VARIOUS KINDS OF FOSSIL PLANTS 17 



vation among fossil plants, but they are of minor interest 

 and importance, and hardly justify detailed considera- 

 tion. One example that should be mentioned is Amber. 

 This is the gum of old resinous trees, and is a well- 

 known substance which may rank as a "fossil". Jet, 

 too, is formed from plants, while coal is so important 

 that the whole of the next chapter will be devoted to 

 its consideration. Even the black lead of pencils pos- 

 sibly represents plants that were once alive on this 

 globe. 



Though such remains tell us of the existence of 

 plants at the place they were found at a known 

 period in the past, yet they tell very little about the 

 actual structure of the plants themselves, and therefore 

 very little that is of real use to the botanist. Fortu- 

 nately, however, there are fossils which preserve every 

 cell of the plant tissues, each one perfect, distended 

 as in life, and yet replaced by stone so as to be hard 

 and to allow of the preparation of thin sections which 

 can be studied with the microscope. These are the 

 vegetable fossils which are of prime importance to the 

 botanist and the scientific enquirer into the evolution 

 of plants. Such specimens are commonly known as 

 Petrifactions. 



Sometimes small isolated stumps of wood or branches 

 have been completely permeated by silica, which replaces 

 the cell walls and completely preserves and hardens the 

 tissues. This silicified wood is found in a number of 

 different beds of rock, and may be seen washed out on 

 the shore in Yorkshire, Sutherland, and other places 

 where such rocks occur. When such a block is cut and 

 polished the annual rings and all the fine structure or 

 "grain" of the wood become as apparent as in recent 

 wood. From these fossils, too, microscopic sections can 

 be cut, and then the individual wood cells can be studied 

 almost as well as those of living trees. A particularly 

 notable example of fossil tree trunks is the Tertiary 

 forest of the Yellowstone Park. Here the petrified 



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