The typical microstructure of Pine wood with plenty of tracheids
The anatomy of oak wood, full of vessels and two-sized rays
Typical bordered pits as in a coniferous wood species
Radial section of wood in which rays are shown
Wood anatomy is a scientific sub-area of wood science,[1] which examines the variations in xylem anatomical characteristics across trees, shrubs, and herbaceous species to explore inquiries related to plant function, growth, and the environment.[2][3]
Extensive study of the wood structure helps also in macroscopically or microscopically identifying the exact wood species for a variety of scientific, technical, historical, economical and other reasons. In recent years, wood anatomy also helps developing new techniques in preventing the illegal logging of forests,[4] that is the harvest, transportation, purchase, or sale of timber in violation of laws, leading to a number of environmental issues such as deforestation, soil erosion and biodiversity loss.
Commonly studied features include the dimensions of lumens and the thickness of walls in the conducting cells (tracheids, vessels), fibers, and various ray properties. The structural attributes of each xylem anatomical feature are largely predetermined upon formation and significantly influence its functionality, encompassing the transport and storage of water, nutrients, sugars, hormones, and mechanical support provision.[5]
These anatomical features are localized within (annual) growth rings, facilitating the establishment of intra-annual structure-function relationships and sensitivity to environmental fluctuations. However, generating large datasets of xylem anatomical data poses numerous methodological challenges.[6]
Optical microscope image showing a pinoid pit, characteristic microscopic feature of Calabrian pine (Pinus brutia) wood in the cross-field area
In botanical terminology, softwoods are sourced from gymnosperms, primarily conifers, whereas hardwoods originate from angiosperms, specifically flowering plants. Within the temperate zones of the northern hemisphere, softwoods are typically represented by needle-leaved evergreen trees such as pine (Pinus) and spruce (Picea), while hardwoods are predominantly composed of broadleaf, deciduous trees like maple (Acer), birch (Betula), and oak (Quercus).
The differentiation between softwoods and hardwoods extends beyond tree categorization to the cellular level. Softwoods exhibit a simpler basic wood structure, characterized by only two cell types and limited variation within these categories. In contrast, hardwoods display increased structural complexity owing to a higher number of fundamental cell types and a considerable degree of variability within these cell types. The primary distinguishing feature lies in the presence of vessel elements, also referred to as pores, which are characteristic of hardwoods and absent in softwoods.
Despite these disparities, softwoods and hardwoods share a cellular similarity – the majority of cells are non-living at maturity, even within the sapwood. These living cells at maturity, identified as parenchyma cells, are present in both softwoods and hardwoods.[9]
Databases
There are several databases relating to wood anatomy. One of them, InsideWood, is an online resource and database for wood anatomy, serving as a reference, research, and teaching tool.[10][11] This database was created by several international researchers, members of the IAWA, mostly botanists, biologists and wood scientists.[12] The database thousands of wood anatomical descriptions and nearly 66,000 photomicrographs of contemporary woods, along with more than 1,600 descriptions and 2,000 images of fossil woods.[13]
Another very important database for wood anatomy, is the so-called, Delta Intkey.[14]
In 2024, a new novel electronic device, named as The XyloTron, was fully developed at the Forest Products Laboratory for fast and reliable identification of wood.
Historical background
The inception of wood anatomy traces its roots back to the 17th century, during which pioneering scientists such as Robert Hooke, Marcello Malpighi, Nehemiah Grew, and Antoni van Leeuwenhoek emerged as the first individuals to utilize simple light microscopes. Hooke, leveraging his high technical expertise, dedicated efforts to enhance the quality of microscopes, focusing particularly on optimizing illumination and refining control over height and angle. Ultimately, he achieved magnifications of up to 50×, conducting examinations on a diverse array of objects. In 1665, Robert C. Hooke authored the seminal work "Micrographia," wherein he provided precise details concerning the porosity of charcoal and the structure of cork.[15]
Antoni van Leeuwenhoek, the third luminary in the field of microscopical plant anatomy, first delineated the characteristics of numerous hardwoods and certain softwoods. Through the utilization of his personally crafted and refined microscope lenses, van Leeuwenhoek demonstrated an exceptional ability to discern intricate details, including bordered pits, perforation rims in vessels, and a macrofibrillar substructure within the cell wall.[16]
As early as the mid-19th century, there was a notable increase in attention directed towards the examination of the structure of woody cell walls. Von Mohl employing polarized light microscopy, was the first to articulate the lamellar composition of a wood cell wall. However, it is important to note that his initial description only differentiated between primary and secondary lamellae, with the recognition of the tertiary lamella occurring later, thanks to Theodor Hartig. Von Mohl also accurately depicted most structural aspects of bordered pits in conifers.Taking a chemical perspective on the woody cell wall, Payen introduced the term "cellulose" to describe one of its constituents, emphasizing its similarity to starch. Carl Nägeli subsequently identified the cell wall as comprising crystalline cellulose, while Mulder introduced the term "lignin" to describe another constituent distinct from cellulose.[17]
The 20th century witnessed significant advancements in technology, influencing the wood anatomy area, and thus enabling a more detailed analysis of microstructural, chemical, and physiological characteristics. Irving W. Bailey using the application of conventional light microscopy and indirect methods such as polarization microscopy, X-ray diffraction, and staining techniques delved into the fine structure of wood tissues. Collaborating with his coworkers, Bailey established the uninucleate condition of fusiform cambial initials. He unveiled intricate details concerning the fine structure of the wood cell wall, particularly highlighting the non-cellulosic nature of the middle lamella. Contributions to the understanding of the fine structure of the wood cell wall were also made by Albert Frey-Wyssling and Reginald Dawson Preston, who employed light microscopy-based techniques. In parallel, Johannes Liese integrated his expertise in wood anatomy and decay mechanisms with extensive studies on wood protection.[18]
The advent of the electron microscope in wood biology around 1950 marked a transformative moment, ushering in a new dimension for the study of structural wood anatomy. Walter Liese, in 1950, captured the inaugural electron micrograph of a pine bordered pit membrane at the Institute of Ernst and Helmut Ruska in Berlin. In 1986, Ernst Ruska was awarded the Nobel Prize in Physics for his foundational contributions to electron optics and the design of the first electron microscope. This first image distinctly reveals the central torus and peripheral margo fibrils of wood with remarkable clarity.[19]
In the 21st century, wood anatomy is strongly being connected and inter-related with molecular biology. A significant milestone in molecular biology transpired between 2000 and 2020 with the accomplishment of sequencing entire genomes of trees. The comprehensive DNA sequences of forest trees marked their debut in 2006 with the publication of the Populus trichocarpa genome, followed by Eucalyptus grandis in 2014. Picea abies, as the inaugural conifer species, underwent sequencing in 2013. Subsequently, numerous other tree genomes have been successfully sequenced.[20]
Related Research Articles
A microscope is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisible to the eye unless aided by a microscope.
Plant cells are the cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagella or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.
Wood is a structural tissue found in the stems and roots of trees and other woody plants. It is an organic material – a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or more broadly to include the same type of tissue elsewhere, such as in the roots of trees or shrubs. In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves. It also conveys water and nutrients between the leaves, other growing tissues, and the roots. Wood may also refer to other plant materials with comparable properties, and to material engineered from wood, woodchips, or fiber.
In biology, tissue is an assembly of similar cells and their extracellular matrix from the same embryonic origin that together carry out a specific function. Tissues occupy a biological organizational level between cells and a complete organ. Accordingly, organs are formed by the functional grouping together of multiple tissues.
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.
Softwood is wood from gymnosperm trees such as conifers. The term is opposed to hardwood, which is the wood from angiosperm trees. The main differences between hardwoods and softwoods is that the softwoods completely lack vessels (pores). The main softwood species also have resin canals in their structure.
Hardwood is wood from angiosperm trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood contrasts with softwood.
A tracheid is a long and tapered lignified cell in the xylem of vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms use another type of conductive cell, called vessel elements, to transport water through the xylem. The main functions of tracheid cells are to transport water and inorganic salts, and to provide structural support for trees. There are often pits on the cell walls of tracheids, which allows for water flow between cells. Tracheids are dead at functional maturity and do not have a protoplast. The wood (softwood) of gymnosperms such as pines and other conifers is mainly composed of tracheids. Tracheids are also the main conductive cells in the primary xylem of ferns.
Pulpwood can be defined as timber that is ground and processed into a fibrous pulp. It is a versatile natural resource commonly used for paper-making but also made into low-grade wood and used for chips, energy, pellets, and engineered products.
The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. This tissue system is present between the dermal tissue and forms the main bulk of the plant body.
Parenchyma cells have thin primary walls and usually remain alive after they become mature. Parenchyma forms the "filler" tissue in the soft parts of plants, and is usually present in cortex, pericycle, pith, and medullary rays in primary stem and root.
Collenchyma cells have thin primary walls with some areas of secondary thickening. Collenchyma provides extra mechanical and structural support, particularly in regions of new growth.
Sclerenchyma cells have thick lignified secondary walls and often die when mature. Sclerenchyma provides the main structural support to the plant.
Aerenchyma cells are found in aquatic plants. They are also known to be parenchyma cells with large air cavities surrounded by irregular cells which form columns called trabeculae
A vessel element or vessel member is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are found in most angiosperms and in some gymnosperms such as cycads and Ephedra, but absent in conifers. Vessel elements are the main feature distinguishing the "hardwood" of angiosperms from the "softwood" of conifers.
Xylan is a type of hemicellulose, a polysaccharide consisting mainly of xylose residues. It is found in plants, in the secondary cell walls of dicots and all cell walls of grasses. Xylan is the third most abundant polysaccharide on Earth, after cellulose and chitin.
The secondary cell wall is a structure found in many plant cells, located between the primary cell wall and the plasma membrane. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding. It is most prevalent in the Ground tissue found in vascular plants, with Collenchyma having little to no lignin, and Sclerenchyma having lignified secondary cells walls.
A stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaves, flowers and fruits, transports water and dissolved substances between the roots and the shoots in the xylem and phloem, engages in photosynthesis, stores nutrients, and produces new living tissue. The stem can also be called the culm, halm, haulm, stalk, or thyrsus.
In woody plants, a tylosis is a bladder-like distension of a parenchyma cell into the lumen of adjacent vessels. The term tylosis summarises the physiological process and the resulting occlusion in the xylem of woody plants as response to injury or as protection from decay in heartwood. It is a key process in wall one of the compartmentalization of decay in trees (CODIT) and other woody plants.
Impregnation resins are slightly viscous, organic liquids that are used in the forest products industry for wood modification. They typically contain formaldehyde and are composed of dimers and trimers of the main molecule. These can become polymer solutions upon curing inside of a wood substrate, imparting stabilizing properties. Impregnation of these resins involves a vacuum chamber procedure that completely disperses the resin into the wood. Once inside of the wood, the resin can diffuse into the cell wall and enhance the physical strength of the wood even further.
Wood science is the scientific field which predominantly studies and investigates elements associated with the formation, the physical and chemical composition, and the macro- and microstructure of wood as a bio-based and lignocellulosic material. Wood science additionally delves into the biological, chemical, physical, and mechanical properties and characteristics of wood as a natural material.
The International Association of Wood Anatomists (IAWA) is an association that studies wood anatomy formed in 1931. Their office is currently based in the Netherlands.
Elisabeth A. Wheeler is an American biologist, botanist, and wood scientist, who is an emeritus professor at the North Carolina State University.
InsideWood is an online resource and database for wood anatomy, serving as a reference, research, and teaching tool. Wood anatomy is a sub-area within the discipline of wood science. This freely accessible database is purely scientific and noncommercial. It was actually created by several international researchers, members of the IAWA, mostly botanists, biologists and wood scientists.
References
↑ Wiedenhoeft, Alex (2010). "Structure and function of wood". Wood Handbook: Wood as an Engineering Material: Chapter 3. Centennial ed. General Technical Report FPL; GTR-190. Madison, WI: U.S. Dept. Of Agriculture, Forest Service, Forest Products Laboratory, 2010: P. 3.1-3.18. 190: 3.1–3.18. Retrieved 14 December 2023.
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