The Root

The part of the root that absorbs minerals is the root hair zone or epidermis.

Explanation:

• Root hairs: Root hairs are tiny, hair-like extensions of the epidermal cells located near the tips of the roots. These root hairs are the primary structures responsible for the absorption of water and minerals from the soil. They significantly increase the surface area of the root, allowing for more efficient absorption.
• Epidermis: The epidermis is the outermost layer of cells on the root and is responsible for the direct contact with the soil. The epidermal cells in the root hair zone facilitate the uptake of minerals and water through processes like osmosis and active transport.
• Root cap: While the root cap does not directly absorb minerals, it protects the growing tip of the root as it pushes through the soil and helps with the directional growth of the root.

How mineral absorption works:

• Minerals in the soil are absorbed by root hairs through both active transport (using energy) and passive transport (such as diffusion). Active transport is often used for minerals like potassium, calcium, and phosphate.
• Once absorbed, the minerals move from the root hair cells to the surrounding cells of the root, eventually reaching the vascular tissue (xylem), which transports the minerals throughout the plant.

Thus, the root hairs, located in the epidermal layer near the root tip, are the primary site of mineral absorption in plants.

Concept:
Root hairs are thin, tubular extensions of the epiblema (epidermal cells) of young roots. They are unicellular structures that greatly increase the surface area available for the absorption of water and mineral nutrients from the soil. Root hairs play a crucial role in plant nutrition because they come in direct contact with soil particles and absorb water through the process of osmosis, while mineral ions are absorbed through both passive and active transport mechanisms.
Step 1: Location of root hairs.
Root hairs are present in the zone of maturation (zone of differentiation) of the root. This region lies just behind the zone of elongation and is specialized for absorption.
Step 2: Characteristics of root hairs.
Root hairs possess several important features:

• They are unicellular outgrowths of epidermal cells.
• They are very delicate and short-lived.
• As the root grows, old root hairs are continuously replaced by new ones.
• They grow between soil particles to absorb capillary water and dissolved minerals.

Step 3: Determining the typical length of root hairs.
Although root hairs are microscopic structures, their length is relatively larger compared to other epidermal cell projections. The typical length of a root hair generally ranges between: This length helps them penetrate small spaces between soil particles and effectively absorb water and nutrients. Final Answer:

Concept:
Plants absorb water mainly through their root hairs. The absorbed water is transported upward through the xylem to different parts of the plant. Water absorption occurs through two major mechanisms:

• Passive absorption
• Active absorption

Passive absorption is driven by the transpiration pull created by evaporation of water from the leaves. It accounts for nearly 95--96% of total water absorption in plants. Active absorption, on the other hand, occurs due to the metabolic activity of root cells. Energy in the form of ATP is used to actively absorb ions, which creates an osmotic gradient that helps in water uptake.
Step 1: Understanding passive absorption.
When stomata are open, transpiration occurs rapidly. This creates a strong transpiration pull in the xylem, which draws water upward from the roots to the leaves. During this condition:

• Water movement is mainly controlled by transpiration.
• Roots do not need to actively pump water.
• Thus, passive absorption dominates.

Step 2: Understanding active absorption.
Active absorption occurs when transpiration is low or absent. This typically happens under conditions such as:

• When stomata are closed
• During night time
• During conditions of high humidity or low transpiration

In such situations, root cells actively absorb mineral ions using ATP. This creates an osmotic gradient that pulls water into the root cells.
Step 3: Identifying when active absorption does not occur.
When stomata are open, transpiration is high and passive absorption supplies most of the water. Therefore, active absorption is usually not required under this condition. Final Answer:
Active absorption usually does not occur when stomata are open because passive absorption driven by transpiration pull becomes dominant.

Concept: Epiphytic plants grow on other plants mainly for physical support and not for nutrition. Orchids are a common example of epiphytes. Since these plants often grow on tree branches and not directly in soil, they must obtain water and nutrients from the surrounding air, rainwater, and organic debris. To adapt to this lifestyle, orchid roots develop a specialized multilayered tissue called velamen. Velamen is composed of several layers of dead cells with thickened cell walls. These cells have large empty spaces that allow them to absorb and store atmospheric moisture quickly. Key functions of velamen include:
• Rapid absorption of atmospheric moisture such as rainwater, dew, or humidity.
• Protection of the inner living root tissues from excessive water loss.
• Mechanical protection of roots from damage and sunlight exposure.
• Temporary storage of water and dissolved minerals. This adaptation allows orchids to survive in habitats where direct soil water is unavailable.

Step 1: Understanding the habitat of epiphytic orchids.
Epiphytic orchids grow attached to tree trunks or branches. Because they do not grow in soil, they cannot absorb water through soil like most terrestrial plants.

Step 2: Identifying the specialized root structure.
The aerial roots of orchids are covered with a thick, spongy tissue known as velamen. This tissue efficiently absorbs moisture from the surrounding atmosphere.

Step 3: Selecting the correct option.
Among the given options:
• Cortex – general storage tissue in roots
• Velamen – specialized tissue in orchid roots
• Endodermis – regulates water entry into vascular tissues
• Pericycle – involved in lateral root formation Therefore, the correct answer is Velamen.

Concept: Root pressure refers to the positive pressure developed in the roots of plants due to the osmotic movement of water into root cells. This pressure helps in the upward movement of water through the xylem, especially during conditions of low transpiration such as at night. The concept of root pressure plays a significant role in understanding water transport in plants.

Step 1: Understanding the term “Root Pressure Theory”.
The term describes the mechanism by which water is pushed upward from the roots due to osmotic forces. This idea was studied and explained in early plant physiology research.

Step 2: Identifying the scientist associated with the term.

• Julius von Sachs contributed to experimental plant physiology but did not coin this term.
• Stephen Hales studied transpiration and water movement but is not credited with this terminology.
• J. Priestley is credited with coining the term “Root Pressure Theory” while studying plant physiology.
• Charles Darwin worked extensively on plant movement, not root pressure.

Step 3: Conclusion.
Thus, the correct answer is J. Priestley.

Step 1: Understanding the Concept:
Root hairs are unicellular, tubular extensions of the root epidermis (epiblema) found specifically in the maturation zone of the root.
Their primary function is to increase the surface area available for water and mineral absorption.

Step 2: Key Formula or Approach:
Recall the factual biological data regarding the standard dimensions of root hairs as prescribed by the curriculum.

Step 3: Detailed Explanation:
Root hairs are incredibly delicate and short-lived (ephemeral) structures.
They are microscopic in diameter but grow to a measurable length that can be seen.
According to standard botanical observations and biological textbooks, the typical length of a root hair ranges strictly from 1 to 10 millimeters.
Any length greater than this would make the single-celled hair too fragile to survive the mechanical friction of soil particles.

Step 4: Final Answer:
The standard length of a root hair is 1-10 mm, matching option (A).

Step 1: Understanding the Concept:
Water absorption in plants occurs through active absorption (requiring metabolic energy) and passive absorption (driven by the transpiration pull).

Step 2: Key Formula or Approach:
Relate the occurrence and dominance of active versus passive absorption to the rate of transpiration and the corresponding state of the stomata.

Step 3: Detailed Explanation:
When stomata are open, the transpiration rate is high, creating a strong transpiration pull.
This pull drives passive water absorption, which is highly efficient and accounts for the vast majority of water uptake, largely dominating over active absorption.
Active absorption only becomes noticeable and prominent when transpiration is very low or absent, such as when stomata are closed or during the night.
Therefore, active absorption usually does not occur (or is negligible) when stomata are open because passive absorption is overwhelming it.

Step 4: Final Answer:
Active absorption is minimal when stomata are open, making option (A) correct.

Concept: Epiphytes are plants that grow on other plants for physical support but do not derive nutrients from them. They commonly grow on tree trunks or branches in tropical forests. Examples include many orchids. Since epiphytes do not grow in soil, they require special adaptations to absorb water and nutrients directly from the atmosphere. One such adaptation is the presence of velamen roots.
• Velamen is a thick, spongy, multilayered epidermis found in the aerial roots of epiphytic plants.
• It helps in rapid absorption of moisture and nutrients from the surrounding air.
• It also protects the roots from excessive water loss and mechanical damage.

Step 1: Understand the adaptation of epiphytic plants. Epiphytic plants grow on other plants and therefore cannot obtain water from soil. They depend on atmospheric moisture, rainwater, and organic debris. To efficiently absorb this moisture, they develop specialized aerial roots with velamen tissue.

Step 2: Analyze the given options.
• Prop roots: Provide mechanical support to plants like banyan trees.
• Pneumatophores: Respiratory roots found in mangrove plants growing in waterlogged soils.
• Clinging roots: Help climbers attach to support structures.
• Velamen roots: Specialized aerial roots found in epiphytic plants for absorbing moisture from the air.

Step 3: Identify the correct option. Therefore, the special type of roots present in epiphytic plants are: