xercise 8. Secondary wood structure



 An overview of secondary wood anatomy     





      Core Objective:                                                 


To illustrate and examine examples of secondary wood structure.







It is generally accepted that secondary xylem  has undergone a long evolutionary history. The main trends can be seen because the various stages are often related to other 'marker' characters in flowers and fruits of the plants concerned. There are instances where habitat has seemingly reversed some of these trends in various species, but overall, their 'direction' can be fairly safely defined.

Taken in its simplest form, the evidence to hand indicates that the tracheid is a dual purpose cell combining properties of both mechanical support and water conduction in evolving groups of plants, which gave rise to fibres with simple mechanical function and to perforate cells, the vessel elements which are concerned with the conduction of water and dissolved salts. This division of labour is seen as a specialization, or advance.

The primitive vessel element shows much similarity to the tracheid as it is axially elongated, with oblique end walls in which are grouped perforations making up the scalariform, reticulate, or otherwise compound perforation plate. The lateral walls bear bordered pits , often in an opposite arrangement. The advanced vessel element is seen as a broad short cell with large, simple transversely arranged perforation plates at either end and alternating bordered pits on the lateral walls. Between these extremes is a variety of forms. In this exercise, we will focus on the structure of wood in stems that have undergone secondary growth. We illustrate the exercise with examples of tropical and subtropical origin, highlighting key features in these examples. 

The species illustrated here are few. However, they serve their purpose as they illustrate the structure of secondary xylem in transverse (TS) radial longitudinal (RLS) and tangential longitudinal (TLS) sections. All three planes are required when researching wood or for that matter, cambium or phloem structure, not just TS.


In general terms, wood is classified as being either 'hardwood' or 'softwood'.

The differences between these is outlined below.



Wood element Hardwoods




ray tracheids


ray parenchyma

axial parenchyma















Thus the major difference between hardwoods and softwoods id that the hardwoods (dicotyledons) contain vessels as the principal conducting conduit and lack ray tracheids, the softwoods (Gymnosperms) lack vessels, and contain ray tracheids.

The examples chosen to illustrate this exercise are deliberately chosen, as being part of important ecosystems, and are usually not illustrated structurally, or discussed. They all illustrate something of the complexity of secondary xylem structure.

Descriptions and sources. The descriptions are typically what would be used to describe the structure and appearance of the wood for reference purposes. We have included a brief description of the wood anatomy, as it may be presented in the literature.

We are particularly grateful to Dr Peter Gasson, from The Royal Botanic Gardens Kew, for kindly and so generously providing the base images that we have used in this exercise.


Useful glossary terms: As in the other exercises in The Virtual Plant, simply click on the underlined text, for a popup definition. Aliform paratracheal parenchyma; apotracheal parenchyma; Aggregate ray;  axial parenchymaDiffuse porous woodtylosis; tylosoid; vestured pit;





























Anthocephalus cadamba, Naucleaceae, (Kadam) Rubiaceae, Metcalfe & Chalk (1972)

Anthocephalus is a member of a large family, with a wide range of habit, which includes trees, shrubs as well as herbs.  The family is widely distributed but is mostly tropical. Anthocephalus grows in dense forests and is deciduous.

Descriptive anatomy of the wood: Wood: diffuse-porous, with the vessels arranged in an indeterminate pattern. Vessels: 90 - 220 m in diameter. Perforation plates are simple, Intervessel pits are alternate, about 5-7 m  in diameter, vestured. Vessel-ray pits distinctly bordered. Fibres: Long (>1500 m). Pits common in radial and tangential walls. Axial parenchyma: Present and is described as banded reticulate apotracheal.

Images above show transverse, radial longitudinal and tangential views of the wood of Anthocephalus cadamba. Note the large-diameter vessels, interspersed with narrow tracheids and parenchymatic elements. Rays are short and 2-4 cells wide.


































Dyera costulata Apocynaceae (Jelutong)

A large family, comprising  tropical and subtropical members. Dyera is a tree, which grows in Thailand, Laos, Vietnam Cambodia, through to Indonesia. Used extensively in furniture making.


Descriptive anatomy of the wood: Wood, diffuse porous, vessels in no specific patterns, Radial rows of  a few (up to four or more) per row. Average vessel diameter is 90-160 m. Perforation plates simple. Intervessel pits are alternate, average diameter is 4-7 m. Vestured pits may be present. Tracheids and fibres present. Fibres non-septate thin-walled, length from 900-1500 m, Fibre pits restricted mainly to the radial walls, simple, to bordered. Parenchyma is predominantly apotracheal


Note the large-diameter vessels, interspersed with narrow tracheids and parenchymatic elements. Rays are
narrow, uniseriate as well as multiseriate rays occur.















Alnus nepalensis  Betulaceae

Alnus nepalensis D. Don. This family is composed of trees and shrubs, which occur mostly in the northern temperate regions, but is also found in South America. Called utis in Nepal, maibau in Burma, and Indian or Nepalese alder in English, is one of 35 species of alder worldwide. Alnus is one of 15 genera of trees that fix nitrogen but are not in the legume family. Utis is a deciduous or semi deciduous tree with a straight trunk that reaches up to 30 m in height and 60 cm (rarely to 2 m) in diameter.

Descriptive anatomy of the wood: Vessels are small, loose oblique pattern. Perforation plates are scalariform (see the radial longitudinal view below). Intervascular pitting moderately large. Parenchyma is diffuse. Rays either exclusively uniseriate to 3-4 cells in width. Fibres with small, distinctly bordered pits.



Images above show transverse, radial longitudinal and tangential views of the wood of Alnus nepalensis. Note the narrow-diameter vessels. These have inclined compound scalariform-reticulate perforation plates. The vessels are interspersed with narrow tracheids and parenchymatic elements. Rays are of variable length and uniseriate. Click on the compound perforation plate for a more detailed view

References: http://www.winrock.org/forestry/factpub/FACTSH/A_nepalensis.html




































Bertholletia excelsa (Lecythidaceae)

The Brazil nut tree is large, frequently attaining 40 to 50 m or more in height, and may live for 500-800.
The tree is called castanheiro do para in Brazil and occurs throughout the Amazon rainforests in Brazil, Peru, Colombia, Venezuela, and Ecuador. The fruit is a large, round woody capsule or pod, approximately the size of a large grapefruit and weighing up to 2.2 kg. Brazil nut oil contains mainly palmitic, oleic, and linoleic and alpha linolenic acids and small amounts of myristic and stearic acids and phytosterols. In addition to protein and fat, Brazil nuts provide the highest natural source of selenium. One single Brazil nut exceeds the U.S. Recommended Daily Allowance of selenium. The tree is conserved in several areas within South America. Descriptive anatomy of the wood: Wood is diffuse porous. Vessels with simple perforation plates. Tyloses common. Intervessel pits are alternate and polygonal about 10 m across. Vessel ray pits are distinctly bordered of similar size to intervessel pits. Fibres  non-septate, long (up to 1500+ m) have simple pits, sometimes narrow borders. Axial parenchyma: in narrow bands, procumbent.



Images above show transverse, radial longitudinal and tangential views of the wood of Bertholletia excelsa.
Note the large-diameter vessels, which show evidence of tyloses formation. Rays are narrow.












































The wood of Gymnosperms differs markedly from that of the Angiosperms. Principally, gymnosperms lack vessels in either primary or secondary wood. They are also of ancient origins and have been gradually replaced in ecosystems. They dominated during the Cretaceous, existing naturally in vast regions of temperate climate, specifically in the Northern hemisphere, where they still dominate.








Fitzroya cupressoides Conifers such as Fitzroya are capable of growing on poor soils. It is a valuable softwood, and is protected in many areas such as in Chile, where logging is strictly controlled. Descriptive anatomy of the wood: Tracheids only, rays narrow, usually a single cell in width. Pits distinctly bordered.











Images above show transverse (TS), radial longitudinal (RLS) and tangential (TLS) views of the wood of Fitzroya cupressoides. The xylem consist of tracheids and parenchymatic elements only. This can be inferred from the uniform-diameter of cells as seen in transverse section.  Note that several growth rings are visible in the transverse section. Rays are narrow, usually one cell in width (uniseriate).  Click here for a detail of the wood in cross section.





















Swietenia mahogoni Meliaceae (mahogany) Mahogany is native to the West Indies and may grow up to 20 m or more (75ft) in height. This is a semi-deciduous tree which looses old leaves at the end of the winter season, at about the time that new leaf growth is being formed. The wood is most valuable for cabinet
making. The wood is hard and reddish in colour. Due to serious overexploitation, this species is listed in the CITES Appendices. (Convention on International Trade in Endangered Species). Descriptive anatomy of the wood: Wood diffuse-porous in other words, wood in which the vessels have more
or less the same diameter throughout the growth ring. Growth rings vary, and may contain distinct growth rings, which have an abrupt structural change to indistinct growth rings. Fibres libriform; with simple to minutely bordered pits or with simple pits or bordered pits with the chambers less than 3 m in diameter. Both septate and non-septate fibres occur in the wood.


Images above show transverse, radial longitudinal and tangential views of the wood of Swietenia mahogoni. Note the large-diameter vessels (XV), interspersed with narrow tracheids and parenchymatic elements. Note also the narrow rays (R).











Pseudotsuga douglasii  Pinaceae -- Pine family Within the Douglas pine, the early to latewood transition is rather sharp, as seen in the transverse section to the left. Descriptive anatomy of the wood: Contains numerous resin canals. Pits: Distinctly bordered.   Rays: Usually uniseriate but multiseriate rays occur.

Images above show transverse (TS), radial longitudinal (RLS) and tangential longitudinal views (TLS) of the wood of Pseudotsuga douglasii. Note the xylem consists of tracheids and parenchymatic elements only. Note sharp demarcation between growth rings, as well as the numerous resin canals visible in transverse section.  Click on the circular structures in the TS, to see the resin canals and junctions between growth rings. Click on the circular structures in the RLS view to see a detail of the bordered pits. Rays are narrow and usually one cell in width (uniseriate). Click on the centre of this image to see a detail of the ray structure.

























Taxus baccata  Yew trees (the genus Taxus) originated during the age of dinosaurs. The English Yew Yew trees are native to Britain and they are common in England and Wales.  In all, there are at least seven species of yews now grow in the temperate forests of Asia, Asia Minor, India, Europe, North Africa, and North America, but yew species around the world can cross-pollinate and interbreed, making possible the large number of ornamental yew cultivars propagated today. These trees are evergreen and grow very slowly and are reported to live for several hundreds of years and some are reported to be over 2000 years old.

Longbows were made from yew. The wood is durable and as a result has been made into wooden bowls. It is also used as a veneer by furniture makers.

Descriptive anatomy of the wood: Tracheid pits in the radial walls generally  uniseriate. Rays homocellular, without ray tracheids. Ray pits cupressoid. Transverse walls of rays thickened, tangential walls of ray cells are thin, indentures present at junction of transverse and tangential walls. Longitudinal tracheids contain conspicuous spiral thickenings.

Medicinally, and unfortunately, yew bark was identified as the most concentrated source of taxol, a novel anti-cancer compound. Many mature yews were destroyed for these trials by stripping their bark to extract taxol. Synthetic derivatives are now used in its place.


Click here to see rays in more detail growth ring transition


Images above show transverse (TS), radial longitudinal (RLS) and tangential longitudinal views (TLS) of the wood of Taxus baccata. Note that the end of season's growth is marked by narrow-diameter tracheids, and spring growth, by wider-diameter thinner-walled tracheids. Click on the transverse section to see the transition between growth rings more clearly. Note that the rays are long and thin, and are only one cell wide (unicellular). Click on a ray in the RLS section, to see this structure in more detail. Follow the link from the ray detail image, to see the septate fibers.































Pinus sylvestris  This Gymnosperm is native to the Europe, from Norway to Spain, and to parts of Asia and is capable of withstanding cold winters.



Images above show transverse (TS), radial longitudinal (RLS) and tangential longitudinal views (TLS) of the wood of Pinus sylvestrus. Rays are variable, from 2 one to many cells high and of variable width. Wider rays may be associated with resin canals.   Like Taxus above, the end of season's growth is marked by narrow-diameter tracheids, and spring growth, by wider-diameter, thinner-walled tracheids.









Juniperus communis is a prickly shrub in the Cypress family with short spiky leaves and one of three conifers which are native to England. In its natural environment, the plant grows in pine woods, or on moors and on the exposed, often wind-swept coastal sand dunes in Scotland. In England, it grows on heath land, or on limestone grassland or cliffs in the south of England.


The images above show  transverse (TS), radial longitudinal (RLS) and tangential longitudinal views (TLS) of the wood of Juniperus communis. Again, end of season growth is marked by very narrow diameter tracheid. The TLS section illustrates that the rays are of variable height, but only one cell wide (unicellular).














EXERCISE 1 Draw up a simple key which describes the anatomy of the wood of the species that have been illustrated here. Try to make use of other reference material to supplement the information that you can summarize from this exercise. For example, use Yahoo or Google to search for additional information. The Table below serves as a guide to the type of information that you need to look for and include information on.










cell type & species


Pinus patula

Evergreen up to 50 m in height

xylem vessels

Vessels do not occur

axial system

Tracheids, fibre tracheids and parenchyma. Tracheids are short and narrow, distinct growth rings.

radial system

Ray parenchyma only rays uniseriate

resin ducts

contain terpenes, oleo-resins, commercial product is turpentine.


Economic use

valuable building timber (if treated)


classified as a softwood as it has no vessels.

add any other features you think are relevant here Major building wood in many countries - grows under variable environmental conditions, can destroy habitat for indigenous plants. Fire prone.



























































EXERCISE 2 This exercise will test your drawing skills! With reference to the transverse, radial longitudinal and radial tangential sections illustrated in this exercise, try to draw a 3-D reconstruction of a block of one or two of the specimens which are illustrated above. Look at the example below. This is a 3-D perspective image.












Note: This 3-D reconstruction is not accurate, as it was not made from registered (matching) sections. It was constructed from the sections of Swietenia mahogani. Drawing a 3-D does not mean you have to try to represent all the cells that you can see, but rather to produce an image that shows the relationship between the three planes of section which are used to understand and explore the structure of wood.