Difference between wood of Gymnosperms and Dicots (Soft Wood vs Hard Wood)

Being a biologist, we can never underestimate the powers of the vascular cambium. It is a type of lateral meristem. A large amount of secondary xylem is produced by the activity of vascular cambium since it is more active on the inner side. It also produces secondary phloem on the outside. We often mistake secondary xylem and wood to be separate but they are the same! You all must be familiar with the woody plants i.e. gymnosperms and dicots. While comparing the difference between the wood of gymnosperms and dicots, it is often concluded that the structure of the secondary xylem is simpler and more homogenous in gymnosperms as compared to the angiosperms. Let’s learn more!

Secondary xylem is also called wood!

Secondary xylem (Wood)

Secondary xylem is a type of secondary vascular tissue formed during secondary growth from the activity of lateral meristem/secondary meristem called vascular cambium. It is restricted to the stem and root of perennial dicots and gymnosperms. It is seen towards the outer side of the primary xylem.  Xylem fibres are usually very abundant in comparison to primary xylem. Tracheary elements are short and are comparatively thick-walled, unlike the primary xylem. Tyloses are present in secondary xylem.

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The secondary xylem has two systems of cells/elements. These differ majorly in the orientation of the plant. The vertical or longitudinal system also known as the axial system has a longitudinal arrangement of tracheary elements (tracheids and/or vessels), fibres and wood parenchyma. The horizontal or transverse also known as the radial system consists of xylem rays or wood rays. 

Secondary xylem in gymnosperms

To begin with, the secondary xylem of gymnosperms is simple and homogenous. Here, axially oriented derivatives of cambium consist of tracheids and parenchyma strands. Tracheids are the main water conducting as well supporting elements. Pits are of two types: simple pits and bordered pits. Ginkgo, Gnetales and Coniferales show a feature where pit membrane of bordered pit-pair gets thickened in centre and this thickened portion is known as torus. Window like pits called fenestriform pits are often seen (example- Pinus). Fibre-tracheids (a type of tracheids) with thin walls and bordered pits are present in gymnosperms. The occurrence of trabeculae and crassulae thickening is often seen. Trabeculae are rod shaped outgrowths of tangential cell walls while crassulae are bar like thickening.  There is absence of libriform fibres.

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Furthermore, axial parenchyma is scantily developed in most of the gymnosperms (conifers). Resin ducts are internal secretory structures often seen in gymnosperms wood. They are schizogenous ducts, which is a characteristic feature of gymnosperms (conifers). The radial system in gymnosperms consist of xylem rays which are uniseriate i.e. only one cell wide.

Secondary xylem in dicots

The secondary xylem of dicot (angiosperms) is seen to be more complex than gymnosperms. The axial system of secondary xylem here includes:

• tracheids
• vessels
• libriform fibres
• fibre-tracheids
• wood parenchyma

Tracheary elements include tracheids and vessels together. Vessels are considered an advanced feature which is absent in gymnosperms (exceptions can be there, for instance, Winteraceae lack vessels). Moreover, dicots have three types of tracheids are present:

Vesseltracheid, Vasicentric tracheids, and Fibre-tracheids.

Vesseltracheids are specialised fusiform tracheids which are arranged in axial strands. These axial strands are seen to be further connected by bordered pits. Vasicentric tracheids are a type of shorter tracheids since they divide through transverse or oblique division. Fibre-tracheids are considered to be the intermediate between tracheids and fibres. Tori are not seen. The occurrence of trabeculae and crassulae thickenings do not exist.

Nextly, fibres in the secondary xylem of dicots are of two types- living fibres and libriform
fibres.

Living fibres- These fibres are septate, have protoplast, contain starch grains and therefore they cannot transport water through apoplasm. Their major function is to provide mechanical support. In
Tamarix aphylla, these fibres live for about 20 years as long as xylem parenchyma.
Libriform fibres– These fibres are very long, characterized by the presence of thick walls and simple pits. They do not have living protoplast. Libriform fibres resemble phloem fibres. Libriform fibres
are present in the species having short cambial fusiform initial.

Wood parenchyma in dicots is differentiated into two types, axial parenchyma and ray parenchyma. Axial- It develops from the long fusiform initials of the vascular cambium and is arranged parallel
to the axis of the stem. Ray- It initiates from short ray initials and arranged radially.

Intercellular gum ducts are often seen in dicots (they contain substances other than gums
such as resins, oils and mucilage). Radially oriented elements in angiosperms are xylem rays (multiseriate– more than 2 cells wide).

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Difference between wood of Gymnosperms and Dicots

While jotting down the difference between wood of Gymnosperms and Dicots, it is essential to make a column of characteristics. It makes things easier to compare. Gymnospermous wood is often called soft wood whereas dicot wood is often called hard wood. Following points can be considered for soft wood vs hard wood:

Conclusion

The secondary xylem of gymnosperms is different from angiosperms because of lack of vessels, absence of libriform fibre, presence of less amount of axial parenchyma, presence of resin ducts and uniseriate rays etc. To conclude, nature can be beautiful in its own ways and it can be very well seen in the different types of secondary xylem in these two different types of plants.

Thanks for reading!

Team MBD

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