432 THE ANATOMY OF WOODY PLANTS 



and dynamic ones. Exemplifications of the former are supplied 

 by the few surviving vascular cryptogams. The latter are rep- 

 resented by the huge aggregation of angiospermous herbs, both 

 dicotyledonous and monocotyledonous, which constitute such a 

 preponderant and aggressive element in the present plant popula- 

 tion of our earth. It seems not unlikely that should present 

 climatic conditions remain for a long period either unchanged or 

 even accentuated, the dynamic herbaceous type will definitely 

 supplant the arboreal, at least in temperate regions. If this view 

 is correct, we must regard the strikingly herbaceous flora of exist- 

 ing prairies, steppes, and pampas of higher latitudes as a fore- 

 runner of a condition which will ultimately become universal. 

 In any case the appearance of the dynamic herb of angiospermous 

 affinities, which is primarily the result of the differentiation and 

 not the degeneracy of the woody cylinder, is an evolutionary 

 phenomenon of the first order. 



