Adventures in Wood Microscopy, with Dr. Duncan Slater

As one rarely gets to see sufficient electron-scanning microscopy (ESM) of wood in one’s daily life, I thought I would share some of mine. These are among the images generated when I did my PhD at the University of Manchester, England. Because I was working on this equipment for the first time, my images are amateur in quality, and most of them relate to fractured surfaces. Better images of wood structures are available from ESM experts, but for teaching I find it advantageous to use my own images, about which I know the full context.

When I first explored wood with an environmental scanning electron microscope?(ESEM), the beautiful structures that came into view took my breath away. Unlike with ESM with a standard microscope, with ESEM the wood does not need to be dried out to be scanned successfully, and it gives you a 3D look into greenwood. This is far superior to making wood slides with a microtome [instrument that cuts extremely thin sections of material for examination] and then looking under a standard microscope, as that process only gives you a 2D view. The images in this pictorial are courtesy of ESEM technology.?

Unlike my previous articles in City Trees, this one has a structure much like a set of presentation slides, building up a picture of some interesting components of the wood of angiosperms and gymnosperms. When I put these slides up in a classroom, I often say to the students, “Welcome to Planet Wood!” I enjoy taking them on a bit of a journey, and hope you enjoy this one — Duncan.

The Vessel

The vessel: the key transport corridor for sap through most woody angiosperms. Seen in 2D, it often just looks like a bigger ellipse than the surrounding ellipses of other cells on a cross-sectional slide made from a woody stem. View a vessel under an electron microscope, though, and you can see each vessel element that makes up the vessel's length, which is similar to small sections of perforated pipe, cut on a slant, aligned to each other to provide a conduit for sap.

From looking at images of vessels like this, first you learn just how perforated a vessel element is—leaking sap into all adjacent areas is a key role of these perforations—then you realize the need for the mechanical support of the adjacent wood fibers, as a cell wall so full of holes is not going to be self-supporting. This image and the next one are from the wood of common hazel (Corylus avellana).

The Scalariform Plate

Where a vessel element meets another vessel element, there is typically a restriction or barrier, in the form of a sieve plate, also known as a scalariform plate. Trees need to be clever about how they do this: they cannot have a significant restriction to sap flow through their vessels, but they need to avoid embolisms, where air seeds into a vessel, rendering it useless.

This combination of needs results in vessel elements with squeezed ends, or with ladder-like plates—as with the [slightly broken] scalariform plate shown—or ones that have a pore-like structure. These end-types represent only a minor restriction to sap flow when the vessel is functional, but can prevent an air bubble expanding into the next vessel element when one element becomes dysfunctional. What amazing and ancient 'tech' this innovation represents—like water pipes with many discrete air valves!

Some authors have suggested that vessel ends act as a barrier to fungal decay too. Well, just look at that scalariform plate—is a fungal thread unable to pass through that? Vessel ends will seem to act as a barrier to decay because the next vessel element is full of sap, and a fungus typically cannot enter a vessel at 100% moisture content. In addition, many trees and shrubs have secondary mechanisms performed by adjacent cells that fill dysfunctional vessel elements with gums, resins, or tyloses. It is really these two mechanisms that are the key barriers to decay passing up through a vessel—not the scalariform/sieve plates, which most often represent little obstruction to a hungry fungus.

The Ray Parenchyma

These images show ray parenchyma cells in the wood of common beech (Fagus sylvatica). Distinct from fibers and vessels, parenchyma cells are much more rounded in shape, and in much of the functional sapwood, these cells are alive, unlike those of fibers and vessels, which are dead cells when they are fully formed. In a tree, the rays make an important living network through the outer sapwood, making this region of the wood able to respond to many challenges.

Ray parenchyma play many roles in sapwood performance: sensing, metabolizing, defending, translocating, and storing non-structural carbohydrates. In the closer-up image, you can see starch grains as small, rounded pellets, stored up in a ray cell—a cache of tree food, if you will.?

It is quite an investment for a tree to develop an extensive ray system, but in general, tree species with a larger ray system in their wood tend to be those that live longer, come back from decline more readily, and are less prone to decay.

Crystals in Storage

This ESEM image shows a few radial parenchyma cells in common hazel and some rather beautiful crystals that have formed inside them. These are not starch grains, which have a rounded shape—so what are they?

A literature search shows that crystals in woody plants have hardly been researched at all. Scurfield et al. (1973) found such crystals to consist of calcium carbonate or calcium oxalate. For conjectural purposes, if these are crystals of a calcium compound, why would this hazel tree store that up for later use? The production of hazel nuts involves the use of significant amounts of calcium; we are, perhaps, seeing nut-growing resources stored up in these cells, ready for use when they are needed.

The Fiber

The key supporting cell in the wood of angiosperms is the fiber cell. This is confused with the industrial term wood fibers, which means a mash-up of all the wood cells to make wood pulp. In plant science, the fiber is defined as a specific wood cell type, not all wood cell types.

Fibers have few perforations, a tapering shape (like a thin bit of plasticine you’ve rolled out on a tabletop), and consist of a complex cellulose and hemi-cellulose matrix that has been lignified, set around a central lumen. Fibers are dead at the point of their functionality. Complex spiraling patterns of cellulose lie within a fiber’s cell wall, influenced by the loading that is being experienced in that location. (A higher proportion of fibers can be formed in highly loaded sections of a tree's anatomy.)

The path of individual fibers through the wood is short but often it is not exactly straight: for improved radial and tangential strength, fibers tend to develop with a bit of interweaving. Where you will find the straightest fiber alignments are where a long vertical stem has developed in shelter, while much higher levels of interweaving fibers are found at branch junctions, bends, and in abnormal growths like burls and galls, where fibers can be found to form arcing or circular patterns. Wood type seen here: common hazel.

The Tracheid

It is a common belief that the wood of angiosperms exclusively contains fibers and vessels and that the wood of gymnosperms contains only tracheids, which are cells that have the dual purpose of sap conduction and support. There are crossover plants that have both fibers and tracheids in their wood, like some species of Magnolia, showing that these contrasting anatomies evolved in stages. Indeed, there is not one ancestral plant that was the first to 'invent' wood; rather, wood has arisen from various plant families independently, which is rather amazing, given all the similar building blocks used to grow the woods of the world.

This image shows tracheids in larch (Larix decidua). On the left, see the uniformity of typical coniferous wood, with a transition across an annual ring (from earlywood to latewood). There is one easy-to-spot larger hole that is a resin duct. On the right-hand side is a closer-up image of the earlywood/latewood section, where you can see the spiraling pattern inside each tracheid. Like the fibers found in the wood of angiosperms, due to the need for tracheids to supply structural strength, there is in this conifer a spiraling matrix of cellulose strands, set within lignin in these tracheids and glued together with pectin.

The Pits

Wood has larger holes for sap flow, spaces in parenchyma cells for storage, and pits—many, many pits, that allow interconnection between wood cells so that sap and chemical signaling can be exchanged. Pictured is the bordered pit of a larch tree—think of this as a control point, allowing liquid exchanges between cells but only if the torus is in the open position. The pit, torus, and associated membrane act like a flexible valve, unsealing only under the right conditions. This micro-engineering at the cellular level is exquisite.

This is a mechanism by which wood cells can prevent air from seeding through functional tissues: once air enters a cell, the associated pits should all automatically close due to the differences in pressure between the aerated cell and the surrounding cells. Such micro-controls are part of the reason that, when you dry out wood, it can take a lot of effort to re-wet it back to its original level of moisture content (if you ever can).

The Pith

The pith of a young twig is best considered as cheap packaging material. Some species produce a very large pith in proportion to their twig diameter—for example, elderberry (Sambucus nigra). As such it risks forming twigs that fail in compression because this species has taken the concept of cheap packaging very far indeed. As with other aspects of tree growth, there are some species that are structural risk-takers because of the competitive advantages this can give them in the woods.

Piths vary in their nature; there are piths that are filled with a sponge-like material, those that are virtually empty of content, and piths that have multiple chambers. Pictured here are two shots of the chambered pith of a common hazel. Note the extreme scale of the pith tissue compared with the surrounding wood cells; it saves a lot of construction costs to have almost nothing in the center of this twig. Oddly, due to its emptiness, the pith can be highly resistant to the ingress of fungal decay. (You will often find decay stopping at the edge of a stem’s pith.)

The Matrix

Pictured here are two similar sections of the wood of an English oak tree (Quercus robur), fractured to reveal a radial view of the xylem. You can see large vessels running up and down both sections—rays transiting left to right— and broken fibers. Viewed collectively, one gets a better conception of the matrix of connections and interactions that occur between wood cells. Each cell type is multi-faceted and attendant to the needs of adjacent cells.

Wood is a very successful 'invention' of plants, occurring independently at least several times in evolutionary history. However, you do come across some academic texts which propose that wood is 'optimized' – and, more specifically, 'mechanically optimized'. But due to the many functions that wood performs (support, conduction, storage, defense), a tree's wood cannot be optimized for any one of these specific functions. Rather, it is an elegant compromise on the many demands upon it as a tissue. This is referred to in studies as 'trade-offs in xylem', of which sap conductance vs. mechanical strength is probably the most studied.

The Fracture

As all my ESEM images of wood are based on looking at wood that has been torn apart, let us consider fractures in greenwood and note some common micro-features. Once you have initiated a crack in a branch or stem of a tree, it is usually quite easy to pull the two pieces further apart. As you do so, you will probably hear a crackle or a 'kck...kck...kck' noise. This is the sound of wood fibers being broken, telling you that the wood grain is not absolutely straight and that there is some resistance to the crack propagating further.

In these ESEM images of fracture surfaces in English oak, the cracking has taken a path of least resistance in places, where you can see flat, exposed ray parenchyma cells. However, in other parts of the scan, you can find twisted and broken wood fibers in small clumps—these are the cells that gave rise to the 'crackle' as you propagated that crack through the wood by bending it.

Wood fibers are strong: try pulling apart a live twig—even of only 8 mm diameter—in a straight line. I bet you cannot do it. So, by having some interweaving along the wood grain of a stem or branch, the sapwood is made more resistant to cracking, and slightly less effective as a conduit for sap. A classic trade-off scenario.

11: The Broken Fork

All my ESEM images not only come from fractured wood; more specifically, they come from in and around the fracturing of branch junctions. There is an intense zone of wood with very different qualities than surrounding wood that forms in the axil of a branch junction and that is marked externally by that junction's bark ridge. This wood is much denser, with a higher proportion of wood fibers and fewer vessels—and it makes tortuous and interlocking patterns with its wood grain. The specialized wood that forms in these axils is called axillary wood.

These two images show the 'spine' of tissues one often finds when breaking apart a young branch junction; on the left is a larch and on the right, a beech.

For a snapped branch junction, the fracture surface along the axillary wood is often a mix of fibers snapped along their length or fibers extracted from the base of one of the two branches that were previously joined. Both effects can be seen in these two images: where fibers are extracted, you get a spur of tissues projecting from the fracture surface; where fibers were snapped, you can see their lumen and spirals of cellulose strands emerging from the fracture surface. Again, to achieve this, one is snapping some fibers along their length, which explains how much of the strength of branch junctions is conferred by this centrally-placed axillary wood.

12 - Axillary Wood

In all these images, I have been showing fracture surfaces of broken branch junctions, so some of the images in this article have included glimpses of some axillary wood, which I have categorized as a reaction wood (Slater et al., 2014).

As with all reaction woods, there is a drift in tissue qualities as there is a graduated transition from 'normal' wood to a reaction wood. Axillary wood is most distinctly different in its center, right in the center of the branch union: cutting further and further into a branch junction is a bit like cutting into the middle of an onion. The tissues are changing qualitatively as you get nearer and nearer to the center.

Pictured is some axillary wood of common beech when I fractured a branch junction. Probably the most important practical aspect of the twisting and turning that the wood grain of axillary wood does is that it causes some wood cells to be orientated so that you must stretch them and break them along their length to break the branch junction. This requires a great deal more energy to break this wood than if all cells were orientated radially or tangentially.

Axillary wood is the latest key finding in wood anatomy, from an arboricultural perspective. It is a key responsive woody tissue that is much denser than surrounding wood, much more crack-resistant, much stronger in the radial and tangential directions—but not much use in the conduction of sap (Slater et al., 2014). As a tissue type formed in all trees which have branches, it warrants much more scientific scrutiny.

It has been my pleasure to introduce its importance into arboricultural texts which have, for many years, ignored its existence or not realized its significance. That is why they awarded me a PhD—that I had identified an important biomechanical role played by this key woody tissue, which helps to form the branching structure of trees. It is easy to find if you know what you are looking for, as it is the denser wood that is typically much more difficult to split with an axe, that lies in the crotch of a branch junction due to the swirling character of the wood grain in that location.

The consequences of understanding the role of axillary wood in trees are many, so I invite readers that found this short journey to ‘Planet Wood’ interesting to follow up with some of my publications that help to define the properties of axillary wood. I wonder how many more lessons can be learnt from close examination of the wood that trees grow.?

*** This article first appeared in City Trees magazine ***