Wood: Softwood, Hardwood, Springwood, Autumn wood, Heartwood and Sapwood, Practice problems and FAQs

Everyone prefers wooden furniture in their home. It is because of the strength and durability of the wood. Wooden furniture is incredibly durable and requires little maintenance. Also, it gives a royal look to the room! But while selecting wooden furniture, we look for wood from specific types of trees like teak, maple, mahogany etc. Do you have any idea why we prefer the wood from specific types of trees?

There are many factors like the climate in which the tree grows, the age of the wood, the strength etc, which makes the wood suitable for making furniture. A certain sort of wood is utilised to make a durable substance, while another type of wood is used to give the product lustre and sheen.

To know more about these we need to know more about the wood. So here we are going to discuss different types of wood and their internal structure.

 Table of contents:

• Wood 
• Classification of wood based on origin
• Classification of wood based on the activity of cambium
• Classification of wood based on the deposition of organic compounds
• Practice Problems
• FAQs

Wood

Wood is the secondary xylem formed by vascular cambium during secondary growth. It is a complex secondary tissue produced mainly in gymnosperms and angiosperms. It is the fibrous material seen in most parts of a tree or shrub. Wood is widely used as a fuel, timber, building material, for making tools, furniture, etc. It is cheap, light and can be easily manipulated with tools.

Being composed of secondary xylem, wood comprises tracheids, vessels, xylem fibres and xylem parenchyma. The tracheids and vessels help in water conduction. Additionally, tracheids also help in providing mechanical support. The wood fibres also help in providing mechanical support. The only living component of wood is the wood parenchyma and it helps in the storage of food.

Wood can be classified on the basis of factors such as origin, activity of cambium and deposition of organic compounds. Now let’s discuss how they are classified. 

Classification of wood based on origin

Based on origin, wood can be of two types - softwood and hardwood. 

Softwood

The wood produced by gymnosperms is known as soft wood or non-porous wood as it lacks vessels. In fact, wood produced by conifers is quite hard but it is still said to be softwood as it lacks wood fibres. Softwood contains 90-95% tracheids and around 5-10% ray cells. Common examples of softwood are - pines, spruces, etc.

Hardwood

The wood produced by dicot plants is known as hardwood or porous wood as it contains large quantities of xylem fibres and vessels. Hardwood contains xylem vessels, tracheids (occasionally), xylem fibres and xylem parenchyma. Examples of such woods are shisham, mulberry, etc.

Classification of wood based on the activity of cambium

The physiological and environmental factors can control the activity of cambium. Did you know that in temperate regions, climatic conditions are not uniform? Hence formation of cambial rings varies in spring season and winter season. As a result two types of wood are formed - spring wood or early wood and autumn wood or late wood. 

Spring wood or early wood

Spring woods are formed in the spring season. It is formed when nutrients are easy to access and when more cambial activity is present. It possesses more xylary elements. Xylem elements are larger and wider in spring wood. It is lighter in colour and is of low density. 

                                                 Fig: Early wood

Autumn wood or late wood

This wood is formed during the winter season. It is formed when the nutrients and water become scarce in the cold and the plant shows less cambial activity. The number of xylary elements in autumn wood is fewer. It possesses narrow vessels and is darker in colour with high density.

                                              Fig: Late wood

Difference between early wood and late wood

Annual rings

If you have observed a cut wood, you might have noticed the ring-like patches on it. What is this exactly? These are the annual rings. Spring wood and autumn wood appear in alternate seasons and as a result alternate rings of wood are formed. These concentric rings are called annual rings. One light band plus one dark band gives one annual ring and it consists of the spring wood and autumn wood produced in one year together. Annual rings estimate the age of a tree. The branch of science which determines the age of trees by counting the number of annual rings is called dendrochronology.

                         Fig: Annual rings

Classification of wood based on the deposition of organic compounds

Secondary xylem becomes dark in old trees. It is due to the deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils. On the basis of this deposition of organic compounds there are two types of wood - heart wood and sap wood.

Heartwood

Heartwood or duramen is located at the centre or heart of the stem and is darker in colour due to deposition of tannins, resins, gums, etc in the cell walls and cell cavities. Heartwood is hard and durable and is resistant to microorganisms and insects. It possesses dead elements. It has highly lignified walls and hence does not conduct water. It provides mechanical support to the tree.

In the wood of some trees, bladder-like outgrowths of neighbouring parenchyma cells form plugs in the xylem elements and block the cavities. These plug-like structures are called tyloses.

Sap wood 

Sapwood or alburnum occurs in the peripheral region of the secondary xylem. It is lighter in colour due to very less deposition of substances. Conduction of water and minerals from root to leaf occurs through this. 

                          Fig: Sapwood and heartwood

Difference between sapwood and heartwood

Practice Problems

Q 1. Which of the following statements are true about the spring wood?

I) The physiological and environmental factors result in the formation of spring wood.
II) It is formed when nutrients are easy to access 
III) Possesses less xylary elements. 
IV) Xylem elements are larger 
V) High density wood

a. I, II, III, IV, V
b. I, II, III, IV
c. I, II, IV

d. All the above

Answer: The physiological and environmental factors control the activity of cambium. In temperate regions climatic conditions vary throughout the year. Hence formation of cambial rings varies in spring season and winter season. As a result two types of wood are formed, they are spring wood or early wood and autumn wood or late wood. Spring woods are formed when nutrients are easy to access and when more cambial activity is present during the spring season. It possesses more xylary elements. Xylem elements are larger and wider in spring wood. It is lighter in colour and is of low density. 

Hence the correct option is c.

Q 2. Match the following:

 
a. I-A, II-B, III-C, IV-D
b. I-C, II-A, III-B, IV-D
c. I-C, II-D, III-B, IV-A
d. I-A, II-C, III-B, IV-D

Answer: Spring woods are formed in the spring season. It is formed in the spring season when nutrients are easy to access and when more cambial activity is present. It is lighter in colour and is of low density. Autumn wood is formed during the winter season. It possesses narrow vessels and is darker in colour with high density. 

Heartwood is the centrally located wood which is dark in colour due to the deposition of tannins, resins, gums, etc. Heartwood is hard and durable and possesses dead elements. It does not conduct water due to highly lignified walls and obstructions in the xylem elements. It provides mechanical support to the tree. 

Sap wood occurs in the peripheral region of the secondary xylem. It is lighter in colour as there is little deposition of tannins, resins, etc. Conduction of water and minerals from root to leaf occurs through this. 

Hence the correct option is b.

Q 3. Which of the following gives hardwood?

a. Pine
b. Shisham
c. Spruce
d. Fir

Answer: Wood obtained from dicotyledonous angiosperms is known as hardwood or porous wood as it contains xylem fibres and vessels. Shisham is an example of a hardwood.

Pine, spruce and fir are gymnosperms and they give rise to softwood that lacks xylem fibres and is non-porous due to the absence of xylem vessels.

Thus, the correct option is b.

Q 4. What are annual rings?

Answer: Spring wood and autumn wood appear in alternate seasons and as a result alternate rings of wood are formed. These concentric rings are called annual rings. It consists of the spring wood and autumn wood produced in one year together. One light band plus one dark band gives one annual ring. Annual rings help to estimate the age of a tree. 

FAQs

Q 1. What is the chemical composition of wood?
Answer: Wood's chemical composition varies depending on the species, but it contains roughly 50% carbon, 42% oxygen, 6% hydrogen, 1% nitrogen, and 1% additional elements like calcium, potassium, sodium, magnesium, iron, and manganese by weight. Sulphur, chlorine, silicon, phosphorus, and other elements are found in minute amounts in wood. Aside from water, wood is made up of three primary elements. They are cellulose, hemicellulose and lignin. About 41–43 percent is made up of cellulose. The next most abundant substance is hemicellulose, which is found in roughly 20% of deciduous trees but around 30% in conifers. Lignin is the third component, accounting for roughly 27% in coniferous wood and 23% in deciduous woods.

Q 2. What are the extractives which are obtained from wood?
Answer: Aliphatic compounds, terpenes, and phenolic compounds are the three main types of extractives obtained from wood. Mono-, sesqui-, and diterpenes are the most common terpenes found in softwood. Meanwhile, the hardwood's terpene composition is quite different, with triterpenoids, polyprenols, and other higher terpenes. Phenolic chemicals are particularly abundant in hardwoods and bark. Stilbenes, lignans, norlignans, tannins, and flavonoids are the most well-known wood phenolic components. The majority of phenolic chemicals are fungicidal, protecting the wood from fungal degradation.

Q 3. Why is wood so strong?
Answer: Wood consists of natural polymers such as parallel chains of cellulose fibres that are held together by lignin molecules. This structure is the main reason behind the strength of the wood as the linear arrangement of the cellulose fibres makes it difficult to break.

Q 4. What is the difference between ring porous and diffuse porous wood?
Answer: Hardwood trees are classified as either ring-porous or diffuse-porous. Wider vessels form in the wood of ring-porous trees in the spring, followed by smaller vessels in the summer. The vessels in diffuse-porous trees have a comparable diameter across the annual ring.

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