Problem 116 - Objective Biology for NEET

Question

In older trees, the greater part of secondary xylem is dark brown due to the deposition of (a) Tannins and resins (b) Oil and gums (c) Aromatic substance and essential oil (d) All of these

Step-by-Step Solution

Answer

The correct answer is (d) All of these

Step 1: Reviewing the Options

Look at each of the combinations of substances proposed in the options (a) Tannins and resins, (b) Oils and gums, (c) Aromatic substances and essential oils, and (d) All of these. Considering your knowledge about substances that could be found in trees is significant in this step.

Step 2: Understanding the Role of Tannins and Resins

Tannins and resins, mentioned in option (a), are well-known substances in trees, especially older ones. They are part of the substances trees use for protection, and they can also affect the color of the wood. Particularly, tannins are known to darken the wood.

Step 3: Evaluating Other Options

In option (b), oils and gums are mentioned. While these substances can be found in trees, they are not typically associated with a darkening effect on wood. Similarly, while aromatic substances and essential oils (option c) can be present in trees, they do not contribute significantly to the darkening of wood.

Step 4: Making the Final Decision

After understanding the possible roles of the substances in all options, it seems like option (a), tannins and resins, is the most likely cause of darkening in xylem of older trees due to their properties and their role in trees. However, option (d) mentions all these substances, which may seem broader. Therefore, to exclude any exceptions and cover all possible causes, option (d) all of these, seems the most appropriate choice.

Key Concepts

Tannins and Their Role in Trees

Tannins are complex organic substances found in many plants, including trees. These compounds play a crucial role in a tree's defense system. They deter herbivores and inhibit the growth of some bacteria and fungi. This defensive mechanism is vital for tree survival, especially as they age.

In older trees, tannins accumulate in the secondary xylem, the layer often referred to as wood. Tannins can cause the wood to take on a darker hue. This characteristic brown color is significant for a few reasons:

• The coloration can help protect the tree from unfavorable environmental factors like UV radiation.
• It enhances the wood’s resistance to decay.
• It contributes greatly to the tree's distinct appearance, often admired in aged timber.

Understanding tannins and their cumulative effect on the darkening of wood is essential for appreciating the natural aging process of trees.

The Importance of Resins

Resins are another group of compounds that are vital to the health and longevity of a tree. They are sticky, viscous substances that trees produce in response to injury or attack, akin to a form of natural band-aid.

In older trees, resins can accumulate in the wood, further contributing to the dark pigmentation of the xylem. Here’s why resins are important:

• They seal wounds and prevent the invasion of pathogens.
• Resins deter insects and other pests that can harm the tree.
• They create a barrier, slowing the decay of the wood.

By working in tandem with tannins, resins play a complex and beneficial role in providing both physical and chemical protection for trees, especially those that have stood for long periods.

How Wood Darkens Over Time

The darkening of wood in trees occurs as they grow older, and this is largely due to chemical changes and the accumulation of certain compounds over the lifespan of the tree. Besides tannins and resins, other minor organic compounds may also contribute to this process, though to a lesser extent.

Here's how the darkening process works:

• The xylem, or wood, develops layers each year, or rings, where these compounds are deposited over time.
• As these substances break down and interact within the tree, they cause a chemical alteration, leading to a richer, deeper color.
• This progression of color can be a visible indicator of the tree's age and health.

This knowledge is particularly useful for those in forestry or carpentry, where the aesthetic and structural qualities of wood are crucial.

Characteristics of Older Trees

Older trees present a unique set of characteristics due to years of surviving and adapting to their environments. One prominent feature is the increased presence of substances like tannins and resins in the wood. These substances, apart from darkening the wood, serve many functional purposes.

Why are older trees interesting?

• They can tell us about the environmental conditions they have endured through their growth rings.
• Such trees provide habitats and food for numerous species, showing their ecological significance.
• The dense, dark wood of older trees is often more valued for construction and craftsmanship due to its durability.

Appreciating the facets of older trees helps us further understand their role in ecosystems and the importance of conserving such venerable natural resources.