Secondary Wood or Secondary Xylem

 

The wood is a product of secondary growth. It is essentially composed of secondary xylem elements namely Trachery elements (Vessel and tracheids), Wood fibres (Fibre tracheids, libriform fibres and gelatinous fibres) and wood parenchyma (axial parenchyma).

Study of wood comes under separate discipline Xylatomy. The wood of Gymnosperms is generally designated as softwood, while that of Angiosperms as hardwood. This distinction is based upon the presence or absence of one important component in the wood i.e., fibres. In gymnosperms, the fibres are absent and have only tracheids, hence the wood of gymnosperm is called as softwood. In angiosperms, the ground tissue is made up of fibres between tracheids and vessels. Fibres are more stronger than tracheids and impart a great mechanical strength. Hence, the wood of angiosperms is called as hardwood.

Further, the wood of gymnosperms is designated as non-porous wood because it lacks vessels, while that of angiosperms is porous wood because of presence of vessels. The gymnosperm wood is more homogenous when compared to angiosperms wood because there are less number of kinds of wood elements in the former than later.

Typically gymnosperm wood consists of following elements- a) Tracheids, b) Xylem rays, c) xylem parenchyma and d) Resin ducts

Elements of angiosperm wood : a) Vessel, b) tracheids, c) fibres, d) xylem rays, e) parenchyma, d) gum ducts, e) Resin ducts.

The wood elements are disposed in two ways 1. Horizontal system and 2. Vertical system .

The elements of vertical system are aligned to the vertical axis of trunk consisting of vessels and tracheids, wood fibres, axial parenchyma, vertical resin canal, etc. 

The horizontal system consists of xylem rays, ray parenchyma and horizontal gum ducts.

The secondary xylem of dicots is more complex than that of the gymnosperms. The arrangement of the vessels in the secondary xylem of dicots is a characteristic feature and is used in the identification of species.

Wood anatomist refers to a vessel in cross section as a pore. Two principal types of woods are recognized on the basis of distribution of pores in a growth layer – diffused porous wood and ring porous wood.

Arrangement of the vessels or pores in the secondary xylem of dicots is a characteristic feature and is used in the identification of species.

When the vessels are more or less equal in diameter and uniformly distributed throughout the growth ring, the wood is termed as diffuse porous wood. Example – Acer, Populus, Betula, Acacia, Olea, Eucalyptus.

When the wood contains vessels or pores of different diameters and in which those produced at the beginning of the season are distinctly larger than those of the late wood, and they are arranged in the form of conspicuous ring at the beginning of the growth ring, the wood is said to be ring porous wood. Example – Fraxinus, Quercus, Pisticia, Morus.

Ring porous wood is thought to be more advanced than diffuse porous wood from the phylogenetic point of view. The former is found only in relatively few species and mainly in plants from Northern Hemisphere.

Parenchyma cells are frequently met within the xylem tissue of most of the plants and are referred as xylem or wood parenchyma. These cells are more or less elongated, placed end to end and may be thick or thin walled.

Ontogenetically, development of xylem parenchyma cells is supposed to be from fusiform initials. In secondary xylem, the xylem parenchyma is of two types: i. Axial parenchyma: parenchyma cells are arranged end to end in vertical rows among the trachery elements. This cells are rectangular to elongated with horizontal end walls. Ii. Radial parenchyma: aligned horizontally or radially.

Parenchyma associated with the vessel is called as paratracheal parenchyma. Parenchyma not associated with the vessel is called as apotracheal parenchyma.

Tectona Grandis

Family: Verbenaceae

Vernacular Name: Teku, Sagwan, Teak

General Characters:

Sap wood is distinct from the heart wood. Sap wood white to pale yellowish-brown. Heart wood golden-brown with darker streaks, turning deep-brown on exposure to air.

Wood with oil feel, strongly and characteristically scented.

Wood moderately hard, moderately heavy. Average weight 650 kg/m³. Medium to coarse texture with straight grain.

Growth rings distinct. Ring porous wood (possess pores or vessles of different diameter with distinctly large pores in the early wood than in the late wood).            

Soft tissue or parenchyma predominantly vasicentric i.e., forming thin sheath around the pores and also delimiting growth rings.

Fibres non libriform, gelatinous, coarse, septate.

Rays distinct, visible to naked eye.

 

Dalbergia latifolia

Family: Fabaceae

Vernacular Names: Shisham, Jitregi, Rosewood

General Characters of the wood:

Sapwood is distinct from heart wood. Wood with pale-yellowish-white with pinkish tinge.

Heartwood purplish-brown to purple with darker streaks.

Wood with faint pleasant odour.

Wood hard, heavy, average weight – 815kg/m³ at 12% moisture content.

Texture coarse, grains straight to shallow inter-locked grains.

Diffuse porous wood (pores that are more or less equal in diameter and uniformly distributed throughout the growth ring).

Growth ring distinct to fairly distinct, pores large to small visible to naked eye..

Growth ring distinct to fairly distinct, pores large to small visible to naked eye.

Soft tissue (parenchyma) mostly around the pores, forming eye-lets, with lateral extensions often connecting the adjacent pores by narrow, wavy, tangential bands.

Rays fine to very fine, distinct only under the lens.

Ripple marks distinct, seen only under the lens.

Wood is categorized as first-class wood and yields the ‘most handsome Indian black-wood or Rose wood’.

Pterocarpus santalinus

Family: Fabaceae

Vernacular names : Rakta-chandanam, Red-sanders, Lal Chandan.

General Characters of the wood:

Sap wood distinct from heart wood.

Sap wood orange-red to claret purple.

Heart wood purplish-black and yields red dye (santalin)

Wood very hard to heavy with inter locked grains with lot of fibres, coarse textured.

Diffuse porous wood.

Pores occluded with reddish brown gum deposits.

Soft tissue (parenchyma) paratracheal.

Rays not visible to naked eye, very fine, closely placed separated by rows of parenchyma.

Fibres present, abundant, semi-libriform to libriform.

Ripple marks present, not visible to naked eyes.

A valuable timber of class with heavy demand.

Termenalia tomentosa

Family: Combretaceae

Vernaular names: Nalla maddi

General Characters of wood:

Soft wood distinct from heart wood.

Soft wood pinkish-white to pinkish grey.

Heart wood walnut-brown to deep brown with darker streaks.

Wood heavy to very heavy with coarse texture with interlocked grains. Average weight – 880 kg/m³ at 12% moisture content.

Growth rings present but visible under the lens, delimited by a fine-line of parenchyma.

Soft tissue (parenchyma) predominantly aliform, forming light-coloured eye-lets around the pores.

Rays fine to very fine, seen only under the lens as numerous closely spaced lines.

 

Pterocarpus marsupium

Family: Fabaceae

Vernacular names: Bijasal, Pedda-egisa.

General Characters of wood:

Sap wood distinct from heart wood.

Sap wood pale-yellowish or nearly white.

Heartwood golden-brown with darker streaks, turning brown with age.

Wood moderately hard to very hard, moderately heavy to heavy, average weight – 800 kg/m³ at 12% moisture content.

Medium to coarse-textured.

A diffuse porous wood contains a yellow dye.

Growth rings distinct or indistinct.

Soft wood (parenchyma) predominantly narrow, wavy, partially enclosing the pores.

Ray very fine, not visible to the eye.

Ripple marks present and visible to eye.

 

Azardirachta indica

Family: Meliaceae

Vernacular names: Vepa, Neem.

Sap wood distinct from heartwood.

Sap wood greyish-white.

Heartwood red first, when exposed turn to reddish brown..

Wood with characteristic taste and aroma, moderately heavy, with narrowly inter-locked grains, medium  to coarse textured.

Growth rings distinct, sharply delimited by narrow brown, concentric lines of terminal parenchyma.

Wood parenchyma paratracheal.

Fibres non-libriform to semi-libriform in radial rows forming extensive tracts between vessels to rays.

Rays visible to naked eye, medium to fine, heterogenous