Question

Which one of the following combinations would you consider suitable for a good commercial wood?
A) Tracheid, Vessels, collenchyma, fibers, medullary rays
B) Tracheid, Vessels, fibers, xylem parenchyma, medullary rays
C) Xylem parenchyma, vessels, middle lamella, fibers, collenchyma
D) Tracheid, vessels, sclerenchyma fibers, medullary rays

Answer 1

Wood is the secondary xylem in the stem of the trees. It is made up of organic matter that is of cellulose which provides tensile strength and the matrix of lignin does not allow compression. Its major role is to provide structural support and also transport water and nutrients to the leaves.

Complete answer:
Wood is one of the renewable sources of energy. It is used as fuel and also used for making weapons, tools, furniture, paper. The wood is yielded by trees which is the result of the secondary growth which increases in the diameter of the tree between the wood and the inner bark. The secondary growth takes place by the division of cells in the vascular cambium, a lateral meristem. These cells undergo thickening of the secondary cell walls which comprises lignin, cellulose and hemicellulose.

The commercially important wood should be durable and have high strength. For this, it should be made up of dead cells whose walls are thick. Tracheids, vessels have lignified walls and the cells die at maturity. They are the structures present in secondary growth. Sclerenchyma fibres is made up of dead cells whose cell walls are thickened. They have economic importance and yields fibres from the fabric.
Medullary rays are dead cells and are important cellular structure present in the wood. They are visible as growth rings.

Let us discuss the other options-
Collenchyma- It is composed of living cells that comprise the non-lignified cell wall. The walls are unevenly thickened. It provides structural support.
Xylem parenchyma- It is made up of living cells of the xylem. Its function is the storage of starch, oil.
Middle lamella- It is the outermost layer of the cell wall. It is composed of pectin.

Thus the correct answer is option ‘D’.

Note: The age of the plant can be determined by the annual growth rings which appear as concentric circles in the woody stem and trunk. This happens because every year the cambium layer produces a xylem layer which is large and thin in the spring and smaller and thick in summer. This counting of the rings helps in determining the age of the tree.