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Diagonal vessel groupings can be readily seen in transections of woods, as in Calycanthus.

DIAGONAL VESSEL GROUPINGS

DIAGONAL VESSEL GROUPINGS [ PDF ]

   For a long time, people had observed that in cross-sections (transections) of woods, in some species vessels were grouped into aggregations that extended in diagonal patterns across the wood.  These were designated by such colorful but inexact terms as “flamelike” patterns.  “Dendritic” is one playful term also used in glossaries, but the diagonal bands really don’t branch like the limbs of a tree (as seen in transectional view)—they intersect as often as they branch.  So “diagonal” is definitely the best term.  Although such vessel groupings had been noticed for many years, apparently nobody had asked why such patterns occurred or looked for correlations.  It turns out that species with odd diagonal patterns of vessels are associated with presence of vasicentric tracheids as well as very narrow vessels.  Narrower vessels embolize less readily than wider vessels (Hargrave et al., 1994).  And certainly vasicentric tracheids provide greater conductive safety than vessel elements.  In the woods with diagonal bands of vessels, the vasicentric tracheids are not abundant: that would deter vessel groupings. Rather, there are smaller numbers of vasicentric tracheids (and some very narrow vessels) intermixed in the diagonal bands.  Thus, in a wood with diagonal groupings of vessels, the conductive safety advantages of grouped vessels and of vasicentric tracheids are combined.  Three-dimensionally, the diagonal bands are not broken by rays, although a wood transection gives that appearance.  One can say, in fact, that most of the vessels, perhaps all, are interlinked with each other by the diagonal vessel groupings with the vasicentric tracheids mixed in.  The imperforate tracheary elements between the diagonal bands are libriform fibers or fiber-tracheids.