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INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



early geological times. Thus in rocks far back in the pre- Cam- 

 brian^ are found supposed signs of Bacteria in the form of filamentous 

 chains and minute spherical bodies, often associated with slender 

 branching filaments and other remains believed to be of Blue- 

 green Algae [cf. Fig. 29, C). What appear to be the oldest known 

 structurally preserved organisms that clearly exhibit cellular differ- 

 entiation and original carbon complexes are in pre-Cambrian sedi- 

 ments of southern Ontario and represent Blue-green Algae and simple 

 forms of Fungi or possibly Algae {see Fig. 29, D). It seems probable 

 that their age exceeds 1,000,000,000 years, and it may quite likely 

 be nearer to 2,000,000,000 years. From later on, and at least 

 beginning with the Devonian around the middle of the Palaeozoic 

 era,^ there are plentiful indications that Bacteria were practically 

 ubiquitous, as indeed they are today. Nor is there evidence of major 

 evolutionary change in their structure through those many millions 

 of years. 



Algae, which among living plants probably rank next in importance 

 to vascular ones in the formation of contemporary vegetation, are 

 also significant for the roles they appear to have played in the 

 formation of petroleum and limestone. As a result of Algae obtain- 

 ing carbon dioxide for photosynthesis from (soluble) calcium 

 bicarbonate, the relatively insoluble calcium carbonate was left as a 



^ See previous page and Fig. 37 concerning the geological eras, etc., with their 

 dominant forms of life and supposed ages. Fig. 37 also indicates the approximate 

 relative development of the main plant groups at different times. It is now thought 

 that the earth may be of the order of 4,500,000,000 years old, and that life may 

 have begun on it about half-way along the time to the present. 



