THE EVOLUTION OF HERBACEOUS PLANTS 305 



Such are the more important geological and climatological 

 inferences to which we are led by a recognition of the recent and 

 predominantly boreal origin of herbs, and of the factors which have 

 determined their origin. Much more complete data must be sedu- 

 lously gathered, particularly by the phytogeographer and the 

 botanical phylogenist, before the full application of this theory 

 to the problems of geology can be definitely determined. That it 

 is possible, however, to draw conclusions of importance to geology 

 and climatology, not alone from the past and present distribu- 

 tion of species, but also from a study of the evolution of the 

 growth of habits of plants, will be readily admitted.^ 



SUMMARY 



1. The earliest angiosperms were woody plants, and herbs have 

 been derived from them by reduction. This is indicated by the 

 facts that: (i) the earliest angiosperm fossil remains are almost 

 entirely of woody plants; (2) the uninterrupted and active cambial 

 ring of woody dicotyledons, which lays down secondary wood pro- 

 vided with well-developed medullary rays and wood parenchyma, 

 is the primitive type and that the much reduced secondary tissues 

 in herbs are clearly derived from this; (3) the ancestors of the 

 angiosperms and the great majority of the primitive members of the 

 phylum are woody plants; and that (4) in all families and genera 

 which contain both woody plants and herbs the latter are more 

 primitive in their constitution. 



2. The differentiation and refrigeration of the climate of the 

 temperate zones since the beginning of the Tertiary, and the conse- 

 quent appearance of a cold winter season, have been the chief 

 factors which have caused the evolution of herbs. Herbs are able 

 to survive adverse conditions of temperature or moisture as under- 

 ground roots and stems or in the form of seeds, and can thus 

 thrive in regions where plants with perennial aerial stems would 

 perish. In general, the lower the winter temperature of a region 

 the fewer the woody plants in its flora. 



3. At least half of the pre- Glacial vegetation of the north 

 temperate zone seems to have been composed of woody plants, 



^ A more complete discussion of the present problem with the presentation of a 

 much larger body of data will be found in a paper by the writers, "The Origin 

 and Dispersal of Herbaceous Angiosperms," Annals of Botany, XXVIII (1914), 547-99. 



