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Fossil gymnosperm wood

Most of the finds of petrified wood belong to this group. Examples are the wood of the Devonian tree Archaeopteris,  the Palaeozoic wood Dadoxylon, het Mesozoic wood Araucarioxylon and the wood of the more 'modern' conifers.
The wood of gymnosperms is of a relatively simple structure and it is recognizable with a loupe when it is sawn and polished. On the other hand the differences between the woods of the different groups can only be seen with the help of thin sections under the microscope. Identifying the species or the genus must be done by specialists.

The differences between Dadoxylon and Araucarioxylon remain unclear even using a microscope. This kind of wood can originate from a range of trees like Cordaites, seed fern trees, Araucaria-like trees and even from Glossopteris, the seed ferns of the huge southern continent Gondwanaland. In most cases the name Dadoxylon is used for Paleozoic wood of gymnosperms and Araucarioxylon for Mesozoic wood with the same characteristics.

Piece of conifer wood Piece of conifer wood with four annual rings.

Meaning of the letters:
TR = transverse section
R = radial section
TA = tangential section

B = bark
AR = annual ring
E = early wood
L = late wood

c = cambium
r = ray
rd = resin duct

After Strasburger (1967): Lehrbuch der Botanik

See the drawing above. Wood is formed by the cambium (c). This is a layer of dividing cells, giving rise to the sieve tubes or phloem outwards and to the (secondary) wood inwards. The cambium is only one cell thick. In the drawing above a sector of a trunk with four annual rings has been depicted. An annual ring (A) is the wood that is formed during one year in an environment with seasons. In tropical conditions and in periods without seasons (like the Jurassic and the Cretacious) the wood shows no annual rings.  In an annual ring one can distinguish the early wood (E) with large, thinwalled cells and the late wood (L) with small, thickwalled cells. The early wood, formed in the spring, has an important role in the transportation of liquids in upward direction. The function of the late wood, on the other hand, is providing stiffness to the trunk. Click here to see an annual ring in fossil wood from Iceland.

Block of Sequioa wood Parenchym Early wood Late wood Bordered pits Ray Ray Ray Ray

Click the words in the drawing
After T.L. Rost et al (1979): Botany, a brief introduction to plant biology.

Schematic drawing of a block of Sequoia wood
(Sequoia sempervivens, Redwood)
as an example of the structure of conifer wood.

Top = transverse section
Side = radial section
Front = tangential section

The tracheids in the late wood have thicker walls than those in the early wood.

Rays run horizontally from the cambium towards the center of the trunk. They are 1 cell thick and 1 to 30 cells high.

The bordered pits are arranged in vertical rows in the radial walls of the tracheids. They are the connection between neighbouring tracheids.

Conifer wood consists for more than 90% of tracheids (wood cells). These are elongated cells that manage the transport of water with dissolved minerals in upward direction.  The tracheids have closed (oblique) endings, and they are connected with the adjacent tracheids through so-called bordered pits. Such a bordered pit is a small circular area with a hole in the middle. Through this the water can move from one cell to another.
From the cambium rays go horizontally towards the centre of the trunk. They consist mainly of parenchym and they are 1 cell thick and 1 to 30 cells high (depending on the species). There are also some ray-tracheids. The task of the rays is the transportation of nutrients which have been produced by the tree, to the central part of the trunk, where these nutrients are stored in empty cells.
The area in which the vertical tracheids and a ray intersect, is called cross-field. The tracheids and the ray-tracheids are in the cross-field in communication by means of very small bordered pits. The form and the way of grouping of these pits is used by specialists to identify the wood.
Some kinds of conifers show long series of parenchym cells in longitudinal direction (see drawing). Parenchym cells can easily be seen because they have a dark content.
Starting in the Jurassic, resin ducts (resin canals) occur in some groups of conifers. There are vertical as well as horizontal resin ducts. Resin ducts can be recognized as relatively large holes surrounded bij parenchym cells. They occur in the genera Pinus (pine), Picea (spruce), Larix (larch) en Pseudotsuga (Douglas fir).

Bordered pits
Nearly all bordered pits are situated on the radial sides of the tracheids, that means: looking from the center of the trunk at the sides of the cells. They lay in vertical rows. In the modern groups of conifers (from the Jurassic on) they are generally arranged in single rows. The bordered pits are circular in most cases but if they crowd, the separation plane can be flat. Occasionally dubble rows occur, in which case the pairs of pits are situated side by side (at the same height).

Bordered pits in conifer wood
'Modern' conifer wood

Araucaroid pitting
Araucaroid pitting

In the wood of Araucaria and in other older kinds of gymnosperm wood like Dadoxylon and cycas wood the bordered pits are arranged in two or more rows. The pits of adjoining rows are now lying at different levels: they alternate (see right picture above). Where the pits crowd together they have a hexagonal form. This is called araucaroid pitting.
As a matter of fact some modern conifers, like Cedrus (Cedar), have also two rows of bordered pits which even sometimes alternate. These pits, however, are always circular.

Microscope Conifer wood from the Jurassic of Portugal

Transverse section
Radial section
Tangential section