may be seen at low tide stumps of trees embedded in a peaty soil con- 

 trasted by its darker color from the sandy beach; the wood of the 

 stumps is dark brown, though not much altered and easily cut with 

 a sharp knife. With the stools and roots of the trees are associated 

 leaves and seeds, and occasionally implements made by prehistoric 

 man; in these submerged forests we have evidence of a sinking of 

 the land surface at no very distant date. Another and more impres- 

 sive illustration of the preservation of plants in places where they 

 grew is furnished by seams of coal, especially by seams containing 

 nodules of hard calcareous rock known as coal balls. Coal is usually 

 described as the altered remains of the vegetation of swamps and 

 lagoons. When a low-lying forest area was submerged and sedi- 

 ments were spread over the vegetable debris, it occasionally hap- 

 pened that water charged with mineral matter in solution, perco- 

 lating through the covering sediment into the black mass of plant 

 refuse below, deposited lime or other preservative substances in the 

 cells and cell walls of the plant fragments and so converted into stone 

 patches of the forest litter. Thin sections of the coal balls prepared 

 by a cutting machine reveal on microscopical examination the minute 

 structure of twigs, leaves, and seeds and enable us to reconstruct 

 many of the plants which flourished in the Coal Age. The tissue 

 and, in some instances, even the contents of the cells, are preserved 

 in amazing perfection ; as we examine under high magnification the 

 minute cells we almost forget that we are looking at scraps of ex- 

 tinct plants which lived probably about 200,000,000 years ago. 



Stems and branches of plants are occasionally found in beds of 

 volcanic origin; in southern Scotland there is abundant evidence of 

 vigorous volcanic activity in the early days of the Carboniferous 



