Plant Tissue: Xylem and Phloem

Plant Tissue: Xylem and Phloem

Overview

Xylem and phloem are two vital components of plant tissue that play crucial roles in the functioning of the plant. Xylem, often referred to as wood, is a type of vascular tissue responsible for transporting water and dissolved minerals from the root to the stem and aerial organs. On the other hand, phloem is another type of vascular tissue that translocates organic materials, primarily carbohydrates, from sites of synthesis to storage sites or sites of metabolic demand.

Xylem

Xylem is a complex tissue made up of a long chain of small tubes, called vessels, which interconnect and allow water to travel through unimpeded. These vessels are supported by other cells that help pull nutrients from the water and transport them to cells within the leaves. The water is propelled upward through the xylem due to pressure at the base of the plant and a phenomenon called transpiration, where water evaporates from the leaf surfaces, pulling more water upwards.

There are two main types of tracheary elements in the xylem: tracheids and vessel elements. Vessel elements, found in angiosperms, are short and wide with perforated end walls that form perforation plates at each end of the cell. Tracheids, present in both angiosperms and gymnosperms, are longer and slender with overlapping vertical files. Both types of tracheary elements are dead cells with thick, lignified cell walls, forming hollow tubes through which water flows efficiently.

Water moves through the xylem via pressure-driven bulk flow, which does not depend on solute concentration gradients and is independent of gravity. Interestingly, the cohesion-tension theory explains how water is pulled from the root to the shoot against gravity, despite water being attracted towards lower energy states (gravity).

Phloem

Unlike xylem, phloem's primary function is the transport of organic materials, particularly sugars, from sites of synthesis to storage sites or sites of metabolic demand. Phloem is typically composed of three cell types: sieve elements, parenchyma, and sclerenchyma. Sieve elements are the primary conductors of sugars and are characterized by having lost their nuclei and other organelles, making them solely dependent on surrounding parenchyma cells for maintenance. These parenchymatous cells support the sieve elements and contribute to the overall function of phloem.

In plants with secondary growth, the secondary phloem is formed, causing the primary phloem to collapse. Secondary phloem is continuously generated, reducing the lifespan of sieve elements within the phloem. Despite its role in sugar transport, phloem plays other roles in the plant body, serving as a pathway for signaling molecules such as mRNAs, hormones, and defense compounds from biotic and abiotic stresses.

Xylem and Phloem Interactions

At specific points, the xylem extends small tubes into the phloem, allowing for efficient exchange of nutrients and other compounds between the two tissues. Although xylem brings water and minerals up from the roots, it works in conjunction with phloem to distribute organic materials throughout the plant.