Question

Select the incorrect statement from the following: (a) The portion of bark above the ring in girdling experiment gets swollen after few weeks. (b) Translocation in phloem is bidirectional. (c) Phloem tissue is composed of sieve tube cells, which form long coloumns with holes in the walls called sieve plates. (d) The process of loading at the source which produces a hypotonic condition in the phloem.

Short answer

The incorrect statement is (d) - 'The process of loading at the source produces a hypotonic condition in the phloem'. The accurate statement should be - 'The process of loading at the source produces a hypertonic condition in the phloem'.

Step-by-step solution

Examine each statement

First, evaluate the underlying science and accuracy of each statement. Statements a, b, and c are accurate.

Identify the incorrect statement

The last statement, statement (d) mentions a hypotonic condition in the phloem due to loading at the source. This is incorrect.

Understand Why Statement D Is Incorrect

In reality, loading at the source of phloem (often leaves) produces a hypertonic condition (not hypotonic) in the phloem. This osmotic condition helps movement of water into phloem from xylem, aiding the flow of sucrose and nutrients.

Key concepts

Girdling Experiment

One classic method to study phloem function in plants is the girdling experiment. Girdling involves removing a ring of bark from around the entire circumference of a tree or plant stem. This bark contains the phloem layer, and by removing it, the downward transport of sugars and other organic materials produced by photosynthesis in the leaves is interrupted.

As a result, above the girdle, where the phloem has been removed, there is an accumulation of sugars leading to swelling, as observed in statement (a) of the exercise. This provides direct evidence that phloem is responsible for transporting nutrients down from the leaves. Below the girdle, there may be a depletion of resources, ultimately affecting the root system. The girdling experiment is a clear demonstration of the role of phloem in translocation and helps students visualize the disruptions in nutrient flow when phloem is compromised.

Bidirectional Translocation

Unlike water transport in xylem which is unidirectional (from roots to shoots), the movement of substances in phloem is bidirectional. This capacity for bidirectional translocation allows the phloem to distribute sugars and amino acids, which are generated through photosynthesis in the leaves, to various parts of the plant where they are needed, including both upwards to growing shoots and downwards to the roots. This concept is highlighted in statement (b) from the exercise.

For instance, during the growing season, the sugars mainly move downwards to provide energy for root growth, while at other times they can be directed upwards to buds and fruits. Furthermore, the phloem can redistribute nutrients to storage organs such as potatoes and bulbs or bring stored nutrients back to leaves during spring growth. The bidirectional nature of phloem transport is integral to the plant's ability to adapt to both its developmental stage and changing environmental conditions.

Sieve Tube Cells

The phloem's structure is crucial to its function in translocation. Phloem tissue primarily consists of sieve tube cells, which are elongated cells connected end-to-end to form tubes, as highlighted in statement (c) from the exercise. These cells have specialized end walls known as sieve plates, featuring pores that allow for the flow of photoassimilates, like sucrose, between cells.

The design of sieve tube cells facilitates efficient transport of nutrients. They work alongside companion cells, which assist with the loading and unloading of molecules into the sieve tubes. It's important to note that sieve tube cells lack a nucleus, ribosomes, and other typical cell components, which reduces resistance to flow and ensures the unhindered movement of sap along the phloem. Through this specialized structure, phloem can transport nutrients over long distances quickly and effectively.