Vascular Tissue

Key Concepts

• Source of energy
• Nuclear fusion
• Photosynthesis
• Transfer of energy
• Transfer of matter
• Types of tissues

 

Introduction

All organisms (plants, animals, and microbes) get energy from their food. Moreover, organisms need the energy to perform various activities and sustain life. Likewise, we humans require energy to perform the process of digestion, reproduction, locomotion, etc. The energy gets transported to various parts of a plant. In this chapter, we will be learning that the process of energy transportation. 

Explanation: 

Plant tissues 

We know that plants consist of various organs such as root, stem, leaves, flowers, and fruits. These organs work together for the growth and development of plants. All these organs are made up of tissues. The tissues are made up of combining cells. Let us discuss the classification of tissues to understand the flow of energy or transportation of nutrients to various parts of a plant. 

Classification of tissues: 

There are two types of tissues, namely meristematic tissue and permanent tissue.  

Meristematic tissues divide continuously. So new cells are formed in the meristematic region of plants. Permanent tissues are derived from meristematic tissues, and these tissues do not divide further as the cells of these tissues lose their division capability. Permanent tissues are of two types, namely – simple permanent tissue and complex permanent tissue. In general, permanent tissues help in giving protection, support and conduction of water and minerals. The main function of complex permanent tissue is the transportation of water, mineral and nutrients to various parts of the plant. Let us discuss the structure and function of complex permanent tissues in detail.  

Complex permanent tissues are of two types: xylem and phloem. These two tissues together are called vascular tissues. Some complex tissues are produced by apical meristem (meristematic tissue). Mostly in woody plants, these are produced by vascular cambium and are generally referred to as vascular tissues. 

Vascular bundle is the arrangement of xylem and phloem tissues. 

Xylem 

It is an important tissue present in the stem and roots. It is present in the non-woody parts of the plant and non-woody plants. The word xylem was introduced in 1858 by Carl Nägeli. Its main function is to transport water and mineral to all parts of the plant, i.e., from root to the tip of the plant. Xylem consists of tracheids, xylem vessels, xylem parenchyma, and xylem fibers. 

a. Tracheids: These are dead and lignified cells. They have thick secondary walls and are pointed at the end. They help in the conduction of water and gives mechanical protection to the plant. The main area of the tracheids cell wall is covered by perforated pits. Pits help to transport water from one cell to the next cell. 

b. Xylem vessels: The other name for xylem vessels is tracheae. It is composed of small and tube-like cells. The xylem vessels consist of vessel segments and vessel elements. It helps in the movement of water from one cell to another. 

c. Xylem parenchyma: These are the only living cells that store fat and starch. The cell wall of the cells is lignified. Xylem parenchyma stores food material. Xylem parenchyma helps in the transportation of water for a short distance. 

d. Xylem fibers: These are long and narrow dead (sclerenchymatous) fibers with pointed ends. They contain lignified walls with a central narrow lumen. The function of xylem fibers is to provide mechanical support and help in the transportation of water. 

Transport of water and mineral through xylem cells 

Plant absorbs water and minerals with the help of roots. Water and mineral enter the root through different channels and merge into the central vascular bundle in the root. 

Cohesion theory explains the upward movement of water against the force of gravity through the xylem cells. Transpiration (loss of water from the leaf surface) is the main part of the movement of water through the xylem, which is unidirectional.  

The water absorbed by roots goes from one cell to another by osmosis till it reaches the xylem cells in the root. Due to this continuous movement of water (due to cohesion – sticking of water molecules to one another and creating a stream of water) in the xylem cells, a column of water gets created, which slowly gets pushed in the upward direction. This process is called root pressure. This pressure replaces the water which is lost through the stomata of the leaf. The process of transpiration creates a suction effect which pulls the water up from the xylem cells of the root. 

The process of transpiration helps in the upward movement of water and absorption of water and minerals, which is also called the ascent of sap.  

Sap: It is the substance being transported throughout the plant by xylem and phloem. 

Phloem:  

Phloem is as important as xylem in the conduction of food material (sugar, organic compounds and minerals). The main function of phloem is to carry dissolved food material to all the parts of a plant. So, it is responsible for translocation. It comprises of living cells. Food material is transported by diffusion between the cells and goes from leaves to root with the help of gravity. Phloem consists of sieve tubes, companion cells, phloem parenchyma and phloem fibers. 

1. Sieve tubes: 

Sieve elements are long and narrow cells that are connected to form sieve tubes. These are living cells, but the nucleus is absent at maturity. The cell wall is a thin structure and is made up of cellulose. The cross-sectional wall of the sieve tube forms a sieve plate. Sieve tube helps in the transport of food material. 

2. Companion cells: 

These are narrow, thin-walled and living cells. They are attached to the side of the sieve element by channels called plasmodesmata. Companion cells help the sieve tube in the transportation of food material. These cells are absent in monocotyledons and some dicots. 

3. Phloem parenchyma: 

These cells are also called transfer cells. The main function of phloem parenchyma is to store food material and other substances such as resins, tannins and mucilage. 

4. Phloem fibers: 

These are also called bast fibers. These are dead cells (sclerenchymatous fibers) with a narrow lumen and a thick wall. 

Difference between xylem and phloem: 

Transport of food material through the phloem  

The process of transportation of food material in plants is called translocation. This process takes place by phloem, which is a conducting tissue. Food materials such as sugar, amino acid and other substances are transported by phloem from leaves to other parts of the plant such as stem, root, fruit and seeds. The two parts of phloem – companion cells and sieve tubes help in the transportation of food in both directions, i.e., upward and downward. Sugar like materials are transported with the help of energy from ATP, and this increases the osmotic pressure of tissue. Water from the adjacent xylem tissue moves into the sieve tubes through companion cells. Due to this, the pressure increases in the sieve tubes, and food material moves in phloem and from there to tissues with less pressure. This activity allows the phloem to move the food material according to the need of the plant in an upward and downward direction. 

Sources: These are the areas of the plant where sucrose is made and where they are delivered to various parts of a plant. 

Sinks: It is the place where food (sugar) is delivered and is used for growth. 

Location of xylem and phloem in roots, leaves and stem: 

Xylem and phloem are present in the stele, a middle section of a root. 

Xylem and phloem enter a plant’s leaves through a short stalk that connects a leaf to a branch called a petiole. 

Xylem and phloem are present in the entire length of stems in distinct threads called ‘vascular bundles.’ 

Cambium: It is a layer between the xylem and phloem that is actively dividing the region. It is responsible for the secondary growth of roots and stems. As cambium is present between the xylem and the phloem, it is also called interfascicular cambium. 

Summary:

• Plants consist of various organs such as root, stem, leaves, flowers and fruits.
• Two types of tissues, namely meristematic tissue and permanent tissue.
• Meristematic tissues divide continuously.
• Permanent tissues are derived from meristematic tissues and these tissues do not divide further as the cells of these tissues lose their division capability.
• Permanent tissues are of two types namely – simple permanent tissue and complex permanent tissue. Xylem and phloem are permanent complex tissues.
• The main function of xylem is to transport water and mineral to all parts of a plant i.e. from root to the tip of the plant.
• The main function of phloem is to carry dissolved food material to all the parts of a plant.