FIBRO VASCULAR TISSUES: PARENCHYMA 51 



of the annual ring and around the vessels in this genus. These 

 are the only situations in which parenchymatous cells are normally 

 present in orders characterized by what may be conveniently termed 

 vasicentric parenchyma. In b the longitudinal view of a vessel with 

 its parenchymatous jacket of vasicentric parenchyma (that is, 

 parenchyma confined to the vicinity of the vessels) is indicated. 

 The vasicentric distribution of parenchyma is, other things being 

 equal, an indication of an advanced systematic position among the 

 dicotyledons and is, as will be shown in the sequel, accompanied 



A a 



FIG. 38. Vasicentric parenchyma in Fraxinus. Explanation in the text 



by other features of organization in the tissues of the secondary 

 wood which are somewhat generally accepted as indicating a high 

 degree of specialization. 



In dicotyledonous woods, with either of the two characteristic 

 modes of distribution of the parenchymatous elements indicated 

 in the preceding paragraphs, degeneracy may occur. As a con- 

 sequence the longitudinal elements may somewhat rarely be ab- 

 sent altogether or may be confined to the end of the annual ring, 

 the latter situation being much more commonly found. Fig. 39 

 illustrates the organization of the wood in Salix or Populus (or 

 equally well that of Liriodendron or Magnolia) as regards the 



