1 9 14] BAILEY &• SINNOTT—PHYLOGENY OF ANGIOSPERMS 49 



are particularly retentive of primitive, unreduced characters. 

 Stunted, suppressed, or poorly nourished cambiums hasten the 

 reduction of wide rays by an incomplete development of specific 

 characters. The sensitiveness of wide rays to changes in the 

 environment or physiological activity of the plant might be anti- 

 cipated when we consider the function of these sheets of parenchyma 

 as storage organs. It is not surprising, therefore, that the effects 

 of conservatism should have been neutralized in certain regions 

 of the plant. 



A brief description of the comparative anatomy of species which 

 occur at various levels in this " regression ' ' series and their behavior 

 under different physiological conditions follows: 



Fagus grandifolia. — This plant illustrates diagrammatically the 

 structure of a species in which the multiseriate rays have suffered 

 little or no reduction. The stem (fig. 1) possesses a ring of primary 

 fibrovascular bundles which are separated by well developed gaps. 

 Those arcs of the cambium which bridge these gaps, the so-called 

 interfascicular cambiums, form ray parenchyma exclusively. As a 

 result of their activity, multiseriate rays are formed which vary in 

 height and width as do the gaps in the vascular cylinder. In sub- 

 sequent secondary growth these "primary medullary rays," which 

 are 2-5 cells wide, increase gradually in width and may eventually 

 attain a breadth of approximately 15-25 cells. Next the pith these 

 rays are in the form of long vertical lines of parenchymatous 

 tissue. As the stem develops, these tall rays or lines of super- 

 imposed high rays are dissected into shallower rays, which are 

 gradually deflected from their original vertical axes. The so-called 

 fascicular cambiums form at first tracheary tissue and uniseriate 

 rays. The latter are also high in tangential section and form long 

 hnes which are dissected in subsequent development into shorter 

 ra ys. Certain of these later increase in width and may in time 

 become as wide as the rays of the so-called interfascicular segments. 

 With the increasing circumference of the stem, new rays are formed 

 continually by the cambium to maintain the proper proportion of 

 ray tissue in each radius. These shorter "secondary" m> lullary 

 r ays which do not extend to the pith are uniseriate at first, but 

 ttay increase in width during subsequent development. In any 



