THE HERBACEOUS DICOTYLEDONS 



401 



FIG. 278. Slender upper region of the stem of 

 //. hirsutus. 



going, but the topography is strikingly different. Here the foliar 

 traces, instead of merely standing flush with the surface of the 

 cylinder, as in the 

 upper region of the 

 more woody herba- 

 ceous types, are actu- 

 ally outstanding, or 

 salient. As a conse- 

 quence of this situa- 

 tion the leaf traces no 

 longer correspond to 

 depressions of the 

 stem, but actually 

 underlie ridges on its 

 surface. This condi- 

 tion is, in fact, highly 

 characteristic of ex- 

 treme herbs, in which 

 the leaf trace has become the dominant factor in the organization 



of the cylinder of the 

 axis. This ana- 

 tomical situation 

 corresponds to a 

 very high degree of 

 assimilative effi- 

 ciency on the part of 

 foliar organs, result- 

 ing in a large 

 amount of food 

 storage or seed pro- 

 duction, as the case 

 may be. 



In Fig. 279 is 

 shown the trace of 

 the herbaceous re- 



FIG. 27 o.-Section showing the leaf trace in the g ion more highly 

 herbaceous region of Helianthus. m a g n 1 fi e d . The 



