382 THE ANATOMY OF WOODY PLANTS 



It must not be supposed, however, that the relation between me- 

 chanical fibrous elements and vasicentric parenchyma is abso- 

 lute, for in groups characterized by this mode of parenchymatous 

 distribution it is present even in genera with tracheary mechanical 

 cells. In other words, the grouping of parenchyma about the 

 vessels has a deeper significance than that of mere convenience 

 to water supply. In not a few instances the storage cells may be 

 confined to the end of the annual ring. This is, for example, the 

 situation found in the Salicales, and it also frequently characterizes 

 genera of the Magnoliaceae occurring in temperate climates. In 

 these instances an examination of the primitive regions, together 

 with experimental investigation, reveals as the original condition 

 either the vasicentric or the diffuse distribution of parenchyma. 

 It may be stated summarily that diffuse storage elements constitute 

 the primitive conditions in the woods of the dicotyledons, and 

 that a later modification is the vasicentric. By reduction either 

 of the two types mentioned may give rise to the terminal condi- 

 tion. Terminal parenchyma is accordingly a phenomenon of 

 reduction in the dicotyledonous series, while in the Coniferales, 

 as has been elucidated at an earlier stage, it represents the primitive 

 state in which all transitions between tracheary and storage ele- 

 ments are frequently and normally found. 



It is in the organization of their wood rays or radial 

 storage tissues that the dicotyledons manifest the most distinct 

 differences from the mass of the gymnosperms. It has been made 

 clear in previous pages that the primitive type of ray organization 

 for the group was the linear or uniseriate ray. In the earliest 

 conditions presented to our investigation, however, that is ac- 

 companied by the aggregate ray, consisting of more or less fused 

 congeries or clusters of rays separated by fibrous elements. This 

 type we must regard as an original one for rays other than unise- 

 riate in the dicotyledons, because it is clearly found in Ephedra, 

 by common consent the most primitive representative of the 

 Gnetales, and because it is extremely persistent in primitive 

 organs and regions in the dicotyledons themselves. The facility 

 with which fibrous elements are transformed into storage cells 

 in the group under consideration has led to the metamorphosis 



