DISTRIBUTION OF FOSSIL VEGETABLES. 341 



If we take a general review of the remains of terrestrial 

 Vegetables, that are distributed through the three great 

 periods of geological history, we find a similar division of 

 them into groups, each respectively indicating the same 

 successive diminutions of Temperature upon the Land, 

 which have been inferred from the remains of the vegetation 

 of the Sea. Thus, in strata of the Transition series, we 

 have an association of a few existing families of Endogenous 

 Plants,* chiefly Ferns and Equisetaceae, with extinct id.nvXves^ 

 both Endogenous and Exogenous, which some modern bota- 

 nists have considered to indicate a Climate hotter than that 

 of the Tropics of the present day. 



In the Secondary formations, the species of these most 

 eaily families become much less numerous, and many of 

 their genera, and even of the families themselves entirely 

 cease ; and a large increase takes place in two families, that 

 comprehend many existing forms of vegetables, and are 

 rare in the Coal formation, viz. CycadecB and Coniferce. 

 The united characters of the groups associated in this 

 series, indicate a Climate, whose temperature was nearly 

 similar to that which prevails within the present Tropics. 



In the Tertiary deposites, the greater number of the 

 families of the first series, and many of those of the second, 

 disappear ; and a more complicated dicotyledonous^ Vegeta- 



occars in Lias at Lyme Regis, and at Boll in Wurtemberg ; and F. Targionii 

 in the Upper Grceiisand near Bignor in Sussex. 



* Endogenous Plants are those, the growth of whose stems takes place by 

 addition from within. Exogenous are those in which the growth takes 

 place by addition from without. 



t Monocotyledonous Plants are those, the embryo of whose seed is made up 

 of one cotyledon or lobe, like the seed of a Lily or an Onion. Dicotyledonous 

 Plants are those, the embryo of whose seed is made up of two lobes, as in 

 the Bean and Coffee-seed. The stems of Monocotyledonous Plants are all 

 Endogenous, i. e. increase from within by the addition of bundles of vessels 

 set in cellular substance, and enlarge their bulk by addition from the centre 

 outwards, e. g. Palms, Canes, and Liliacepus plants. The stems of Dicotyle- 

 donous Plants are all Exogenous, i. e. increase externally by the addition of 



^9* 



