CHAEACTEES AND AEEANGEMENT OF FOSSIL PLANTS. 721 



which is often the case, a minute examination of its tissues is the sole 

 method of determining its position in the Natural series. There must 

 also be some general ideas of the vegetation both of the tropics and 

 cooler latittides, of mountain-chains, table-lands, valleys, and estuaries; 



Fig 904 



Fig. 905. 



Fig. 906. 



Fig. 907. 



more especially of countries characterised by equable, and by excessive 

 or extreme climates, as compared with eontuients, and of humid and 

 desert districts ; in short, of all the complex associations with, or 

 dependence of botanical characters upon, surface, soil, and climate, 

 which the globe presents." 



Many of the fossil plants of the tertiary or recent strata may be 

 referred to genera at present existing, and merely present specific dif- 

 ferences ; such as pines, elms, beeches, maples, etc. Those of the 

 secondary strata may, in general, be referred to known families, but 

 in most instances require the formation of new genera ; whUe those 

 of the older strata, ia numerous instances, cannot be classed in exist- 

 ing families, and must constitute new groups. From all the iavesti- 

 gations of fossil botanists, however, it appears that the same great 

 types existed in a former state as at the present day, viz. Thallogens, 

 Cellular Cryptogams, Acrogens, Vascular Cryptogams, Monocoty- 

 ledons, and Dicotyledons (including Angiosperms and Gymnosperms). 

 The relative proportion of these classes, however, has been dif- 

 ferent from that of the present day, and the predominance of certain 

 forms has given characters to the vegetation of different epochs. 



Brongniart gives the following division of fossil plants : — 



1. Amphigenous Cryptogamous Plants, Celliilar Cryptogams, or Thallogens, 

 which he subdivides into two classes, Fungi and Algae. 



Figs. 904-907. Tlie structure of wood in recent Coniferae, to illustrate the appearances 

 presented T>y some fossil woods. Fig. 904. Transverse section of a piece of Coniferous 



wood, of the natural size, Fig. 905. A section of the same wood seen under the 



microscope. The medullary rays and woody tubes seen without any large porous vessels. 

 Fig. 906. Longitudinal section of the same, in the direction b c. magnified. A medullary 

 ray seen crossing the woody tubes, which are marked by discs, in one or more rows. 

 Fig. 907. Section of the same in the direction a b, perpendicular to the medullary rays, 

 which are seen at intervals between the woody fibres. 



3 A 



