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Portion of a drawing by Nehemiah Grew (1685) of oak wood; he accurately shows the vessels as solitary.

Portion of a drawing by Nehemiah Grew (1685) of wood from a wormwood (Artemisia) plant. Some vessels are isolated, but many are in groups.

A transection of the wood of Myoporum laetum, a New Zealand shrub. The vessels are typically in groups of three or four.

A transection of wood of a desert shrub, Krameria. The vessels are basically solitary, but because the vessels are so numerous and crowded, a very small number touch each other. Such a small proportion is distinct from woods in which vessels characteristically are grouped. If Krameria had libriform fibers or fiber-tracheids instead of tracheids, it would have large vessel groupings, like other desert shrubs that have those cell types in wood instead of tracheids.

In Ulmus, the elm, large earlywood vessels are large and mostly solitary, but the latewood vessels are small and grouped into extensive bands.

Oaks from temperate regions often have very wide earlywood vessels but narrow latewood vessels. Study of transections of oak wood shows that vessels occur within patches of tracheids, which thereby should be called vasicentric tracheids.

VESSEL GROUPING

VESSEL GROUPING [ PDF ]

Answering the 300 year old question (which nobody asked) of why vessels are grouped in some woods but not in others.  When I studied wood anatomy of composites (Asteraceae), culminating in the 1966 summary paper, I noticed that when cross-sections (transections) were studied, one could see that in the wood of species from dry localities, vessels occur in large groupings, whereas in species from relatively moist localities, relatively few vessels are in contact. Species of Olearia from contrasting habitats show this. The correlation seemed obvious to me—had nobody ever noticed the correlation before?  Apparently not.  One can find illustrations as far back as 1685 showing vessels in cross sections of various woods—Grew’s “Anatomy of Plants” shows solitary vessels in oak wood, grouped vessels in “wormwood” (Artemisia) wood, but doesn’t call attention to this.  Eventually wood anatomists noticed that some woods had solitary vessels and some had grouped vessels.  The only interpretation ever offered was that solitary vessels were more primitive.  But if so, how could vessels in Olearia range all the way from mostly solitary in some species to numerous per group in other species?  To be sure, I knew that in California, woods of some chaparral and desert shrubs had solitary vessels (Krameriaceae, Rosaceae), whereas shrubby Asteraceae of such localities (Artemisia, Chrysothamnus)had vessels in remarkably large groups.  Primitiveness could not be the key, otherwise there would not have been such a wide range of vessel grouping within Asteraceae.  Ecology had to be the key: groupings of vessels make the conductive system safer.  If one vessel in a group is disabled by an air embolism due to water stress, neighboring vessels may retain water columns and the conductive pathway remains intact.  The larger the number of vessels in a group, the greater the chance that some vessels will retain water columns.  In my 1966 survey of woods of Asteraceae, there was an excellent correlation between moisture availability in the habitat and number of vessels per group in a wood—the drier the habitat, the more numerous vessels per group.  But there are exceptions, such as Krameria, and those exceptions show that degree of vessel grouping is controlled by what other cells are available for conduction in a particular wood.
     The plants tell you whether they have tracheids or not, you don’t have to look at a glossary.  Metcalfe & Chalk’s 1950 “Anatomy of the Dicotyledons” presents wood data for most woody families, and included in their nice descriptions is whether solitary or grouped vessels occur in particular families.  More importantly, they mentioned in families with both conditions which genera or subfamilies had solitary vessels and which had grouped vessels.  After comparing their data for a few families and looking at the descriptions, I thought I had the answer—and, in mad enthusiasm, I went through the whole of the two volumes in one night seeing if my idea worked.  It did.  Beginning on page 97: Cistaceae have solitary vessels in species with tracheids, but grouped vessels in the species of Helianthemum with fiber-tracheids.  And so it went in family after family.  In those families or genera that had tracheids (“fibres with bordered pits” in the Metcalfe & Chalk terminology) in addition to the vessels, vessels were not grouped.  In the families or genera with fiber-tracheids (“fibres with pits minutely bordered”) or libriform fibers (“fibres with simple pits”) in addition to vessels, the vessels were grouped to a lesser or greater extent..  In the case of the species with these latter two categories of “fibres,” the genera with the larger vessel grouping seemed to be from drier localities.  So what was happening was that tracheid presence was deterring vessel grouping very effectively.  Tracheids must be a conductive system that had greater safety (= resistance to spreading of air embolisms in the water columns) and therefore tracheids could be a functioning conductive system, their lumina filled with water even if the vessels accompanying them in a wood filled with air and failed.  In a wood without tracheids, the fiberlike cells (fiber-tracheids or libriform fibers) are not conductive, so that the only mechanism for safeguarding a failed vessel would be to have other vessels adjacent (in a grouping)—at least some of those vessels would stand a chance of not failing and therefore maintaining the conductive pathways.  Such a simple idea.  And there was also some experimental work that suggested that tracheids were conductive whereas fiber-tracheids and libriform fibers were not (H. J. Braun’s 1950 book in which he showed photographs of some dye uptake patterns in several woods—he did not mention any correlations or explanations for the patterns, however).  But not all tracheids are alike.  Some tracheids have numerous small bordered pits, some have fewer, larger bordered pits, it depends on the plant.  Those who depend on simplistic definitions will want to define a tracheid in terms of so-many-bordered-pits of such-and-such-a-size on a certain kind of wall.  But plants tell you the function of the cells; you don’t need to look at a glossary!  Look at a transaction of a wood, and if the vessels are essentially all solitary, tracheids are likely to be present (a quick glance at a radial section or a maceration can confirm the presence of tracheids.  If various degrees of grouping are present (smaller groupings in woods of wetter localities, larger groupings in woods of drier localities),   the wood has either libriform fibers or fiber-tracheids.  The difference between fiber-tracheids and libriform fibers can then be easily discerned from a longitudinal section: fiber-tracheids have pits with reduced borders, libriform fibers have no perceptible borders on pits.
    This scheme works because of an interesting principle: tracheids are more effective than vessel grouping in providing safety for the conductive system.  If a wood has a background of tracheids, the number of pathways for conduction should all vessels embolize is enormous.  But if a wood has fiber-tracheids or libriform fibers, there is no “back-up” system in case of failure of a vessel: the pathway represented by any particular vessel is lost.  If vessels are grouped, the chances that at least one vessel in that grouping will retain water columns despite water stress are greatly increased.  So tracheids are superior to non-conductive tracheary elements in safeguarding the vessel pathways.  Would an occasional tracheid near a vessel deter vessel grouping?  No, a small number is not enough, as Rosell et al. (2007, Bot. Jour. Linnean Soc. Bot. 154:331-351) have confirmed statistically.  However, a large number of tracheids around a vessel will function to deter grouping of vessels, as the oaks (Quercus) show.  This is also true in some Myrtaceae Sapotaceae.  But only these two families have large sheaths of tracheids around vessels, so they form the exception that proves the rule that a sufficient number of tracheids around a vessel can form a “back-up” conductive tissue better than grouping of vessels can. 
    I felt that this was one of my best discoveries.  So I submitted the paper to American Journal of Botany.  Surely they would want to publish such an exciting series of discoveries.  After nine months, no reviews had been obtained by American Journal of Botany; I told the editor that if there were no reviews within a year, I would withdraw the paper, which I did.  I submitted it to Aliso, and it was reviewed and published in 1984, about 18 months after my original submission to American Journal of Botany.  Some people think that some “prestigious” or “visible” journals have great papers and other journals publish just leftovers.  Not true!  Incidentally, 19 months after I submitted the manuscript to American Journal of Botany, a reviewer for that journal finally did get a review to the editor, who forwarded it to me (because the paper had already appeared in Aliso, that was an entirely irrelevant action).  Few today understand that this paper was a wonderfully simple and excellent piece of scientific discovery.  However, IAWA has sponsored a system of definitions in which the differences between tracheids, fiber-tracheids, and libriform fibers are artificial, despite my putting forth the functional basis for such definitions (which had been used by IAWA and by such noted workers as Metcalfe and Chalk, I. W. Bailey, etc, and were accepted worldwide prior to 1989).
   There are even broader implications.  Why are vessels grouped into bundles in petioles and leaves?  In leaves, wouldn’t conduction to parenchyma be enhanced if vessels fanned out diffusely into a lamina from the petiole bundles?  Theoretically yes, but then the safety of the conductive system would be lowered.  If one vessel in a leaf bundle embolizes, adjacent vessels in a bundle can maintain conduction in that bundle.  Conduction is a matter of compromises between safety and efficiency.  Little surprise that vessels are large and not prominently grouped in earlywood of Ulmus, the elm, but narrower and in large groupings in the latewood.  Do we know that the big vessels in earlywood of oaks are embolized when seasonal water stress commences?  Yes, there are tyloses, parenchyma cells that fill the lumen of an earlywood vessel when it becomes nonconductive, but latewood vessels of that same growth ring lack the tyloses, and are functioning when the earlywood is no longer conducting.  If tracheids are so good at providing conductive safety, why are there any woods with nonconductive tracheary elements (fiber-tracheids and libriform fibers)?   The simple slit-like pits of libriform fibers offer minimal loss in mechanical strength compared to bordered pits.  In conditions where soil moisture not strongly seasonal—as in a rain forest—a wood composed of vessels plus libriform fibers is ideal for a tall tree.  So why are there any intermediate types of fiberlike cells, the fiber-tracheids?  As with many vestigial characteristics, complete extinction of a trait like borders on a pit does not occur abruptly.  The mechanical strength of fiber-tracheids is probably not significantly less than the mechanical strength of comparable libriform fibers.  Can woods that have no tracheids adapt to dry or desert situations?  Yes, other mechanisms may evolve in wood, in addition to vessel grouping and narrowness of vessels.  Woods may evolve vasicentric tracheids or vascular tracheids secondarily.  They may have narrow vessels, helical thickenings in vessels, etc [ PDF ].  There are many pathways to wood xeromorphy and conductive safety—but vessel grouping is certainly a conspicuous and pervasive one.