THE PLANT BODY 55 



in diameter, new cells are added by change in function of cambium 

 initials. 



The ray cells of vascular plants were probably derived originally 

 from tracheids— the conifers show the origin of prostrate ray tracheids 

 from erect tracheids— but the origin of ray cells must go far back in 

 the history of vascular plants. The high, upright cells of uniseriate rays 

 appear to be specialized types, which approach wood-parenchyma cells 

 in form and perhaps in function. 



The xylem of angiosperms, both as a whole and by its constituent 

 cell types, provides excellent series in evolutionary modification. Ad- 

 vance to greater complexity and reduction to simpler structure are 

 clearly shown. Specialization in wood structure is correlated with ad- 

 vance in flower structure; the wood of sympetalous families is more 

 highly specialized than that of polypetalous families. 



Broadly considered, the various types of wood cells advance in 

 specialization together: the higher types of vessels usually accompany 

 the highest types of fibers; scalariform vessels commonly accompany 

 liber tracheids and are not present with libriform fibers. But there are 

 conspicuous exceptions; for example, fiber tracheids accompany simply 

 perforate vessels in Quercus. 



SpeciaHzation in the ray appears to be chiefly in simplification and 

 reduction. The vesselless woods have both uniseriate and multiseriate 

 rays, and many advanced genera have only one type. Heterogeneous 

 rays, present in some of the more primitive genera, are present in lower 

 seed plants — Pteridospermae, Bennettitales, Cycadales. 



In wood structure, generic differences are usually evident; specific 

 differences, rarely; well-marked differences may exist between sub- 

 genera or sections, as in Quercus. (Some anatomists believe that, in 

 structure, the wood may be as conservative, or even more conservative, 

 than the flower.) Within families, wood structure ranges from remark- 

 ably uniform to only fairly uniform and even diverse; in orders, there 

 may be uniformity or an almost complete lack of resemblance (prob- 

 ably evidence that the order is unnatural under the present interpreta- 

 tion). Comparisons of advance in wood structure with specialization in 

 flowers frequently show high correlation in groups of genera commonly 

 assumed to be closely related, but there are examples of major dis- 

 crepancies in large families and orders. Broadly considered, the struc- 

 ture of wood is one of the most important characters in the determina- 

 tion of natural relationships. 



Phloem. The phloem of angiosperms has not been studied as 

 thoroughly as the xylem, but the story of its evolutionary modifica- 

 tion is known. It closely parallels that of the xylem. The fundamental 



