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Vasicentric tracheids are defined as tracheids present adjacent to vessels in a wood which also possesses libriform fibers or fiber-tracheids as imperforate tracheary elements. Vasculartracheids would correspond with this definition, but are defined as occurring only in latewood, whereas vasicentric tracheids occur throughout a growth ring or in wood of a diffuse-porous species. V asicentric tracheids offer a subsidiary conducting system which can supply stems and leaves when the vessels to which vasicentric tracheids are adjacent fail because of air embolisms. True tracheids (present as the sole imperforate tracheary element type in the woods in which they occur) have much the same physiological effect, maximizing potential safety in conduction. Vasicentric tracheids, vascular tracheids, and true tracheids are alike in being imperforate tracheary elements bearing relatively large bordered pits approximately like those on lateral walls of vessels in density. Libriform fibers or fiber-tracheids also occur in woods in which vasicentric tracheids or vascular tracheids are present. Vascular tracheids, by virtue of occurrence only at the end of a growth ring (where they grade into narrow vessel elements), maintain water columns in stems, which thereby persist through dry seasons, but may not adequately supply leaves, which may wilt as drought progresses. Vasicentric tracheids, by safeguarding the conduction of all parts of a growth ring, seem related to the evergreen habit which many shrubs possessing vasicentric tracheids have. Phylads with true tracheids are relatively few in dicotyledons, and only a small part ofthese groups have evidently succeeded in adapting to dry climates. The numerous phylads which instead have libriform fibers or fiber-tracheids have developed comparable safety by evolving vasicentric tracheids. Therefore genera with vasicentric tracheids bulk large in the floras of areas in which evergreen drought-tolerant shrubs are adaptive, the Mediterranean-type areas of the world. In southern California, 26 families (17 reported for the first time) have vasicentric tracheids; notable genera include Arctostaphylos, Ceanothus, Prunus, Quercus, and Salvia. Desert shrubs have vasicentric tracheids to a somewhat lesser degree. Other Mediterranean-type areas are surveyed for both vasicentric tracheids and true tracheids. The regions so analyzed are central Chile, southern Australia, the lands adjacent to the Mediterranean Sea, and southern Mrica. Genera in these areas with vasicentric tracheids include Banksia, Carissa, Eucalyptus, Grevillea, Hakea, Protea, Quercus, Rosmarinus, etc. New Zealand and Japan are wetter and have fewer genera with vasicentric tracheids but more with true tracheids. Many drought-tolerant evergreen shrubs have vasicentric tracheids. A new listing of families and genera with vasicentric tracheids is provided for the world flora, with new reports documented and pertinent literature cited. This new listing includes 68 families (33 newly reported in this paper), which represents a considerable advance on the 32 families listed by Metcalfe and Chalk in their compendia of 1950 and 1983, and includes some large and diverse families for the first time (e.g., Ericaceae, Pittosporaceae, Rosaceae). The lack of reports on occurrence of vasicentric tracheids may in part relate to difficulty in identifying these cells; suggestions for analysis are given.

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© 1985 Sherwin Carlquist

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