A maceration of the wood of Hedera helix shows two vasicentric tracheids (vertical) as well as a number of libriform fibers. The vasicentric tracheids have bordered pits, the libriform fibers have simple pits and are darker because of their wall thickness.
A tangential section of Ceanothus thyrsiflorus wood shows vasicentric tracheids adjacent to the vessel. The helical thickenings occur in both the vessel elements (which are wider) and the vasicentric tracheids.
A transection of the wood of a globular cactus, Mammillaria mystax. The wood consists of vascular tracheids except for a few vessels, which appear to have thin walls and have parenchyma cells adjacent to them.
A longitudinal section of the wood of Mammillaria mystax shows the difference between a vessel (narrower helices) and vascular tracheids (helices composed of wide bands). The helical secondary walls of these cells permit expansion and contraction with change in water content of this cactus.
VASICENTRIC TRACHEIDS [ PDF ]
When I was doing the work on vessel grouping, I realized that I needed to understand the role of vasicentric tracheids as well as that of “ordinary” tracheids. By “ordinary” (or “true”) tracheids, one means tracheids that form the entire “background” of a wood. Vasicentric tracheids surround a vessel, but farther away from vessels in woods that have vascientric tracheids, there are fiber-tracheids or libriform fibers. If tracheids form a conductive system that can work even when the vessels amid the tracheids are embolized, couldn’t vasicentric tracheids serve the same purpose? Obviously they must, I thought. The way in which they sheathe vessels would form a way of safeguarding the conductive pathways that the vessels represent. And in fact, if vasicentric tracheids are sufficiently abundant, they do deter vessel grouping, as in oaks and some Myrtaceae. If they aren’t so abundant, they still serve the function of safeguarding conductive pathways. Tracheids as a ground tissue of the wood offer maximal conductive safety, but most of that advantage is offered by tracheids nearby the vessels—then the ground tissue of the wood can be libriform fibers or fiber-tracheids, both of which have better strength characteristics than tracheids. So I did a new compilation of families that have vasicentric tracheids. The list was much larger than that of Metcalfe & Chalk (1950). I define vasicentric tracheids exactly as they did. I saw more instances than they did because I used macerations, which can prove whether a fusiform cell has bordered pits in it, and whether such a cell has a perforation plate in it. With care, one can see vasicentric tracheids in sections, but the confirming role of macerations is valuable.
Because I live in southern California, I am more familiar with woods of this region than of most other regions of the world. As it turns out, the two most important genera of the chaparral, Ceanothus and Arctostaphylos are excellent examples of vasicentric tracheid presence. Other chaparral shrubs (Adenostoma, Cercocarpus) have tracheids throughout the wood. Obviously conductive safety is an important feature of dryland woods, and there has been a selective advantage for this feature. Either “true” tracheids or vasicentric tracheids are a highly adaptive feature in such habitats. The advantage of either of these conditions seems clear. They can maintain the conductive pathways within a wood when vessels embolize. This tends to protect foliage at all times, so the evergreen habit of chaparral plants is understandable. There is another condition, vascular tracheids, in which tracheids like very narrow vessel-like elements but lacking perforation plates are formed at the very end of a growth ring. These tend to occur in drought-deciduous plants in southern California, and the distribution of vascular tracheids, at the end of the growth ring, supports the adaptational nature of these cells. In the driest part of the year, there would be a layer of functioning tracheids that would protect the cambium and protect it from dehydration. Globular cacti form an interesting case. There are a few vessels in the earlywood of cacti, but most of the conductive cells in the remainder of the growth ring may be vascular tracheids with helical thickenings.
Some authors disagree with the above definitions of true tracheids, vasicentric tracheids, and vascular tracheids. They mention the occurrence of distorted cell forms as a criterion of vasicentric tracheids. This really is not a valid distinction. Vasicentric tracheids near big vessels in oaks and some Myrtaceae such as Eucalyptus, and because enlarging vessels distort the form of cells surrounding them, vasicentric tracheids in those particular instances can have some peculiar forms. But latewood vessels of oaks may be accompanied by perfectly fusiform vasicentric tracheids. And major genera of Myrtaceae such as Melaleuca and Verticordia also have ordinary fusiform vasicentric tracheids adjacent to vessels. So obviously, the distorted-shape characteristic is not a valid characteristic, but the position of tracheids within the wood is an easily used and physiologically relevant criterion. Oak and eucalyptus wood from trees is familiar to wood anatomists, but because Melaleuca and Verticordia are shrubs, wood anatomists trained on the basis of wood anatomy of forest trees are unfamiliar with them. Vasicentric tracheids, as well as true tracheids, may have provided the mechanism whereby some families, such as Epacridaceae, Ericaceae, Fagaceae, Myrtaceae, Proteaceae, Rhamnaceae, Rosaceae, and Zygophyllaceae have been able to speciate into dry areas—especially evergreen species in these families.