The slender helical thickenings of Prostanthera rotundifolia (Lamiaceae) are slender and almost horizontal. What one notices upon looking carefully is that they fade out where a pit aperture interrupts them, or else they are extremely close to the pit aperture. One might have expected the helical thickenings to run midway between pit apertures, but they do not.
The helical thickenings of Olea cunninghamii intersect and branch; they fade in and fade out of the wall surface in an unusually intricate pattern. They are also much more slender than most helical thickenings.
The helical wall thickenings of Sabia japonica are very slender, and run more nearly vertically than diagonally. The pits on the vessel walls are aligned vertically, probably in accordance with the tracheids adjacent to the vessel, but the pattern of the thickenings is not related to the pits at all.
In earlywood vessels of the bush poppy (Dendromecon rigida) the helical thickenings are broad but shallow. There is no evident relationship between the thickenings and the pit apertures, and some of the pit apertures occur within the thickenings.
In general, helical thickenings of the latewood vessels tend to be coarser and more conspicuous as compared to the thickenings of the earlywood vessels within a single wood. Here is a latewood vessel of Dendromecon rigida.
Helical thickenings in the tracheids of Fabiana viscosa. The magnification of this photo is greater than that of the vessel wall picture, so that the helical thickenings appear to be coarser than those of the vessel, but they probably are about the same. One does not find helical thickenings in wood cells that are not conductive in nature.
Grooves interconnecting pit apertures in the vessel walls of Operculina palmeri (Convolvulaceae). The grooves are relatively shallow, and the pit apertures are elongate in the direction of the grooves, which is typical. As a further generalization, one may say that the grooves follow the pattern of microfibril deposition (statement based on direction of rifts seen in walls when broken).
The inside surface of a vessel element of Evolvulus alsinoides (Convolvulaceae) shows vestures of various sizes in the pit apertures. Also, notice the irregularity in deposition of the secondary wall, evidence as streaks. These are not helical thickenings.
Vestured pits of Parkinsonia aculeata (Fabaceae), seen from the outside of a vessel. A few remnants of the pit membranes can be seen in one corner of the photograph. At the opposite end, pit apertures appear to have fewer vestures. This appearance is due to removal of tips of vestures by sectioning.
The vestures in vessel wall pits of Cadaba (Brassicaceae s. l.., Capparaceae) are few in number Most seem confined to the pit cavity and do not seem to extend very far towards where the pit membrane (removed by sectioning) is positioned.
The inside surface of a vessel of Metrosideros tomentosa (Myrtaceae). Most of the vestures seem to touch each other, or else they are interconnected by minute threads. The vestures fade into the wall surface. Vessel wall surfaces in Metrosideros bear small vestures or warts of various sizes.
Vesturing is found not just in pits of vessels of Metrosideros, but in pits of tracheids as well. The vesturing of this tracheid pit (seen from the outside of the tracheid, pit membrane removed by sectioning) is relatively abundant, but does not extend very far toward the pit membrane.
A view of the inside surface of a vessel of Triplaris surinamensis. The vestures are very small in size and extend well out onto the vessel wall surface from the pit cavities. What explains this pattern?
The inside surfaces of vessels of Cercidium (Fabaceae) are amazingly rich and varied (see section on Systematic Wood Anatomy in this website). The pits of this species, C. australis, are vestured, but vesturelike structures are grouped into polygonal patterns. Around each of the pit cavities is a collarlike rim; the appearance has been designated as “crateriform.”
Pits or perforation plates? SEM is required to make the distinction in these narrow tracheary elements of Ipomoea arborescens, because pit membranes often don’t stain and are therefore not readily visible with light microscopy. These only look like somewhat large pits with the light microscope, but SEM reveals absence of pit membranes, so they are perforation plates.
The inside of a vessel of Corynocarpus laevigatus (Corynocarpaceae). The juncture between two vessel elements is shown, and the simple perforation plate is bordered, appearing like a pair of ridges. Notice also the fine thickenings on the vessel wall surfaces.
An SEM photograph of a thin section of a vessel and adjacent cells of Barbeuia madagascariensis (Barbeuiaceae). The perforation plate is nonbordered: the rims of the adjacent simple perforation plates fuse into a pointed ridge.
When highly magnified, the surface of this tracheid of Ephedra trifurca proves to be covered with tiny crystals (calcium oxalate). Where sectioning has knocked crystals away, angular recesses in the wall surface are evident.
Calcium oxalate crystals are present on the outer surfaces of ray cells of Ephedra ray cells. SEM also reveals layering in the thick cell ray cell walls in this tangential section of E. przewalskii wood.
VESSEL WALL SCULPTURING
Many types of vessel (and tracheid) wall sculpturing can be seen with light microscopy, given careful observation. However, other structures on walls of vessels and tracheids are so fine that they can be resolved adequately only by scanning electron microscopy. Now that SEM is available, the nature of these structures is being described in detail, and some features can now be seen accurately for the first time. As we know more about these structures, we realize that they are more widespread systematically and more diverse than had been suspected. Just clarifying those variations would be a life’s work, and a fascinating one. However, an even more intriguing question is formed by the function of the structures one sees inside vessels with a scanning electron microscope. Because these structures are so tiny, we cannot watch them function. Experiments might be devised to demonstrate their function in indirect ways.
My own viewpoint has always been that woods are like an experimental material, divided into thousands of replicates and then put into different conditions. Plants are rather economical organisms, and are in competition with each other. If a number of different kinds of plants have a particular structure, like vestured pits, they probably are expending cellulose for something that enhances the survival of that plant. Comparative anatomy shows the distribution of structures in terms of ecology and systematics. These correlations don’t explain functions, but they do show directions in which to look rather clearly.
Some will leap to the conclusion then that if a structure like a vestured pit aids survival in dry conditions, all plants in dry conditions ought to have vestured pits. Wrong. Apparently vestured pits are something that a few groups of plants have invented—it’s a rather intricate structure. Let’s say it is related to dryness. Then shouldn’t it be lost as plants with vestured pits evolve into wetter conditions? Not necessarily. You can see what the interpretive problems involved are. Vestured pits, if they help in, say, preventing embolisms of vessels and tracheids, should not be harmful in plants not subject to the most extreme drought.
Thickenings and grooves. There is a tendency use the term “helical thickenings” for all forms of sculpturing on a vessel wall. Some sculpturing does take the form of thickenings, some takes the form of grooves, and in some plants, both phenomena are present together. These can co-occur within a family or a genus. First, some examples of helical thickenings. The more one sees these, the more one thinks that there is no such thing as a typical helical thickening. The photographs presented here do not represent all of the possible variations.
Grooves interconnecting pit apertures are shown clearly here for Operculina. There are no apparent thickenings. In Clethra luzonica, such grooves (rather narrow ones) are present. In some cases, as shown for that species, the grooves are related to a single pit aperture, in some they are related to several pit apertures. The widely-used term “coalescent pit apertures” is a misnomer where only one pit aperture is involved, obviously. In Clethra hondurensis, both grooves and thickenings (ridges) that flank the grooves are present.
Several examples of grooves combined with ridges—a phenomenon frequently overlooked—are shown here. In some species, grooves are more conspicuous than the ridges (Walafrida). In some species the ridges are more conspicuous (Clematis). In some species the ridges and grooves may be about equally prominent (Poliomintha).
It’s interesting to see that latewood vessels have more helical sculpturing than earlywood vessels within any given species. Data for these two categories were collected for all of the species included in the 1985 survey [ PDF ]. Authors beginning with Kanehira in 1921 saw that helical thickenings in vessels were more common in colder areas. That appears to be true. But what is even more striking is the increase in helical thickenings with increasing drought. The percentages of woody species with helical thickenings in southern California is highest in the desert shrubs, chaparral, and coastal sage areas; the woods of alpine areas have helical sculpture somewhat less abundantly [ PDF ].
Amazingly, I was the first person to report helical sculpture (mostly helical thickenings) in vessels and tracheids of Ephedra (wood-and-bark-anatomy-of-the-new-world_1989 ??? [ PDF ], [ PDF ]. Ephedra has been a plant of great interest to botanists, why didn’t they look at the wood? Most of the New World species have helical sculpture in vessels and tracheids, and some of the Old World species do. Apparently botanists shy away from studying wood anatomy! Unfortunate, because it’s so fascinating—the most complicated tissue in plants (that’s probably why they shy away from it). Another reason why wood of Ephedra had not been studied is that Ephedra is not a tree, and most of the woods that have traditionally been represented in xylaria are trees. And then, there is the tendency for many wood anatomists to stay in laboratories and not collect materials in the field.
The presence of helical sculpturing in not only in vessels, but in tracheids, such as those of Ephedra and Fabiana, is interesting: it shows that tracheids are conductive cells, and that the helical sculpture must be some kind of mechanism that functions in retaining water columns during periods of drought, or possibly, refilling them once embolized. Wood structure, as I see it, much more often helps to avert disasters than to remedy them once they have happened. This is true, I believe, for both the conductive and mechanical functions of wood. Wood can respond to damage, of course. After frost damage to a cambium, one often sees groups of callus parenchyma cells (misleadingly called “pith flecks” in anatomical glossaries), which represent proliferation of some surviving parenchyma cells that soon develop an organized cambium). Helical sculpturing was thought by Jeje and Zimmermann (1979) to lessen resistance to flow in vessels. One may be able to devise experiments that have this as an experimental finding, but that doesn’t mean that this is why helical sculpturing has evolved in wood cells—an interesting lesson in how science works! In fact, helical sculpturing is more abundant in conducting cells that are conducting water very slowly, and less abundant or not at all present in wood cells that conduct water rapidly.
Vesturing: a curious and varied phenomenon. Vesturing is something best seen with SEM, although it was noticed long ago with light microscopy. It is quite characteristic of certain families (notably those of Myrtales), or genera, as mentioned by those who study wood identification. Fine vestures and warts on walls of vessels and tracheids may be difficult to see with light microscopy. Much remains to be found about the full range of expressions of the phenomenon.
From the inside of a vessel as seen with SEM, vesturing most commonly appears like small wartlike lobes of wall material around a pit aperture, sometimes appearing to be attached at about the same level, as in Cercidium floridum, although more often not at a single level (as in Evolvulus). To see vesturing from the outside of the vessel, one must find an area where the pit membranes are cut away by sectioning of a wood. An appearance frequently encountered, as shown here for Parkinsonia, is of warts that terminate at about the same level, just short of the pit membrane, to which they do not appear to be attached.
This appearance invites an interpretation. One that has attracted some support says that vestures are a mechanism that prevents excessive deflection of the pit membrane, so that if pressure on one side of the pit membrane is greater than on the other, the pit membrane will not rupture, allowing a flow of air (presumably) into a vessel, disabling it. An appealing idea in some respects. One can ask why, then, vestured pits are not universal in dicotyledons of dry areas. Or cold areas. The obvious answer one might think of is that any wood structure that is unusual and precise, like the vestured pit, may require a complicated genetic program than has not been evolved in all clades of dicotyledon.
But then we are led to look at various occurrences of vestures and vestured pits. Tracheids of those Winteraceae that grow in frosty areas have warty inner surfaces, (or vesturing). The same should apply to conifer tracheids. Although some conifers do have warty surfaces on tracheids, others do not, and degree of cold (or for that matter, drought) particular species experience does not appear correlated with degree of wartiness. And while warts are present within pit cavities of some conifers, they are very small and would do nothing to deflect movement of a pit membrane—indeed, the torus-margo means of blocking a pit aperture when a pressure differential occurs would outweigh any action of the tiny warts in a conifer tracheid pit aperture to prevent movement of a pit aperture.
In a number of dicot woods, the vestures appear clustered at the mouth of a pit aperture, and do not appear to form a platform (Parkinsonia-style) to prevent deflection of the pit membrane. Cadaba is shown here as an example of such a limited extent of pit apertures. Some pit apertures are very coarse, some are very fine—one is amazed at the variety.
In Erysimum insulare, vestures are clearly present as one looks at the vessel wall from the inside of the vessel. When one looks at the pits from the outside of the vessel in that species, only tiny dotlike warts, unlikely to have much action on movements of a pit membrane, can be discerned.
Vestures could certainly function in preventing deflection of pit membranes if they formed connections with the pit membrane—but they never do!
And what is one to think of the vestured pits of Epilobium caucasicum? On the inside of the vessel, one sees long slitlike vestured grooves. Only the central portion of these grooves overlies the pit cavities, which are circular. Thus, we have vesturing in portions of the groove that are far away from the pit membrane and cannot possibly contact it.
Although the pits of vessels of Metrosideros tomentosa shown here have vestures that tend mostly to be clustered around the pit aperture, only perhaps half of those vestures would not contact the pit membrane. In other species of Metrosideros, the vestures coat the inner surface of the vessel wall. If we look at the tracheids of Metrosideros, we see that their pits are vestured, but in none of the pits are vestures close enough to the pit membrane or concentrated enough to affect the movement of a pit membrane. If we look at the inner surface of the vessels of Triplaris surinamensis, the vestures are very fine and extend well onto the vessel wall surface—most have no possible relationship with a pit membrane.
Vesturing (of a very coarse kind) on the walls of Cericidium (not all species of which have the phenomenon, although most do) is very curious. In vessels of C. australis, the pits are vestured. But in addition, the inner wall surfaces in C. australis bear a curious pattern of giant warts. In addition, C. australis vessels have what have been called “crateriform pits.” They are collar-like formations of secondary wall material around the pit apertures (see Systematic Wood Anatomy heading in this website).
In addition, one can cite such unusual instances as Ixerba (Ixerbaceae), in which warts are present in tracheids, but not on vessel walls. And Sophora tetraptera (Fabaceae), in which prominent helical thickenings are present on vessels, and vestures are present on the helical thickenings but not elsewhere on the inner surfaces of vessel walls.
Plants are relatively efficient with cellulose deposition, a process that represents a considerable expense of photosynthates. Vestures are sufficiently common in woods of some angiosperms and some gymnosperms so that one must ascribe some function to them. Those species must derive some benefit from them. Methodology in testing a function or functions is obviously not difficult because of the microstructural nature of vestures. The association with stress in water economy of a plant with relation to freezing or drought seems clear. The same appears to apply to helical thickenings. Quite a number of species that have helical thickenings in vessels also have them in tracheids—perhaps in most species the structures are shared by the two cell types. The same is true with respect to vesturing, as shown here in Metrosideros. Libriform fibers do not have helical thickenings, and they do not have vesturing. Presence of vesturing or helical thickening might be considered, along with other features, as characteristics of tracheids, and correlations with the conductive nature of tracheids (as opposed to fiber-tracheids and libriform fibers). The stresses that operate in water columns of vessels must operate in tracheids as well.
The more we know about various forms of wall sculpturing in vessel elements and tracheids, and the ecological distribution of the plants that have them, the better we will be able to investigate the functions of these structures. One must remember that ecology alone cannot dictate interpretation, however. The wood anatomy of a succulent desert plant, the water economy of which is buffered by water storage, will be quite unlike that of a nearby desert shrub that lacks succulence. A wood anatomist should take all of these factors into consideration.
And some miscellaneous things. SEM is a wonderful instrument for demonstrating some things about vessels (and other wood cells) that light microscopy cannot reveal. For example, many Convolvulaceae have what appear to be tracheids with various sizes of pits. Are the large pits simple perforation plates? Apparently yes, and thereby the cells are fibriform vessel elements, according to the micrograph shown here for Ipomoea arborescens.
One feature that has been overlooked by some wood anatomists is the presence of borders on simple perforation plates. Simple perforation plates are like large pits that have lost their pit membranes as the vessel elemnt matures, so having a bordered rim, as shown here for Poliomintha and Corynocarpus, is to be expected. But in fact, quite a number of families have nonbordered perforation plates! Shown here is an SEM of a longisection of a vessel of Barbeuia. The rim, instead of forming two points as seen sectional view, forms one point, and thus the perforation is nonbordered.
The presence of crystals in woods may not seem to be related to wood cell walls, and it usually isn’t. However, the outside surfaces of vessels, tracheids, and ray cells of wood of Ephedra are densely coated with crystals. Where crystals are knocked away by sectioning, one can see small recesses in the outer wall surface where they were attached.
The crystals also occur abundantly on outer surfaces of cells in the bark. Interpretation?
Probably wood and bark cells are protected from chewing insects such as weevils. Crystals on the outer surfaces of cells in wood and bark of Ephedra were not noticed prior to the use of SEM. The size range of these crystals makes them difficult to resolve with light microscopy. More such instances are to be expected as SEM studies of woods proceed.
In the illustrations at left, the long axis of the vessel is shown oriented vertically except in the photos of Clethra, Corynocarpus, Erysimum (inside of vessel) and Operculina.