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6 THE GEOLOGICAL HISTORY OF PLANTS. 



fossil plants that changes of this kind have occurred so 

 great as, on the one hand, to permit the plants of warm 

 temperate regions to exist within the Arctic Circle ; and, 

 on the other, to drive these plants into the tropics and 

 to replace them by Arctic forms. It is evident also that 

 in those periods when the continental areas were largely 

 submerged, there might be an excessive amount of moist- 

 ure in the atmosphere, greatly modifying the climate, in 

 so far as plants are concerned. 



Let us now consider the history of the vegetable king- 

 dom as indicated in the few notes in the right-hand 

 column of the table. 



The most general subdivision of plants is into the two 

 great series of Cryptogams, or those which have no mani- 

 fest flowers, and produce minute spores instead of seeds ; 

 and Phaenogams, or those which possess flowers and pro- 

 duce seeds containing an embryo of the future plant. 



The Cryptogams may be subdivided into the following 

 three groups : 



1. Thallogens, cellular plants not distinctly distin- 

 guishable into stem and leaf. These are the Fungi, the 

 Lichens, and the Algae, or sea- weeds. 



2. Anogens, having stem and foliage, but wholly cel- 

 lular. These are the Mosses and Liverworts. 



3. Acrogens, which have long tubular fibres as well as 

 cells in their composition, and thus have the capacity of 

 attaining a more considerable magnitude. These are the 

 Ferns (Filices), the Mare's-tails (Equisetacece), and the 

 Club-mosses (Lycopodiacece), and a curious little group 

 of aquatic plants called Khizocarps (Rhizocarpece). 



The Phaenogams are all vascular, but they differ much 

 in the simplicity or complexity of their flowers or seeds. 

 On this ground they admit of a twofold division : 



1. ^ymnospermSy or those which bear naked seeds 

 not enclosed in fruits. They are the Pines and their 

 allies, and the Cycads. 



