Wood anatomy, the study of woody cells and tissues, has made significant strides since the early descriptive accounts were made, which mainly involved cataloging the observable features of woody plants. Anatomical data have been utilized to gain a better understanding of the interrelationships between woody plants, confirming evidence of the natural relationships between plant families through combined analyses.
This book will serve its purpose well for undergraduate students of Forestry. Wood is primarily composed of elongated, hollow cells arranged parallel to each other along the trunk of a tree. The characteristics of these fibrous cells and their arrangement affect various properties, including strength, appearance, resistance to water and chemical penetration, resistance to decay, and more. Just beneath the bark of a tree is a thin layer of cells called the cambium, which is not visible to the naked eye. Here, cells divide and eventually differentiate to form bark tissue on the outside of the cambium and wood or xylem tissue on the inside. This newly formed wood, known as sapwood, contains many living cells and conducts sap upward in the tree. Eventually, the inner sapwood cells become inactive and are transformed into heartwood. This transformation is often accompanied by the formation of extractives that darken the wood, make it less porous, and sometimes provide more resistance to decay. The center of the trunk is the pith, which is the soft tissue around which the first wood growth takes place in the newly formed twigs. This book will be helpful in imparting theoretical knowledge to undergraduate students, academicians, and teaching faculty in the field of woody plants in the disciplines of Forestry and Agriculture.
Wood anatomy, the study of woody cells and tissues, has advanced considerably since the early descriptive accounts were made which consisted mainly of cataloguing what was ‘out there’. Anatomical data have been applied in better understanding of the interrelationships of woody plants, confirming evidence of natural relationships of plant families in combined analyses. This book will serve its purpose well, for undegraduates of forestry. We realized that with the passage of time, many new disciplines had been developed, and older ones expanded to a point where a much revised and updated book of this type could play an important part. Wood is composed mostly of hollow, elongated, spindle-shaped cells that are arranged parallel to each other along the trunk of a tree. The characteristics of these fibrous cells and their arrangement affect strength properties, appearance, resistance to penetration by water and chemicals, resistance to decay, and many other properties. Just under the bark of a tree is a thin layer of cells, not visible to the naked eye, called the cambium. Here cells divide and eventually differentiate to form bark tissue to the outside of the cambium and wood or xylem tissue to the inside. This newly formed wood (termed sapwood) contains many living cells and conducts sap upward in the tree. Eventually, the inner sapwood cells become inactive and are transformed into heartwood. This transformation is often accompanied by the formation of extractives that darken the wood, make it less porous, and sometimes provide more resistance to decay. The center of the trunk is the pith, the soft tissue about which the first wood growth takes place in the newly formed twigs.
Anatomy The study of gross internal structure of plant organs by the technique of section cutting is called anatomy .The term anatomy was derived from two words ‘ana’ means as under and ‘temnein’ meaning to cut. Anatomy is the branch of science which deals with internal structures and organization of the organism. It is also known as internal morphology. Plant anatomy also describes tissue and tissue systems of the plant body.
Arrangement of plants and animals into groups according to their close resemblance, naming and describing them is called classification. Classifying the plants into groups is called systematic botany. The main aim of classification is to name the plant to describe them and to bring out their resemblances in order to throw light on their relationship and common ancestor. It is to indicate primitive and advanced plants systematically. Need for Classification It is not possible for anyone to study all the organisms. But if they are grouped in some convenient way the study would become easier as the characters of a particular group or a family would apply to all the individuals of that group. Classification allows us to understand diversity better. Classification of plants Entire plant kingdom is divided into four main groups. 1. Thallophyta 2. Bryophyta 3. Pteridophyta 4. Spermatophyta or Phenerogams
Development of Plant Body A vascular plant begins in existence as a morphologically simple unicellular zygote (2n). The zygote develops into the embryo which in the long run develops into the mature sporophyte. The development of the sporophyte (2n) involves division and differentiation of cells and organization of cells into the tissue systems. The embryo of the seed plant possesses relatively simple structure as compared to the mature sporophyte. The embryo bears a limited number of parts containing generally only axis bearing one or more cotyledons. The cells and the tissues of this structure are less differentiated. However, the embryo grows further, because of the presence of the meristems, at two opposite ends of the axis, of future shoot and root. After germination of the seed during development of the shoot and root new apical meristems appear which cause a vegetative growth and reproductive stage of the plant attained. Fundamental Parts of the Plant Body The plant body consists of a number of organ i.e., root, stem, leaf and flower. Each part has different functions for plant growth.Flower consists of sepal, petals, stamens, carpels and sometimes also sterile members. Each organ is made up of number of tissue systems; each tissue system consists of number of tissues. Tissue consists of group of cells that are alike in origin, structure and function.
Meristematic Tissues These are composed of cells that are in state of division or retain the power of dividing. These cells are essentially alike and isodiametric. They may be spherical, oval or polygonal and their walls thin and homogeneous; the protoplasm in them abundant and active with large nuclei, and the vacuoles small or absent. Classification of Meristems Meristems are classified in different ways on the basis of certain factors. A) According to their origin and development 1. Promeristem or primordial meristem 2. Primary meristem and 3. Secondary meristem.
Primary Dicot Stem A stem with only primary permanent tissues is called primary stem. It represents a young stem. The different tissues present in dicot stem are arranged in concentric layers. e.g. Sunflower I) Epidermis Epidermis is the outermost layer of the stem. It is made up of compactly arranged parenchymatous cell, which look rectangular in transverse section. The cells are transparent and devoid of chloroplasts. The outer walls are convex, thickened and cutinized. On the outer side they possess a layer of cuticle. The internal walls of the epidermal cells are thin. The radial walls are thick towards the outer side and gradually become thin towards the inner side. Pits occur in the radial walls. The epidermis of sunflower bears several unbranched multicellular hair. At place the epidermis contains minute pores called stomata or stoma. Each stomata or stoma has a pair of specialized kidney shaped cells called guard cells. The guard cells have a few chloroplasts. By their swelling, the two guard cells can form a pore between them.
Primary Dicot Root A young dicot root which does not possess secondary tissues but only the primary permanent tissues (tissues derived from the apical growing point) is called primary dicot root. It is generally cylindrical in outline and possesses the following tissues. I) Epiblema or Piliferous Layer It is the outermost layer of the root. It is made of compactly arranged thin walled and slightly elongated parenchymatous cells. Epiblema of a root differs from the epidermis of the stem in being devoid of distinct cuticle and stomata. Some cells of the epiblema give rise to thin – walled tubular outgrowths called root hairs. In some plants the root hair mother cells are shorter in comparison to other epiblema cells. The root hairs and piliferous layer (pilus – hair, ferre to carry). The root hair and thin walled epiblema cells absorb water and mineral salts from the soil. Root hairs commonly do not live for more than one week. With their death the epiblema cells become suberised and cutinised. II) Cortex It lies below the epiblema. The cortex is made up of many layers of thin walled parenchyma cells. The parenchyma cells may be rounded or angular. They enclose inter-cellular spaces for diffusion of gases.The cells of the cortex store food. They also conduct water from the epiblema to the inner tissues.
Secondary Growth in Dicot Stem The formation of secondary tissue from the lateral meristem is called secondary growth. It increase in diameter of the stem. A stem with secondary tissues is called secondary stem. Secondary tissues are formed by two types of lateral meristem, vascular cambium and cork cambium or phellogen. Vascular cambium produces secondary vascular tissues while phellogen form periderm. Secondary growth occurs in perennial dicots such as trees and shrubs. It is also found in the woody stems of herbs. In such cases, the secondary growth is equivalent to one annual ring, e.g. sunflower. Formation of secondary vascular tissues They are formed by the vascular cambium. Vascular cambium is produces by two types of meristems: intrafascicular and interfascicular cambium. 1. Intrafascicular cambium is a primary meristem which occurs as strips in vascular bundles. 2. Interfascicular cambium arises secondarily from the cells of medullary rays which occur at the level of intrafascicular strips. 3. These two types of meristematic tissues get connected to form a ring of vascular cambium
The tissue system A group of tissues performing a similar function irrespective of its position in the plant body is called a tissue system. In 1875, Sachs recognized three tissue systems in the plants. a) Epidermal tissue system b) Vascular tissue system c) Fundamental tissue system. a) Epidermal tissue system Epidermal tissue system is the outermost covering of plants. It consists of epidermis, stomata and epidermal outgrowths. Epidermis is generally composed of single layer of parenchymatous cells compactly arranged without intercellular spaces. But it is interrupted by stomata. In leaves, some specialized cells which surround the stomata are called the guard cells. Chloroplasts are present only in the guard cells of the epidermis. Other epidermal cells usually do not have chloroplasts. The outer wall of epidermis is usually covered by cuticle. Stoma is a minute pore surrounded by two guard cells. The stomata occur mainly in the epidermis of leaves. In some plants such as sugarcane, the guard cells are bounded by some special cells. They are distinct from other epidermal cells. These cells are called subsidiary or accessory cells. Trichomes and root hairs are some epidermal outgrowths. The unicellular or multicellular appendages that originate from the epidermal cells are called trichomes. Trichomes may be branched or unbranched. Rhizodermis has two types of epidermal cells - long cells and short cells. The short cells are called trichoblasts. Root hairs are produced from these trichoblasts.
The macroscopic characteristics of wood give the clues to the conditions under which the wood is grown, provide an indication of physical properties and serve as an aid in wood identification. Growth rings do not always appear as distinct alternating bands of earlywood and latewood. Some hardwoods, for example, form large-diameter pores early in a growing season and much smaller pores later in the year; such woods are called ring-porous. Discontinuous rings are occasionally found in trees having one-sided crowns and in heavily defoliated, suppressed, and over mature trees. Examination of a stem cross-section often reveals a dark-colored center portion surrounded by a lighter-colored outer zone. The dark center area is known as heartwood and the lighter tissue as sapwood. Trees in which fibers are spirally arranged about the stem axis are said to have spiral grain. Wood can often be identified by macroscopic characteristics, particularly by colour, gloss, odour, weight and structure. Wood have two types of features: A) Macroscopic or Gross feature B) Minute structure feature
The microscopic features reveals that wood is composed of minute units called cells. The fundamental differences between different kinds of woods are founded on the types, sizes, proportions, pits and arrangements of different cells that compose the wood. Softwoods have a simpler basic structure than do hardwoods due to the presence of only two cell types, and relatively little variation in structure within these cell types. Hardwoods have greater structural complexity because they have both a greater number of basic cell types and a far greater degree of variability within the cell types. In each case, however, there are fine details of structure that could affect the use of a wood. The structure of a typical softwood is relatively simple. The axial or vertical system is composed mostly of axial tracheids, the radial or horizontal system and the rays, are composed mostly of ray parenchyma cells. According to estimates, 1 cubic metre (about 35 cubic feet) of Spruce wood contains 350 billion to 500 billion cells. The basic cell types are called tracheids, vessel members, fibres, and parenchyma The structure of a typical hardwood is much more complicated than that of a softwood. The axial or vertical system is composed of fibrous elements of various kinds of vessel elements in various sizes and arrangements, and axial parenchyma cells in various patterns and abundance. Like softwoods, the radial or horizontal system are the rays, which are composed of ray parenchyma cells, but unlike softwoods, hardwood rays are much more diverse in size and shape.
Physical properties of wood The versatility of wood is demonstrated by a wide variety of products. This variety is a result of a spectrum of desirable physical characteristics or proper ties among the many species of wood. In many cases, more than one property of wood is important to the end product. For example, to select a wood species for a product, the value of appearance-type properties, such as texture, grain pattern and color, may be evaluated against the influence of characteristics such as machinability, dimensional stability and decay resistance.
Cell walls mainly consist of cellulose, hemicellulose, and lignin in a 4:3:3 ratio. This ratio differs from sources such as hardwood, softwood, and herbs. Besides these three components, natural lignocellulosic materials contain a small amount of pectin, nitrogenous compounds, and the secret ash. 1. Cellulose 2. Hemicellulose 3. Lignin Cellulose Cellulose is a glucan polymer consisting of linear chains of 1,4-ß -bonded anhydroglucose units. Cellulose is the structural component of the primary cell wall of the green plants.About 33% of all plant matter is cellulose (the cellulose content of cotton fiber is 90%, that of wood is 40-50% and that of dried hemp is approximately 45%. Cellulose is a straight chain polymer.The number of sugar units in one molecular chain is referred to as the degree of polymerization (DP).Cellulose is insoluble in most solvents including strong alkali. It is difficult to isolate from wood in pure form because it is intimately associated with the lignin and hemicelluloses.
Ring Porous • having vessels more numerous and usually larger in cross section in the springwood with a resulting more or less distinct line between the springwood and the last season’s wood— compare diffuse-porous. • In some species (e.g. Oak (Red Oak, White Oak) Teak and ash elm, hickory), the largest pores are in the earlywood while those in the latewood are more evenly distributed and uniform in size. • These woods typically have distinct figures and patterns, and the uneven uptake of stain (the large pores soak up more colour) make the figure more pronounced. • These are also known as open-grain woods Semi-ring porous or Semi-diffuse porous • In some species (e.g. Butternut, Black Cherry, Black Walnut, Pecan, Tanoak), pores are large in the earlywood and smaller toward the latewood, but without the distinct zoning seen in ring-porous woods. • Also, some species that are usually ring-porous (e.g. Cottonwood) occasionally tend toward semi-ring porous.
