4OO 



THE ANATOMY OF WOODY PLANTS 



lower regions of the aerial axis, while in the upper parts the con- 

 fronting parenchyma is eliminated from the cylinder as a result 

 of its progressive reduction in thickness, precisely as in Vitis 

 and Urtica. 



The Compositae, so generally conceded a very high systematic 

 position among the dicotyledons, exemplify exactly the same 

 principles as have been noted above. Fig. 277 represents a low 



region of a somewhat 

 woody axis of Helianthus 

 hirsutus. The leaves, 

 opposite as in Urtica, 

 have three traces each. 

 The foliar strands corre- 

 spond to depressed seg- 

 ments of the stem, a 

 situation which, as has 

 been noted in an earlier 

 chapter, is commonly 

 found in the vicinity of 

 large storage rays and 

 which results from the 

 greater amount of vege- 

 table substance present 



in such segments and a consequent retarded rate of growth as com- 

 pared with the more woody regions of the cylinder. In H. tuberosus 

 the fluting of the lower part of the stem in the region of the storage 

 rays related to the leaves becomes extremely marked. As there are 

 six leaf traces at a given node, it follows that there are six cor- 

 responding furrows in the internode below. Further, since the 

 traces of successive internodes alternate, the furrows of necessity 

 show a similar alternation. The narrow bundles of greater diam- 

 eter in the figure represent the foliar traces of the next higher 

 node, and the masses of sclerenchyma subtending the median 

 region of the phloem of the broadest bundles correspond to the 

 now fused traces of a still higher node of the axis. Fig. 278 

 reproduces a high and herbaceous region of the aerial axis of 

 H. hirsutus. The figure is equivalent in nodal relations to the fore- 



FIG. 277. Thick stem of Helianthus hirsulus 



