Wood and Stem Anatomy of Bergia suffruticosa: Relationships of Elatinaceae and Broader Significance of Vascular Tracheids, Vasicentric Tracheids, and Fibriform Vessel Elements

Abstract

Bergia suffruticosa Fenzl provided sufficient lower stem and upper stem material to compare wood and stem anatomy to those of putatively related families: Clusiaceae (including Hypericaceae), Frankeniaceae, Lythraceae, and Haloragaceae. Of these, only Clusiaceae proves close; resemblances to Bergia include presence of simple perforation plates, presence of tracheids as the imperforate tracheary element type, presence of vasicentric tracheids and fibriform vessel elements, predominance of uniseriate rays with some biseriates, vertical orientation of scalariform vessel-ray pitting, absence of intraxylary phloem, abundance of brownish compounds in parenchyma cells, and occurrence of druses and solitary crystals. Other features such as seed anatomy confirm the relationship of Elatinaceae and Clusiaceae. The presence and nature of vasicentric tracheids in Bergia suffruticosa and Clusiaceae prompts an examination of this cell type. Vascular tracheids are defined here as: (1) present in latewood only (except for some genera in which all wood is like latewood); (2) not always present near vessels; (3) characteristic mostly of drought-deciduous shrubs; (4) about the same length as vessel elements in any wood in which they occur; (5) with pits like those of the vessel elements; and (6) present in families so specialized that the imperforate tracheary element type is the libriform fiber (rarely the fibertracheid). Vasicentric tracheids, in contrast, are (1) distributed in earlywood as well as in latewood and can occur in diffuse porous; (2) always occur near vessels; (3) are wider, somewhat shorter, and more densely pitted than associated imperforate tracheary elements although they always bear pits like those of vessels; (4) may be associated with fiber-tracheids (with various types of pitting) or libriform fibers, depending on the species; and (5) occur in trees or shrubs that may be evergreen or winter-deciduous, but tend to be unlike the shrubs (predominantly from families in which herbaceous species are frequent or preponderant) in which vascular tracheids occur. Vascular tracheids as defined here do not have vestigial perforations; cells with such perforations are regarded as vessel elements by definition. Fibriform vessel elements (perforate but intermediate in size and shape between tracheids and wide vessel elements) have been reported in other groups, notably vines, where fibriform vessel elements may result from vessel dimorphism (lateral enlargement of narrow vessels preempted by the great widening of a few vessels). Fibriform vessel elements may also occur in some nonvining families, such as Hydrophyllaceae, however.