Problem 175 - Objective Biology for NEET

Question

Assertion: when soft herbaceous stem cut near the base from sharp blade, early in the morning, drop of solution ooze out from cut stem. Reason: Root pressure develops in herbaccous stem.

Step-by-Step Solution

Answer

The assertion is correct, soft herbaceous stems ooze out a solution when cut near the base with a sharp blade, particularly early in the morning because of root pressure. The root pressure forces the fluid or solution up into the plant stem. When the stem is cut with a sharp blade, the solution exudes out due to the force of the root pressure.

Step 1: Understanding root pressure

Root pressure refers to a force or pressure that develops in the roots of plants, as a result of active distribution of mineral nutrient ions into the root xylem. This causes water to move in the same direction by osmosis, resulting in a pressure gradient. This pressure forces fluid up into the plant stem.

Step 2: Role of sharp blade and timing

The blade is used to cut the stem, in order to observe the phenomenon of root pressure. The action is done early in the morning because during this time, transpiration is usually at its lowest and root pressure is at its highest. Therefore, you will more likely observe sap being pushed out when the cut is made near the base of the stem.

Step 3: Reason for oozing of solution

The oozing or exuding of the solution from the cut stem is due to the root pressure. As the root pressure builds up, it forces the solution upwards. When the stem is cut, the route of the solution is interrupted and it oozes out.

Key Concepts

Plant Physiology

Plant physiology encompasses the study of how different parts of a plant function and work together to sustain life processes. It involves understanding phenomena like root pressure, which is crucial for the movement of water and nutrients from the soil to the rest of the plant.

Herbaceous plants have soft stems due to their high water content, which is maintained by the continuous flow of water from the roots to the leaves. This water movement is driven partly by root pressure, particularly during the early hours when the external forces like transpiration are minimal. By studying and understanding this pressure in plant physiology, we gain insights into the ability of plants to transport essential resources without the need of a pump system as seen in animals.

Xylem Function

The xylem is an essential tissue in plants responsible for the transport of water and dissolved minerals from the roots to the rest of the plant. The xylem vessels are like tiny pipes that run throughout the plant's structure.

During the process where root pressure occurs, the xylem plays a critical role, acting as the conduit for the upward movement of the nutrient-rich water. This function of the xylem is a testimony to plant efficiency in resource distribution. Without the xylem, tall trees wouldn't be able to transport water to their uppermost leaves, and all plants would have to rely on direct water absorption from their immediate environment, which is not viable for growth or survival.

Transpiration

Transpiration is the process by which water is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration plays a crucial role in plant health and root pressure dynamics.

It is essentially like the plant's 'breathing' process, and it also creates a pull, called the transpiration pull, which assists in the upward movement of water and nutrients. Early in the morning, when transpiration rates are lower due to cooler temperatures and higher humidity, root pressure is more noticeable because the transpirational pull is less dominant, making it a perfect time to observe the effects of root pressure.

Osmosis in Plants

Osmosis in plants is the process of water movement through a semipermeable membrane (such as the cell wall) from a region of low solute concentration to a region of high solute concentration.

This process allows plants to maintain their turgidity and is critical for the uptake of water by the roots. Root pressure is a direct result of osmosis. As minerals are actively taken up into the roots from the soil, the osmotic pressure increases inside the root cells, causing water to rush in. This creates the upward force that can push water to the rest of the plant, which can be observed when a stem is cut and fluid oozes out.