CHAPTER XXIII 
THE EVOLUTIONARY HISTORY OF PLANTS 
368. The Earliest Plant Life. — What sort of plants first 
appeared on the earth has never been positively ascertained. 
The oldest known rocks contain carbon (in the form of 
black lead or graphite) which may represent the remnants 
of plants charred at so high a temperature and under so 
great pressure as to destroy all traces of plant structure. 
Some objects supposed by many to be the remains of large 
algze have been found in rocks that date back to a very 
early period in the life history of the earth, before there 
were any backboned animals, unless possibly some fishes. 
Judging from the way in which the various groups of 
plants have made their appearance from the time when 
we can begin clearly to trace their introduction upon the 
earth, it is probable that some of the simplest and lowest 
forms of thallophytes were the first to appear. Decaying 
animal or vegetable matter must have been less abundant 
than is now the. case, so that a plant that could make 
part or all of its food from raw materials would have had — 
a better chance than a saprophyte that could not. Water- 
plants are usually simpler than land-plants, so it is highly 
probable that some kind of one-celled aquatic alga was 
the first plant. 
369. Fossil Plants. — Fossils are the remains or traces 
of animals or plants preserved in the earth by natural 
processes. Fossil plants, or parts of plants, are very 
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