This idea was enunciated by Bailey (1944) who thought of the primary xylem as a refuge for "primitive" xylem characteristics in woody angiosperms.
Even in primary xylem of conifers and Gnetales, one sees circular bordered pits intercalated into the helical bands
Primary xylem and, in the stems, even secondary xylem of Sarcandra is vesselless, as claimed by Bailey and Swamy (1950).
Primary xylem elements and element associations of angiosperms.
In the same way, the distribution of xylem features might also be explained by adaptation to current functional contexts, with statements such as Mauseth's (1988) decidedly rare in the literature: "The primary xylem
was even described as being a refugium of primitive characters rather than being a tissue that is highly adapted to the needs of seedlings, leaves, flowers, fruits, and the tips of roots and shoots" (Mauseth, 1988, p.
Secondary xylem is bounded centripetally by primary xylem and sclerified pith (Figure 2) and centrifugally by vascular cambium and phloem (Figure 3).
Secondary xylem (wood) is either deposited uniformly about the primary xylem and pith or is deposited in an eccentric fashion (compare Figure 1 and Figure 5).
Rays emanating from the primary xylem ridges become especially wide as diameter of the secondary xylem tissue increases (Figures 3).
Primary xylem consists of both proto- and metaxylem (Figure 23).
These activities widen the more peripheral part of each of the medullary vascular bundles with the secondary tissues to impart a wedge shape, the narrow end of which is occupied by primary xylem