i66 



THE ANATOMY OF WOODY PLANTS 



universally accepted procedure in comparative anatomy is to judge 

 the former organization of the walls of the stelar tube from 

 pertinent conditions presented by the leaf traces which take their 

 origin from it. In other words, ancestral conditions which have 

 disappeared in the stele of the highly progressive and easily modi- 

 fied stem may continue almost indefinitely in the fibrovascular 



FIG. 120. Diagram showing degeneracy of internal phloem and endodermis 

 in the siphonostelic central cylinder. Explanation in the text. 



structures of the leaf. The great value of this principle is 

 quite generally admitted in the case of the gymnosperms, but 

 its validity for the fernlike forms is unfortunately not so uni- 

 versally conceded in spite of cogent logical considerations in its 

 favor. At c is represented a further condition of reduction. Here 

 not only the internal phloem lias disappeared in the stele, but 

 likewise there is no internal endodermis, although both are present 

 in the leaf trace. At d in the same figure is seen the anatomical 



