THE HERBACEOUS DICOTYLEDONS 407 



Fig. 2846 is shown a diagrammatic view of the stem in the genus 

 Aster. Obviously exactly the same general conditions are found 

 as in the axis of the oak. 



The result of all the evidence considered in the previous para- 

 graphs of the present chapter is to show that the herbaceous type 

 of stem is distinctly derived from the woody one in the dicotyledons. 

 Its earlier expression is in the rather thick stem of herbaceous 

 texture, in which the storage parenchyma especially related to 

 the foliar traces both subtends and flanks the fibrovascular bundles 

 of the leaf in that part of their course which lies within the cylinder 

 of the stem. With subsequent thinning of the cylinder, clearly 

 connected with greater efficiency in the production of seed and 

 coupled with a more strictly annual duration, the subtending 

 parenchyma is wiped out, and only the flanking storage tissues 

 continue to exist. Obviously a very high potency in the produc- 

 tion of seed, correlated with high assimilative power, ultimately 

 makes for a type of annual stem in which storage is effected mainly 

 in the seeds and no longer in the axis itself. Under these circum- 

 stances we have the herbaceous type reaching its extreme sim- 

 plification. The situation just pictured involves a final marked 

 reduction in the organization of the fibrovascular strands which 

 is expressed in its completest form in the bundle system of the 

 axes of the monocotyledons. This group, however, will be con- 

 sidered in the following chapter. 



The result of the appearance of the herbaceous type in the 

 angiosperms has been momentous for the development of higher 

 organisms. It is a fact of very obvious significance that the 

 highest vertebrates and the highest seed plants have had a nearly 

 contemporaneous existence. The warm-blooded mammal is in 

 reality rendered possible by the appearance of the herbaceous 

 type in the angiosperms, which directly or indirectly supply the 

 most important part of the food of all the higher animals. It 

 should further be emphasized in the present connection that the 

 dicotyledonous herbs are very different indeed in their mode of 

 origin from those exemplified by lower types and in particular 

 by the vascular cryptogams. Here the herbaceous condition is 

 the result of mere degeneracy in the tissues of the xylem. In 



