CHAPTER XV 

 GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 



WHEN the first plants appeared upon the earth it is impossible to 

 say, since these have left no traces in the rocks, and the earliest 

 recognizable plant-remains belong to forms already much specialized. 

 The perishable nature of the simplest plants, like most Algae, is such 

 as to preclude their preservation in a recognizable condition in the 

 fossil state. Consequently the geological history of the lower plants 

 is exceedingly unsatisfactory. 



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Thallophytes 



Schizophyta. While certain evidence of the existence of the 

 Fission-algae and Bacteria are wanting in the oldest rocks, there are 

 nevertheless suggestions of their presence in the occurrence of cal- 

 careous nodules, perhaps due to the growth of Schizophyceae, and 

 evidences of decomposition in the tissues of fossil plants, evidently 

 the result of the activity of Bacteria. Indeed, actual bacterial cells 

 have been described in a fossil condition, and there is every rea- 

 son to suppose that Bacteria were among the very earliest organisms 

 to appear upon the earth. 



Algae. The perishable nature of most Algae accounts for their 

 rarity in a fossil condition, but there are certain fossils which have 

 with some reason been supposed to be the remains of large Algae, 

 similar to the Kelps. Of these supposed Phaeophyceae, the best 

 known is the genus Nematophycus, of Devonian age. 



Green Algae. Of the Green Algae, the Siphoneae are especially well 

 represented in a fossil state, owing to the deposit of lime with which 

 they are incrusted. At the present day, these calcareous Siphoneae 

 are important agents in the building of coral-reefs, where their cal- 

 careous skeletons add materially to the growing reef. It is now 

 known that similar forms occurred in Silurian, and possibly in older 

 formations. These ancient forms were apparently similar to some 

 of the existing types. 



Rhodophyceae. Another group of calcareous Algae, of possibly 

 equal antiquity, are the Corallines, a group of the Rhodophyceae, 

 which also are still of importance as reef-builders. They are the so- 

 called "Nullipore" corals, formerly supposed to be animals. 



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