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 

 algae 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 wa};- 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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