Diagonal and Tangential Vessel Aggregations in Wood: Function and Relationship to Vasecentric Tracheids

The list of families with diagonal ( dendritic or flamelike of other authors) patterns of vessel aggregation is similar to the list of families that have vasicentric tracheids. This paper attempts to deal with apparent exceptions. Because of recent reports of vasicentric tracheids, the families with diagonal vessel aggregations are all also on the list of families with vasicentric tracheids with the exception of four families. Genera of those four families are studied to see if a relationship between vasicentric tracheids and diagonal vessel aggregations does hold. Of the families not on both lists, Leitneriaceae (Leitneria), Melastomataceae (Mouriri), and Moraceae (Madura) do prove to have vasicentric tracheids in appreciable numbers. Small numbers of vasicentric tracheids and small degrees of vessel aggregation occur in Asimina (Annonaceae), Morus (Moraceae), and various Araliaceae. Vessels may not appear diagonally grouped if libriform fibers accompanying them are few, or if vessels are separated by large numbers of vasicentric tracheids. Diagonal grouping appears most marked in woods from drier and colder localities. Extensive diagonal vessel aggregations are apparently a reliable indication ofvasicentric tracheid presence, but only a portion of the woods with vasicentric tracheids have diagonal vessel aggregations. By having few, large, and intersecting aggregations of vessels mixed with vasicentric tracheids, a wood has greater potential safety (failure of fewer water columns by air embolisms) in space (within wood) and time than a wood with smaller, more numerous vessel groupings. Vessel grouping and vasicentric tracheid presence are considered additive in their safety effects