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Wood anatomy

The document provides an overview of wood anatomy, detailing various components such as secondary vascular tissues, growth rings, heartwood, and sapwood. It explains the structural differences between heartwood and sapwood, types of heartwood, and the role of parenchyma in wood. Additionally, dendrochronology is introduced as a method for determining the age of trees through the counting of annual growth rings.

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Wood Anatomy Presented by; Sumra kazbanu A. Msc sem 2 Roll no. - 215 CBO 405 Department of life sciences, H.N.G.U. Patan
CONTENTS • Introduction • Ontogeny of secondary vascular tissues, • Growth Rings, • Heart wood, • Types of Heart wood, • Sap wood, • Wood parenchyma
Introduction In most dicotyledons and gymnosperms, the stems increase in girth by the activity of lateral meristems. A tree stem consists of three areas; pith, xylem and bark. The central pith is usually bare visible and does not increase in size through the life of the tree. A cylinder of wood, known scientifically as xylem ries in diameter with age and rate of growth. And finally, the bark sheath can be subdivided into inner bark and outer bark.
• New wood and inner bark are added each year by the activity of a layer of dividing cells sandwiched between the inner bark and sapwood. New bark production is relatively small compared with new wood production, and bark is continually being shed to the outside of the stem, thus in older trees the greatest volume of the stem is wood. Since New wood is added to the outside of existing wood the oldest wood is close to the pith, and the most recent is close to the bark.
Ontogeny of secondary vascular tissues • The initiaation of secondary vascular tissue occurs after the maturation of primary xylem and phloem. At this stage the cambium is represented by a single layer of cells. • The primary tissues lying outside the secondary vascular tissues are subjected to pressure from the inside by the enlarging secondary vascular cylinder. • The primary phloem is crushed and becomes non- conducting.
• The cortex persists for a longer time as it increases in circumference by cell division and cell expansion. • The epidermis may also persist through cell division and cell enlargement to accmmodate the diameter increase as the result of secondary growth. • However, in most plants with continued secondary growth these outer tissues are completely crushed by the formation of perdierm within them during secondary growth. These dead peripheral layers are eventually eliminated by decay or by abscission.
Growth Rings • The secondary xylem in the stems of parennial plants commonly consists of concentric layers, each one of which represents a seasonal increment. • In transverse section of the axis,these layers appear as rings ,and are called annual rings or growth rings. • Each layer represents the growth of one year. The width of growth rings varies greatly and depends upon the rate of growth of tree.
• In regions with a pronounced cold season, the activity of the cambium takes place only during the spring and summer seasons thus giving rise to the growth in diameter of woody plants. • In spring or sunmer the cambium is more active and forms a greater number of vessels with wider cavities. In winter ot autumn season,however there is less need of vessels for sap transport,the cambium is less active. • EARLY WOOD:- The wood developed in the summer or spring season is called spring wood or early wood. • Early wood is inner layer of growth rings. The early wood zones of a growth rings typically consists of thin walled, larger diameter cells and also appear lighter in colour.
• Late wood:- The wood formed in winter or autumn season is known as autumn wood or late wood.Late wood is a outer layer of growth rings. • Late wood zones thick walled , smaller diameter cells and also appear darker or brownshade in colour.
• Dendrochronology:- Each annual ring corresponds to one year's growth, and on the basis of these rings the age of a particular plant can easily be calculated. The determination of age of a tree by counting the annual rings, is known as dendrochronology.sometimes two annual rings are formed in a single year, and in such cases the counting of the annnual rings does not show the correct age of the tree.this happens perhaps because of the drought condition prevailed in the middle of a growing.
• Tyloses:- In many plants, the walls of the xylem vessels produce ballon like outgrowth into the lumen of the vessels, are called Tyloses. • Usually these structure are formed in secondary xylem. Tyloses are formed by the enlargement of the pit membranes of the half bordered pits present in between a paranchyma cell and a vessek or a tracheids.
HEART WOOD & SAP WOOD Sap wood Heart wood The outer region of the old trees is sapwood or alburnum. The central region of the old tress is called heart wood or duramen. The sapwood consists of recently formed xylem elements. It is filled up with tannins,resins, gums and other substances. It is not hard and durable. It is hard and durable. This is of light colour and contains some living cells also in the association of vssles & fibres. It looks black or dark brown due to the presence of various substances in it. Generally the vessels are not plugged with tyloses. Usually the vessels remain plugged with tyloses.
This part of the stem performs the physiological activities ,such as conduction of water and nutrients, storage of food etc.. The function of heart wood is not of conduction, it gives only mechanicak support to the stem.
Types of heart wood 1) Ring-porous hardwoods: - for some groups of species (oaks and elms) the earlywood/ latewood transition occurs abruptly and is very distinct. Within each growth ring, a band of large earlywood vessels (pores) is clearly visible to the naked eye, after which a band of latewood vessels appears much smaller and requires the use of a hand lens to see.
2) Semi-ring-porous hardwoods :- For another group of species (black walnut, butternut and hickory) the pore transition from large to small diameter within a growth ring is gradual. The pores in the earlywood zone have a largediameter that gradually decreases in size as pores enter the latewood zone.
• (3) Diffuse-porous hard woods :- the last group of species has vessels (pores) that are uniform in size across the entire growth ring (yellow poplar, gum and maple). These vessels are usually small, uniform in size and are very difficult to see with the naked eye (a hand lens is needed).
• Ring porus heart wood also divided follow types:- • Solitary pores : single pores that do not touch any other pores evenly spaced across cross section (maples). • Pore multiples : arrangement where two to five pores appear grouped together.Pore multiples usually occur in radial rows (cottonwood), but can occur in both radial and tangential directions (Kentucky coffeetree). • Pore chains : arrangement where pore multiples appear in radial direction only. • Nested pores : when larger numbers of pores contact each other both radially and tangentially (cluster). • Wavy bands : Pores are arranged in irregular concentric bands. Also called ulmiform because this characteristic is distinctive of all elms(also seen in strawberry)
Sap wood • Resin Canals :- The first step after making the determination that a wood specimen is softwood due to the absence of pores is to inspect the cross-section surface for the presence of resin canals. Resin canals are tubular passages in wood that exude pitch, or resin, to seal off wounds that occur due to insect or mechanical damage. Resin canals most often occur in or near the latewood zone of the growth rings. Softwoods can be separated into two classifications based on the presence or absence of resin canals.
• Examples:- Species that have resin canals include pines, spruces, larches and Douglas-fir. The species that do not have resin canals include firs, hemlocks, cedars, redwood, baldcypress and yew.
• Woods with resin canals are further separated into two groups: (1) those with large resin canals pines, and (2) those with small resin canals Douglas fir, spruce and larch. Using a sample wood identification set to compare the size and number of resin canals of different species is useful in determining how much they can differ between species. • For example, most pines have quite large and numerous resin canals that can be seen without the aid of a hand lens.
Wood parenchyma • Parenchyma are small, thin-walled, longitudinal cells that provide food storage. These cells are sparse in softwoods but are often quite significant in hardwoods. Parenchyma are often very small and difficult to see. • However, there are many species with visible and unique arrangements of parenchyma cells that offer a clear structural feature for decisive identification.
• There are two basic types of parenchyma: paratracheal and apotracheal. • The major difference between them is that paratracheal parenchyma make contact with the pores or vessel elements, while apotracheal parenchyma are separated from pores by fibers or rays. In most species, apotracheal parenchyma are not be useful in identifying wood with just a hand lens. Paratrachal parenchyma appears in many forms, and is often more useful for identification. For example, in hickory the banded parenchyma looks like a reticulate, Òweb-likeÓ formation as it connects between the rays and pores. This Òweb- likeÓ appearance is unique to hickory.
References • A Textbook of botany - Angiosperms By :- B.P. Pandey • http://wood 280.forestry.ubc.ca/files/2012/08/wood-anatomy. • http://les.bf.uni-lj.si/fileadmin/datoteke- asistentov/kcufar/english.
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