ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 431 



tropical arboreal Leguminosae are common, while herbs belonging 

 to this family are much less abundant in equatorial latitudes. 



The geological conditions so far as they are clearly displayed 

 seem to correspond closely with those indicated geographically in 

 the present era. For example, the Paleozoic, which was a period 

 of average high temperature compared with later geological eras, 

 was characterized by the prevalence of arboreal cryptogams belong- 

 ing to the groups whose survivors are practically entirely herba- 

 ceous. In the cooler but still warm Mesozoic the prevailing type 

 of seed plant vegetation was gymnospermous and in particular 

 coniferous, and was consequently arboreal and not herbaceous in 

 habit. Concerning the proportion of herbs and trees in the 

 angiospermous vegetation of the later Mesozoic and of the warmer 

 part of the Tertiary we are unfortunately not well informed. It 

 is certain from still unpublished investigations that herbaceous 

 types were present, although not abundant, in the Upper Cretaceous 

 of the Eastern United States. As the data in this case are sup- 

 plied by remains preserved from obliteration by charring and 

 subsequent sweeping into open water, it is not unlikely that they 

 indicate, approximately at any rate, the proportion of herbaceous 

 and woody forms in the floras of the later Cretaceous. 



The general evidence points conclusively toward the herb as a 

 product of cooler climatic conditions and of later geological times. 

 It should not be forgotten in this connection that the appearance 

 of wood parenchyma, with which in the last analysis the evolution 

 of herbaceous angiosperms is clearly linked, is definitely related 

 to the phenomenon of annual rings, in turn the result of climatic 

 refrigeration in later geological epochs. The impulse toward the 

 formation of tangential parenchyma was thus obviously supplied 

 by climatic cooling. Tangential parenchyma united with radial 

 storage tissues furnishes the explanation of the broad or compound 

 ray as exemplified by the oak on the one hand and by vines and 

 herbs on the other. It will be apparent from the conditions 

 elucidated above that climate has had a paramount influence in 

 molding the organization of plants and that refrigeration has 

 always favored the appearance of herbaceous types. A careful 

 distinction must, however, be made between degenerate herbs 



