6 THE GEOLOGICAL HISTORY OP PLANTS. 



fossil plants that changes of this kind have ooourred so 

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

 temperate regions to exist within the Arotic 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 ns 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 PhfBnogams, 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. Thalloffeiis, cellular jilants not distinctly distin- 

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

 Lichens, and the Algro, or soa-weods. 



2. Anogena, 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. Those are tlie 

 Ferns (Fi fires), ilie Maro's-tails {Equisetacem), and the 

 Olub-mosses {Lycopodiacpce), and a curious little group 

 of aquatic plants called Rliizocarps {Rlmorarprm). 



The Phronogams are all vascular, but they dilTer much 

 in the simplicity or complexity of tlieir flowers or seeds. 

 On this ground they admit of a twofold division : 



1. Oymnosperms, or those whicli boar naked seeds 

 not enclosed in fruits. They are the Pines and their 

 allies, and the Oycads. 



