PERIODS OF VEGETATION AMONG FOSSIL PLANTS. 



Periods of Vegetation among Fossil Plants. 



On taking a general survey of the known fossil plants, 

 Brongniart thought that he could trace three periods of vege- 

 tation, characterised by the predominance of certain marked 

 forms of plants. In the ancient period there is a predomi- 

 nance of Acrogenous Ciyptogamic plants ; this is succeeded 

 by a period in which there is a preponderance of Gymno- 

 spemious Dicotyledons ; while a third period is marked by the 

 predominance of Angiospermous Dicotyledons. There is thus 

 — 1. The reign of Acrogens, which includes the plants of the 

 Devonian, Carboniferous, and Permian periods. During 

 these periods there seems to be a predominance of Ferns, 

 and a great development of arborescent Lycopodiacese, such 

 as Lepidodendron and Sigillaria, and with them are associated 

 some Gymnospemis, allied to Araucaria, and some anomalous 

 plants, as Noeggerathia. 2. The reign of Gymnospemis, com- 

 prehending the Triassic and Jurassic periods. Here we meet 

 with numerous Conifera? and Cycadaceee, while Ferns are 

 less abundant. 3. The reign of Angiosperms, embracing the 

 Cretaceous and the Tertiary periods. This is characterised 

 by the predominance of Angiospermous Dicotyledons, a class 

 of plants which constitute more than three-fourths of the 

 present vegetable productions of the globe, and which appear 

 to have acquired a predominance from the commencement of 

 the Tertiary formations. These plants appear sparingly even 

 at the beginning of the chalk formation in Europe, but are 

 more abundant in this formation as developed in North 

 America. 



