Wood Structure and Identification PDF

Republic of the Philippines Mindanao State University at Naawan College of Agriculture, Forestry and Environmental Sciences 9023 Naawan, Misamis Oriental, Philippines...

Republic of the Philippines Mindanao State University at Naawan College of Agriculture, Forestry and Environmental Sciences 9023 Naawan, Misamis Oriental, Philippines Wood Structure and Identification Fundamental concepts/terminologies on wood identification and Relevance of wood identification to forestry Wood - a material of plant origin characterized by a fibrous structure and largely composed of lignin, which provides rigidity to the structure and cellulose which serves as the main framework of the structure. - technically called xylem - the xylary portion of the tree (fibrovascular tissue), consists of sapwood and heartwood; and the principal strengthening and water conducting tissue of stems and roots in gymnosperms and angiosperms. - hard, tough substance that forms the trunks of trees, and that has been used for thousands of years as a fuel and as a material of construction. Technically, the term wood includes similar materials in other parts of the plant, including even the so-called veins in leaves (Encarta Encyclopedia) - The tissues of the stem, branches and roots of woody plants lying between pith and the cambium (ASTM, 1995) Functions of Wood in Plant 1. transports water from the roots to the different part of the plants 2. provides mechanical support to the crown of the plant 3. stores food in the form of starch Properties Common to all Woods 1. Wood, regardless of source, is cellular in structure (made up of cell walls). 2. All wood is anisotropic in nature that is they exhibit different physical properties when tested along the three directional axes or cardinal planes. 3. Wood is a hygroscopic substance that can gain and lost moisture easily. 4. Wood is susceptible to attack of fungi, insect, and other wood-destroying organisms. 5. As a construction material: a. Wood may be cut and work in various shapes with the aid of simple hand tools or with power-driven machinery; b. It can be joined with nails, crews, bolts, or connectors, all of which require the simplest kinds of tools and produce strong joints; c. Dimensional variations with changes in moisture content are relatively insignificant in the direction parallel to the grain (the direction of the grain most important in structures); d. Dimensional changes that may take place as a result of rise in temperature are less significant in wood construction than they are in construction utilizing metal structural members; e. Wood is a combustible material but if used in large-enough size, such as wood structure built with heavy timbers, provide an element of safety in fire not possessed by most other construction materials. i. When wood is subjected to extreme heat, wood decomposed with evolution of combustible gases and tarry substances leaving behind a charcoal residue, in the process known as pyrolysis. The low thermal conductivity and high specific heat of wood keep the pyrolysis reaction from spreading rapidly into the interior of a piece of wood as charcoal which forms on the surface, until it reaches the glowing stage, is an excellent heat-insulating material. f. Wood is lighter than most of the materials used for construction and fabrication; g. Wood is a poor conductor of heat, sound, and electricity (6 times less conductive than bricks; 8 times less conductive than glass; 15 times less conductive than concrete made of sand and gravel aggregates; 390 times less conductive than steel; 1700 times less conductive than aluminum); and h. Wood is effective against cold in winter and heat summer. 6. Wood is biological material and is a renewable resource. 7. Wood retains paints, lacquers, varnish, etc. being porous and fibrous. 8. Wood has outstanding mechanical properties, resilient and about 15 times more efficient in bending the steel structure. 9. Wood possesses superior vibration-dumping characteristics than any other substances. FPU031 (Wood Structure and Identification) Variations in Wood 1. variations within a tree (anatomical or physical properties) The variation can be described in terms of: a. Those patterns of change which occur in the radial direction, i.e. across adjacent growth increment; and b. Changes that occur along the axis. 2. variations within trees of the same species Influencing factors: a. Age or maturation changes within the cambium itself (which are associated with the variance within trees of a species) b. Genetical factor (that are one of the basic causes of between-tree variation) c. Environmental factor, i.e. rainfall, temperature, and silvicultural treatment (which affect the net water and nutrient supply to the cambium and influence both within and between tree variations). Sources of Commercial Wood (Timber) 1. Gymnosperms – known as conifers (one of the orders: Coniferales), evergreens, or softwoods or non-porous woods (without vessels but tracheid’s) which are the source of the softwood lumber of the trade; with the simple structures. Seeds are borne naked or not enclosed in an ovary; leaves are small, scale-like, or needle- like. 2. Angiosperms – are divided into monocotyledons and dicotyledons. It is commercially known as hardwoods. They are otherwise known as porous woods which generally contain different types of cells such as vessel elements, tracheid’s, axial parenchyma, fibers, and rays. Seeds are enclosed in an ovary. Characteristics Common to Woody Plants 1. Woody plants must be vascular plants, i.e. must possess specialized conducting tissue consisting of xylem (upward) and phloem (downward). 2. They must be perennial plants. 3. They must possess a stem that persist from year to year; and 4. Typically, woody plants exhibit secondary thickening. Kinds of Woody Plants 1. tree – a woody plant that attains a height of at least 15-20 feet or more at maturity in a given locality and usually (not always) has but a single self-supporting stem or trunk (may become a shrub if grown in adverse conditions). 2. shrub or bush – a woody plant that seldom exceeds 20 feet in height in a given locality and usually (not always) has a number of stems. Examples: gumamela, bougainvillea, santan, rose, duranta etc. 3. woody lianas – climbing woody vines may climb by twining, clambering, aerial roots, or tendrils. Examples: sampaguita, grapes, and rattan. Wood Identification It is a branch of forestry which deals with the determination of an unknown wood by comparing it with an already known wood sample. It is the process of establishing the gross features of wood, the identity, their botanical characteristics, and their uses. Importance of Wood Identification 1. Basis for proper forest charges collection or taxes levied for timber 2. For checking fraud/deceit in buying lumber 3. Important in the field of research in wood science and other related fields 4. For proper conversion and utilization 5. Aids in identifying herbarium specimens devoid of flowers, fruits, or sometimes leaves 6. Aids in criminal cases involving the use of wood 7. Important in differentiating timbers or group of timbers FPU031 (Wood Structure and Identification) Plant Anatomy, Parts and Functions of the Stem, Photosynthesis, Types of Growth Plant Anatomy Plant anatomy (phytotomy) is the study of the shape, structure, and size of plants. The plant body is divided into two (2) basic parts: shoot and roots. The shoot grows above the ground while the roots grow below the ground. The transition zone between shoot and roots is called the root collar. The shoot is composed of the leaves, stem, and reproductive parts. The leaves are often the primary site of photosynthesis; the stem holds leaves, transports and stores water and nutrients, and is sometimes photosynthetic; and the reproductive parts include flowers, fruits, and seeds. On the other hand, the roots anchor the plant, absorb water and minerals from the soil, and often serves for storage. Parts and Functions of the Stem The stem supports the buds and leaves and serves as conduit for carrying water, minerals, and sugars. The three major internal parts of a stem are the xylem, phloem, and cambium. The xylem and phloem are the major components of a plant's vascular system. A typical tree stem in cross section (transverse surface) shows the periderm (outer bark), phloem (inner bark), vascular cambium, and xylem. 1. The bark is everything outside the vascular cambium wherein the outer bark is referred as the periderm while the inner bark is known as the phloem. a. Periderm is subdivided further into phellogen, phellem, and phelloderm. i. Phellogen (cork cambium) is the region of cell division (lateral meristem) that forms the periderm tissues. Phellogen development influences bark appearance. It produces the phellem (cork cells) to the outside and phelloderm to the inside. ii. Phellem (cork cells) is composed of waxy, densely packed cells covering the surfaces of mature stems and roots. Photosynthesis can take place in some trees both through the phellem and in fissures (cracks). Phellem is dead at maturity and form waterproof, insulating layers that protect plants; and replaces the epidermis as the tree increases in girth. iii. Phelloderm consist of living parenchyma cells, which may be photosynthetic and store nutrients. 2. Phloem (also called bast) tissue makes up the inner bark. However, it is a vascular tissue formed from the vascular cambium. It is composed of sieve tube elements and companion cells, phloem parenchyma cells, and phloem fibres. a. Sieve tube elements – actively transport photosynthates down the stem. Softwoods have sieve cells instead. b. Companion cells – provide sieve tube elements with needed metabolites. Softwoods have albuminous cells instead. c. Phloem parenchyma cells – located near the finest branches and terminations of sieve tubes in leaf veinlets, where they also function in the transport of foods. These are called transfer cells and border parenchyma cells. d. Phloem fibres – flexible long cells that make up the soft fibres (e.g., flax and hemp) of commerce. 3. The vascular cambium is the primary meristem producing radial growth. The cambium has two types of cells: fusiform initials and ray initials. a. fusiform initials – derived from the Latin word fusus which means spindle. These are vertically elongated spindle shape cells which form the secondary xylem and secondary phloem. b. ray initials – more cubical in shape and leads to the formation of parenchyma cells. 4. The xylem includes everything inside the vascular cambium. The three (3) main functions of wood in plants: conduction of water from the roots to the leaves, mechanical support of the plant body, and storage of FPU031 (Wood Structure and Identification)  biochemicals. The xylem is composed of tracheids, vessel elements (hardwoods only), fibers, axial parenchyma, and rays. a. Tracheids and vessel elements are cells used for conducting water and minerals from the roots to the leaves. Hardwoods have both tracheids and vessel elements while softwoods do not have vessel elements. b. Fibers are cells with heavily lignified walls making them stiff. Many fibers in sapwood are alive at maturity and can be used for storage. c. Axial parenchyma is living tissue. Parenchyma cells can be used for storage and cell division. d. Rays (multiseriate & uniseriate) are radial parenchyma cells. Parenchyma cells give rise to adventitious (external) tissues. Photosynthesis Photosynthesis is one of the physiological processes in the plant body. It is the process of converting light energy to chemical energy and storing it in the bonds of sugar. Plants are autotrophs, which means they can manufacture their own food (sugar/ carbohydrate). Plants need only the following materials to manufacture sugar: light energy, CO2, and H2O. The overall chemical reaction involved in photosynthesis is: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2. Photosynthesis takes place in the chloroplast, specifically using chlorophyll - the green pigment that captures light through the stomata (stomates) of the leaves. The chloroplast is in the mesophyll cells of the leaves. The food manufactured from photosynthesis is used for the growth of the plant. Types of growth  Primary growth (apical growth)– the growth responsible for the elongation of stem, branches, and roots traceable to the activities of apical growing points.  Secondary growth or secondary thickening (lateral growth) – the growth responsible for the increase in diameter (girth) of trees, traceable to the activities of the lateral cambium. Secondary growth means wood. What is a meristem and how is it classified? A meristem is a plant tissue containing cells that can continually divide throughout the plant’s life resulting to its continual growth. Meristems are of two types: apical meristems and lateral meristems. Apical meristems are embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length (primary growth). On the other hand, lateral meristems are a cylinder of dividing cells that runs most of the length of stems and roots and is responsible for secondary growth. What are the two (2) types of lateral meristems? o vascular cambium – a type of lateral meristem that produces phloem and xylem. o cork cambium (phellogen) – a lateral meristem that generates cork, the waterproof outer part of the bark of trees and shrubs. FPU031 (Wood Structure and Identification) The Wood's Cross-Section, Methods and Types of Wood Identification The Wood's Cross-Section The diagram shows the types of wood found in a typical tree trunk in a cross-section The wood at the center of the trunk, the heartwood, is older, darker, and more durable than the younger wood surrounding it. As a tree grows, a thin layer of cells called the cambium generates new wood, called sapwood, just under the bark. Sapwood is softer and tends to be lighter in color than heartwood. As the sapwood ages, natural substances called extractives invade the sapwood and gradually convert it to heartwood. Wood Identification The identification of wood can be of critical importance to the primary and secondary wood using industry, government agencies, museums, law enforcement, and scientists in the fields of botany, ecology, anthropology, forestry, and wood technology. Wood identification is the recognition of characteristic cell patterns and wood features and is generally accurate only to the generic level. Because woods of different species from the same genus often have different properties and perform differently under various conditions, serious problems can develop if species or genera are mixed during the manufacturing process and in use. Lumber graders, furniture workers, those working in the industry, and hobbyists often identify wood without laboratory tools. Features often used are color, odor, grain patterns, density, and hardness. With experience, these features can be used to identify many different woods, but the accuracy of the identification is dependent on the experience of the person and the quality of the unknown wood. If the unknown wood specimen is atypical, decayed, or small, often the identification is incorrect. Examining woods, especially hardwoods, with a 10× to 20× hand lens, greatly improves the accuracy of the identification (Panshin and deZeeuw 1980, Hoadley 1990, Brunner et al. 1994, CITES 2002, Richter and Oelker 2002, Wiedenhoeft 2011). Some foresters and wood technologists armed with a hand lens and sharp knife can accurately identify lumber in the field by examining cell patterns on the transverse surface. The accepted technique for scientifically rigorous, accurate identification requires that wood to be sectioned and examined with a light microscope. With the light microscope, even with only a 10× objective, many more features are available for use in deciding. Equally important as the light microscope in wood identification is the reference collection of correctly identified specimens to which unknown samples can be compared (Wheeler and Baas 1998). If a reference collection is not available, books of photomicrographs or books or journal articles with anatomical descriptions and dichotomous keys can be used (Miles 1978, Schweingruber 1978, Core et al. 1979, Gregory 1980, Ilic 1991, Miller and Détienne 2001, Ogata et al. 2008). In addition to these resources, several computer-assisted wood identification packages are available and are suitable for people with a robust wood anatomical background, such as the on-line searchable resource InsideWood (http://insidewood.lib.ncsu.edu/) or Richter and Dallwitz (2000). Methods of Wood Identification 1. Comparing the unknown wood samples with samples found in wood libraries 2. Comparing the unknown samples with pictures found in wood handbooks, manuals, and illustrations in books FPU031 (Wood Structure and Identification)  3. Consulting the opinion of the expert or authorities in wood identification 4. Using keys (dichotomous key card-sorting key) 5. Using software’s or program with the advent of computers 6. Chemical methods a. Franklin method – uses equal volume of glacial acetic acid and hydrogen peroxide in sufficient quantity to cover the splinters in test tube; and b. Schultze method – employs concentrated nitric acid and a few crystals of potassium chlorate at room temperature until the splinters appear white. Types of Test Used in Wood Identification 1. Splinter test – also called ash test. It involves burning a splinter of the heartwood, of matchstick-size, in still air and noting the appearance of the residue, maybe charcoal partial ash, or complete/full ash. Complete whitish or brownish ash: Tanguile; charcoal with fine threads of blackish-black ash in small quantities: Red Lauan; brownish to whitish ash: Dalingdingan; white to whitish, white to whitish to grayish ash: Manggachapui and Gisuk-gisuk 2. Frothing test – ash or residues of species if sprinkled with water will produce whitish mass of bubbles, i.e. Sapotaceae Family 3. Fluorescence test – when wood is soaked in water will produce certain color, i.e. Narra soaked in water would turn the water to bluish color Wood Features for Identification (Physical, Anatomical, and Structural) Physical Features of the Wood 1. Color – due mainly to mainly to extractive in wood. a. bright yellow or orange – Bangkal, Kalamansanai b. light red to blood red – Smooth Narra, Tindalo c. grayish – Batitinan d. black or black stripes – most of Ebeneaceae family 2. Odor and taste – may be caused by infiltration products in the heartwood or by the action of fungi, bacteria, or molds; or it could be due to the amount of starch deposit in wood. a. bitter taste – Batino b. salty taste – Pagatpat c. agreeable odor – Batikuling and Kalantas d. leather-like smell – Malatae e. sawdust of Akle, Akleng-parang, and Malugai can irritate the mucous membrane causing one to sneeze, and in some cases could cause severe headache. 3. Weight – can, sometimes, be of great value in identification. It is a function of (1) the amount of cell wall substance per unit volume, (2) the amount of infiltration, and (3) the quantity of moisture present in wood. a. very heavy – Mangkono; b. heavy – Malabayabas, Dangula c. light – Malakalumpang; d. very light – Kapok 4. Luster – the property of wood that enables it to reflect light, or property of wood exhibiting sheen. It is affected by 1) angle at which light strikes the wood surface, (2) types of cells exposed on that surface, (3) amount of extractives and oily or waxy substance present in wood. Loss of luster is a good indicator of the incipient stage of decay in wood. 5. Grain – refers to the arrangement, direction, and alignment of woody elements comprising the woods. a. Straight – when the fibers and the other elements lie in the direction parallel to the longitudinal axis. b. Cross – when the alignment of the fibers and the other elements deviate from the direction parallel to the longitudinal axis. FPU031 (Wood Structure and Identification)  6. Texture – refers to the size and the proportional amount of wood elements. a. very fine – when pores are visible only with the aid of a magnifier b. fine – when pores are just barely visible to the naked eye c. moderately coarse – when the pores are readily visible to the naked eye d. coarse – when the pores are very distinct to the naked eye 7. Hardness – is estimated by cutting it with a knife or by applying pressure to the longitudinal surface of the wood with a fingernail; related to the density of the wood. The denser the wood, the harder is the wood. Anatomical Features of the Wood 1. vessels – are axial series of cell formed together to form an articulated tube-like structure. These are called pores as viewed in the cross-section. 2. axial parenchyma – thin-walled cells generally with simple pits whose function are primarily for storage and distribution of food materials. 3. wood rays – ribbon-like aggregation of cells extending in the radial direction, such that from bark toward the pith. These are thin-walled cells which function for transport of food from the inner bark to the core of the trunk. FPU031 (Wood Structure and Identification)  These serve as storage compartment of unconsumed food. On the radial surface, these appear as flecks, and in tangential plane, if visible, show up as a short and usually staggered lines extending with the grain. Types: a. According to cell composition i. heterocellular – when more than one type of cell, e.g. ray parenchyma, ray tracheid, and resin canal comprise the wood ray, i.e. fusiform ray of Benguet Pine ii. homocellular – when one type of cell comprises the wood ray b. According to width i. uniseriate – one-cell wide wood ray ii. multiseriate – more than one cell wide wood ray c. According to arrangement i. storied – when rays are arranged in tries or in echelon as viewed on the tangential surface; responsible for the formation of ripple marks i.e. Narra d. According to size i. narrow ii. broad 4. resin canals and oil ducts – are openings surrounded by epithelial cells that are capable for scooping resinous, gummy, or oily substances. These may be filled or empty (Mayapis). These may be smaller than pores and either be diffuse or concentric (short to long tangential series) which appear as patches in the cross section. The diffuse type can be found in Benguet Pine, Apitong, Narig, and Palosapis. These are distinct in Dipterocarp species: concentric lines in Hopea and Shorea. 5. cross-field pits – pits formed by the walls of a ray cell and a longitudinal tracheid as seen in the radial section. Types: a. window-like/ fenetriform – large pits with broad aperture and an overhanging boarder on the tracheid side so narrow that careful focusing is required to determine its presence, i.e. Benguet Pine b. pinoid – smaller than window-like, more variable in size, and more numerous per cross field, i.e. Mindoro Pine c. piceoid – small boarder pits, generally elliptical in shape, with the narrow and slightly extended aperture, i.e. Igem-dagat d. taxodioid – possess aperture which range from oval to circular, are included and much wider than the narrow, fairly even border. e. cupressoid – resemble piceoid but differ from them in that aperture is included and elliptical rather than lineal as in piceoid type, i.e. Almaciga longitudinal window-like pinoid piceoid tracheid cross field taxodioid cupressoid ray cell 6. tracheids – fibrous, lignified cell with bordered pits, and perforated ends Types: a. softwood tracheid – very long (7+mm) equipped with large prominent border pits on their radial walls b. hardwood tracheid – shorter (1.5mm) and as long as the vessel elements with which they are associated FPU031 (Wood Structure and Identification)  7. other features a. ripple marks – fine striations found across the grain on the tangential surface of wood, produced by the storied arrangement of the wood rays and other elements; also caused by the presence of interlock grain in wood. i.e. Narra, Mahogany, Manggis, Siar, Vidal’s Lanutan, Dungon, Bayok, Bayok-Bayokan, Anilao, Balobo b. included phloem – when the barks are included in the wood due to the malfunction of the cambium for some time, i.e. Api-api Structural Features of Wood Although wood can be cut in any direction for examination, the organization and interrelationship between the axial and radial systems give rise to three main perspectives from which wood should be viewed to glean the most information. These three perspectives are the transverse plane of section (the cross section), the radial plane of section, and the tangential plane of section. Radial and tangential sections are referred to as longitudinal sections because they extend parallel to the axial system (along the grain). The transverse plane of section is the face exposed when a tree is cut down. Looking down at the stump one sees the transverse section; cutting a board across the grain exposes the transverse section. The transverse plane of section provides information about features that vary both in the pith to bark direction (called the radial direction) and those that vary in the circumferential direction (called the tangential direction). It does not provide information about variations up and down the trunk. The radial plane of section runs in a pith-to-bark direction, and is parallel to the axial system, thus it provides information about longitudinal changes in the stem and from pith to bark along the radial system. To describe it geometrically, it is parallel to the radius of a cylinder, and extending up and down the length of the cylinder. In a practical sense, it is the face or plane exposed when a log is split exactly from pith to bark. It does not provide any information about features that vary in the tangential direction. The tangential plane is at a right angle to the radial plane (Figure 2.3a, top). Geometrically, it is parallel to any tangent line that would touch the cylinder, and it extends along the length of the cylinder. One way in which the tangential plane would be exposed is if the bark were peeled from a log; the exposed face is the tangential plane. The tangential plane of section does not provide any information about features that vary in the radial direction, but it does provide information about the tangential dimensions of features. All three planes of section are important to the proper observation of wood, and only by looking at each can a holistic and accurate understanding of the three-dimensional structure of wood be gleaned. The three planes of section are determined by the structure of wood and the way in which the cells in wood are arrayed. The topology of wood and the distribution of the cells are accomplished by a specific part of the tree stem. 1. Three cardinal directional planes of wood a. Cross or transverse section – face exposed when the cut was made perpendicular to the tree length b. Radial section – face exposed when the cut was made parallel to growth rings. c. Tangential section – face exposed when the cut was made parallel to the bark. FPU031 (Wood Structure and Identification)  2. Growth rings/annual rings/growth increments – successive layers of wood formed within a year or growing season. a. Latewood/summerwood – formed at the late period of the growing season (abundant in sunlight): wood is denser, heavier, darker, with thick-walled cells. b. Earlywood/springwood – formed at the early stage of the growing season when water is abundant or plentiful: wood is quite porous, with thin-walled cells, less dense, lighter in color and weight. 3. Sapwood and Heartwood a. Sapwood – light colored wood tissue composed of living cells which continue to conduct water from the roots to the different part of the plant; contains less extractive and no tyloses. b. Heartwood – dark-colored wood tissue composed of physiological dead cells whose primary function is for mechanical support; contains plenty of extractives and tyloses. The Cell Wall Structural Components of the Cell Wall Cell walls in wood impart most of the properties discussed here. Unlike the lumen, which is a void space, the cell wall itself is a highly regular structure, from one cell type to another, between species, and even when comparing softwoods and hardwoods. The cell wall consists of three main regions: the middle lamella, the primary wall, and the secondary wall. In each region, the cell wall has three major components: cellulose microfibrils (with characteristic distributions and organization), hemicelluloses, and a matrix or encrusting material, typically pectin in primary walls and lignin in secondary walls (Panshin and deZeeuw 1980). In a general sense, cellulose can be understood as a long string-like molecule with high tensile strength; microfibrils are collections of cellulose molecules into even longer, stronger thread- like macromolecules. Lignin is a brittle matrix material. The hemicelluloses are smaller, branched molecules thought to help link the lignin and cellulose into a unified whole in each layer of the cell wall. To understand these wall layers and their interrelationships, it is necessary to remember that plant cells generally do not exist singly in nature; instead, they are adjacent to many other cells, and this association of thousands of cells, taken together, forms an organ, such as a leaf. Each of the individual cells must adhere to one another in a coherent way to ensure that the cells can act as a unified whole. FPU031 (Wood Structure and Identification)  Cell wall – made up of millions and millions of microfibrils. a. Primary wall – associated with the meristematic and enlarging cells; very thin with random arrangement of microfibrils (irregular network) which is 0.1 to 0.2 microns thick consisting of cellulose and hemicellulose. b. Middle lamella – intercellular material which serves the function of binding the cells together; highly lignified; called compound middle lamella if formed with two primary walls on either side. c. Secondary wall – is formed by apposition or an increase in thickness of the cell wall caused by the addition of cell wall material to the inside of the primary wall; incapable of further surface enlargement; relatively dense and contain high proportions of cellulose, thus constitute the major source of strength for the woody cell. Layers of Secondary Wall 1. S1 layer – the outermost layer, comprises less than 10% of the wall thickness; the microfibrils are lying parallel to one another in two distinct spirals, one left-handed and the other right-handed (50-70 degree to the vertical). 2. S2 layer – the middle layer, comprises some 35% of the wall thickness; the microfibrils are lying parallel to each other in spiral oriental (10-30 degree) with is closely associated with the performance of wood (variation in strength, stiffness, and dimensional stability is possibly related to the presence of moisture, and fracture morphology with the variation in this angle). 3. S3 layer – the innermost layer, comprises 1% of the wall thickness and has similar arrangement with the S1 layer. d. Fibrils – the smallest aggregation of cellulose molecules that are found in plant cell wall; the thread – like structure in the cell wall. Microfibrils – bundles or layer of strands of elementary fibrils; the smallest natural unit of cell wall Cut-away drawing of the cell wall, including the structural details of structure that can be distinguished with an electron a bordered pit. The various layers of the cell wall are detailed at the top of the drawing, beginning with the middle lamella (ML). The next microscope. layer is the primary wall (P), and on the surface of this layer the random orientation of the cellulose microfibrils is detailed. Interior to the primary wall is the secondary wall in its three layers: S1, S2, and S3. The microfibril angle of each layer is illustrated, as well as the relative thickness of the layers. The lower portion of the illustration shows bordered pits in both sectional and face view. Modifications of the Cell Wall The following are the modifications of the cell wall: 1. pits 6.) crassulae 11.) tyloses 2. perforations 7.) trabeculae 3. cell wall thickening 8.) septation 4. dentation 9.) wart structures 5. callitroid thickenings 10.) vestured pitting 1. pit (plural: pits) – a recess in the secondary wall of the cell, open of the lumen on one side and including the membrane closing the recess on the other side. Parts of a pit: a. pit aperture or orifice – the opening of a pit into a cell lumen or into a pit chamber b. pit chamber – the space between the pit membrane and the pit aperture or orifice c. pit cavity – the entire space within a pit from the pit membrane to the lumen. FPU031 (Wood Structure and Identification)  d. pit membrane – consists of primary cell wall and middle lamella e. pit border – the overhanging margin in a bordered pit f. pit canal – the passage from the cell lumen to the pit chamber in bordered pits g. torus – the thickened portion of the pit membrane of bordered pits h. margo – the space between the torus and the pit border Types of pits: a. simple pit – a pit in which the pit cavity is wider or has a constant width. It does not have pit border b. bordered pit – it has pit border in which the pit cavity becomes abruptly constricted during the thickening of the secondary wall Two (2) complementary pits of adjacent cells is called a pit pair. Types of pit pairs: a. simple pit pair – a pairing of two (2) simple pits in adjacent cells b. bordered pit pair – two complementary bordered pits in adjacent cells c. half-bordered pit pair – when a bordered pit is opposed by a simple pit d. blind pit – a pit that has no complementary pit e. aspirated bordered pit pair – a bordered pit pair in which the torus is displaced against one or the other of the pit borders so that the aperture is blocked simple pit pair bordered pit pair half-bordered aspirated pit pair bordered pit pair cell wall pit membrane pit border pit aperture pit aperture/ pit chamber torus orifice margo pit membrane Types of pit arrangement: a. linear pitting – where there is a lateral elongation of the pit structure. b. scalariform pitting – the borders pits are arranged in ladder-like series. c. opposite pitting – the bordered pits are arranged in transverse rows extending across the cell; when crowded, the outlines of the pits become rectangular in surface view. d. alternate pitting – the bordered pits are arranged in diagonal rows across the cell; when crowded, the pits become polygonal in surface view. 2. perforations – are opening in the common end wall of adjacent vessel elements for rapid liquid conduction in hardwoods. Vessel elements are stacked one on top of the other to form vessels. Where the ends of the vessel elements meet one another, a hole is formed called a perforation plate. Types of perforation plates: simple scalariform foraminate reticulate FPU031 (Wood Structure and Identification)  a. simple – single opening (without bars), surrounded by rim only b. scalariform – several to numerous elongated pores with bars between them (ladder like) c. foraminate – numerous circular pores d. reticulate – netlike 3. cell wall thickening – localized ridges of microfibrils that are helically oriented about the cell axis in the interior surface of the secondary wall. Secondary wall is thicker and more rigid than the primary cell wall due to lignin deposition. These depositions occur in such a way that various many peculiar patterns of ornamentation are formed on the cell wall. Types of cell wall thickening: pit xylem parenchyma xylem vessel simple bordered annular spiral scalariform reticulate pitted dentation tyloses a. annular – ring like thickening at regular intervals b. spiral – spiral or helical thickening c. scalariform – irregularly in such a way that forms a staircase-like structure d. reticular – network-like structure e. pitted – cavity-like structures referred as pits 4. dentation – analogous to cell wall thickening which appear toothlike or dentate 5. callitroid thickenings – local thickening of the cell wall that are typical of the genus Callitris (conifer in Australia). These are pairs of thickening bars across the pit border. In tangential view, these appear as arches adjacent to the pit aperture. 6. crassula (plural: crassulae) – a pair of dark, curved zone lying above and below the pits themselves; characterized by slightly pit field – an area in the wall of a plant cell in which one or more pits develop 7. trabecula (plural: trabeculae) – rod-like extension across the cell lumen 8. septation – transverse partition which is formed entirely of secondary cell wall layers 9. wart structures – small wart-like protuberances (outgrowths) which are cytoplasmic remains deposited over the inner face of the cell wall facing the lumen warty layer – is the area where formation of these wart-like protuberances occurs 10. vestured pitting – development of wart structures into large simple or branched forms associated with pit chamber. It is also known for a long time under the name cribriform pitting. 11. tyloses (singular: tylosis) – are the balloon-like outgrowth of parenchymatous cells to the lumen of tracheids or vessels of the secondary xylem which store resins, gums, and many ergastic substances. tylosoids – are like tyloses, except that they form in gum and resin canals as outgrowths of the thin-walled epithelial cells lining the canal. FPU031 (Wood Structure and Identification)