104 PAL^ONTOLOGICAL BOTANY. 



of the woody stems can be easily seen in thin sections under 

 the microscope. 



8. The determination of fossil plants is a matter of great difficulty, 



and requires a thorough knowledge of structure, and of the 

 markings on stems, roots, etc. 



9. The rocks containing organic remains are called fossiliferous, and 



are divided into Primary, Secondary, and Tertiary, or into 

 Palseozoic, Mesozoic, and Cainozoic, each of these series being 

 characterised by a peculiar facies of vegetable life. 



10. The mere absence of organic remains will not always be a correct 



guide as to the state of the globe. 



11. The number of fossil species has been estimated at between 3000 



and 4000 ; but many parts of j^lants are described as separate 

 species, and even genera, and hence the number is perhaps 

 greater than it ought to be. 



12. Brongniart divides the fossil flora into three great epochs : — 1. 



The reign of Acrogens ; 2. The reign of Gymnosperms ; 3. The 

 reign of Angiosperms. 



13. The reign of Acrogens embraces the Silurian, Carboniferous, and 



Permian epochs, in which there was a predominance of plants 

 belonging to the natural orders Filices, Lycopodiacese, and 

 Equisetacese, associated, however, with others of a higher class. 



14. The reign of Gymnosperms embraces the lower and middle 



Secondary periods, and is characterised by the presence of 

 numerous Coniferse and Cycadacese. 



15. The reign of Angiosperms includes the Cretaceous and Tertiary 



periods, and is marked by the predominance of Angiospermous 

 Dicotyledons. 



16. Coal is a vague term, referring to all kinds of fuel formed from 



the chemically-altered remains of plants. 



17. When there is a great admixture of mineral matter, so that it will 



not burn as fuel, then a shale is produced. 



18. The microscopic structure of Coal probably varies according to 



the nature of the plants of which it is composed, and the 

 changes produced by pressure, heat, and other causes. Cell- 

 ular tissue, punctated woody tissue, and scalariform vessels, 

 have been detected in it. 



19. Certain temporary and local floras seem to have given origin to 



peculiar layers of coal. 



20. During the Carboniferous epoch we meet with Ferns, Sigillarias, 



and their roots called Stigmarias, Lepidodendrons, Uloden- 

 _drons, Calamites, Gymnosperms, etc. 



