Author(s): Sherwin Carlquist
A hitherto unappreciated correlation exists between nature of vessel grouping and nature of imperforate tracheary elements in wood of dicotyledons at large: families and genera with true tracheids (large fully bordered pits common on both radial and tangential walls) have solitary vessels. Presence of true tracheids as a subsidiary conductive system is hy- pothesized to render vessel grouping a superfluous adaptation. Vessel group- ing does occur to various degrees in taxa with fiber-tracheids or libriform fibers; the degree of grouping is related to likelihood or seriousness of vessel failure by air embolisms because of either drought or frost. Grouping of vessels is regarded as a way of providing alternate conduits whereby water can be carried in the same pathways in case one or several vessels in a group are incapacitated by air embolisms. Presence of vascular tracheids, if suffi- ciently abundant, is held to be correlated with smaller degree of vessel grouping because vascular tracheids can form a subsidiary conductive sys- tem; small numbers of vascular tracheids do not affect vessel grouping pat- terns. Species which possess vasicentric tracheids possess a subsidiary con- ductive system ideally located around vessels and have solitary vessels or else (if vasicentric tracheids are less common) a low degree of vessel grouping. Species with very large vessels at the beginning of growth rings tend to have little grouping in the earlywood vessels but more grouping in latewood ves- sels; this dimorphism is held to relate to enhanced safety of latewood vessels, since earlywood vessels have little safety and the latewood is thereby the wood portion where safety mechanisms are concentrated. Fiber-tracheids do not have sufficient conductive capabilities to form a subsidiary conductive system; borders on pits of fiber-tracheids are rapidly lost during evolution, and such loss generally precedes appearance of septate or nucleated condi- tions or is simultaneous with it. Relative selective value of the various vessel grouping types (clusters, radial multiples, diagonal bands, tangential bands) as well as of larger aggregations remains a topic for more investigation, as does significance of grouping of primary xylem vessels.