Continuous Transverse Section of a Person
Dicots (Dicotyledonous) Plants
Dicots, i.e. dicotyledonous plants contain:
- Network of veins
- Seeds that have two seed leaves, i.e. cotyledons
- Leaves that usually contain broad blades (leaf surface) and petioles (stalks)
- Taproot has lateral branches
Herbaceous dicotyledonous plants comparatively have a shorter life span (a single growing season only) and non-woody tissue.
In the next section of the article, we will discuss the transport systems of the plant.
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Transport Systems
- Plants require transport systems to fulfil their metabolic demands such as hormones, mineral ions, and glucose. These substances are needed for different processes within the plants.
- The transport system in plants also assists to move substances up and down efficiently.
The Vascular System in Plants
Plants contain a vascular system that includes a network of vessels, i.e. vascular tissue that runs through leaves, stems, and roots. These three organs play the primary role in transport.
The vascular system in plants contains:
- Xylem: It transports water and mineral ions from the roots to the remaining parts of the plants
- Phloem: It is involved in the transport of the substances from the leaf (source) to the sink (root)
Both xylem and phloem are arranged together in vascular bundles that are laid out differently in the leaves, stems, and roots.
In the next section of the article, we will discuss xylem and phloem distribution in plants.
Xylem and Phloem Distribution in Plants
Xylem
The xylem tissue in plants performs the following functions:
- It transports water and dissolved minerals up the plant
- It provides structural support to the plant and helps in food storage
The type of organ determines the location of the vascular bundles in plants:
- In roots: The vascular bundles are located at the centre and centre core. The xylem tissue is present at the centre core of it.
- In stems: The vascular bundles are found around the outside and the xylem tissue is located around the inside, i.e. nearest to the stem's centre
- In leaves: The vascular bundles create midrib and veins and hence they stretch from the leaf's centre in a parallel line. The xylem tissue in leaves is located at the upper side of the bundles
Phloem
Phloem tissue performs the following main functions:
- It helps in the movement of organic compounds especially sucrose from the source (leaf) to the sink (roots). These compounds are transported up and down the plant.
Phloem is a complicated tissue that is composed of different types of cells such as:
- Its bulk is composed of sieve tube elements that are the primary conducting cells and the companion cells
- Other kinds of cells in phloem tissue include parenchyma which helps in storage and strengthening fibres.
The location of vascular bundles depends on the type of organ they are in:
- In roots: The vascular bundle present in the centre and on the edges of the centre core is the phloem tissue
- In stems: The vascular bundles are present around the outside and the phloem tissue is located on the outside (close to the epidermis)
- In leaves: The vascular bundles create the veins and midrib, hence they extend from the centre of the leaf in a parallel line. The phloem tissue is located on the lower side of the bundles.
In the next section of the article, we will discuss xylem vessel elements and phloem sieve tube elements.
Structure and Function of Xylem Vessel Elements
In plants, four types of cells make up the xylem tissue which performs their functions collectively. These cells include:
- Tracheids are long, narrow tapered cells having pits
- Vessel elements are large with thick cell walls. They have no end plates as they mature
- Xylem parenchyma
- Sclerenchyma cells, i.e. fibres and sclereids
The majority of the xylem tissue is composed of tracheids and vessel elements. Both of these are types of water-conducting cells.
Phloem Sieve Tube Elements
- Phloem tissue in plants transports organic compounds especially sucrose from the leaf to the roots. These substances are transported up and down the plant.
- The organic compounds are dissolved in water to create sap.
- The complicated phloem tissue is composed of sieve tube elements which are the primary conducting and companion cells.
- Other kinds of phloem tissue cells include parenchyma which is meant for storage and strengthening fibres
- Unlike xylem tissue, mature phloem tissue has living cells.
Sieve Tube Elements
Sieve tube Elements in Phloem
Sieve tube elements are arranged end to end to create a continuous tube.
The structure and functions of sieve tube elements are given below:
- Sieve plates and sieve pores: They enable the non-stop movement of organic compounds
- Cellulose cell wall: It provides strength to the cell wall to withstand the hydrostatic pressure that moves the assimilates
- In mature cells: Nucleus, vacuole, and ribosomes are not present
- Thin cytoplasm: Minimizes friction to assist the movement of assimilates
Phloem C ompanion C ells
Each sieve tube element contains a companion cell related to it because companion cells control the metabolism of their related sieve tube member. They also take part in loading and unloading sugars in the phloem.
Structure and function of phloem companion cells
The structure and function of phloem companion cells are given below:
- Nucleus and other organelles: They provide metabolic support to sieve tube elements and assist with the loading and unloading of assimilates
- Transport proteins in the plasma membrane: They help to move assimilates into and out the sieve tube elements
- The huge number of mitochondria: They provide ATP for active transport of assimilates into or out of the companion cells
- Plasmodesmata: It is the link to the sieve tube elements which enables organic compounds to move from the companion cells into the sieve tube elements.
In the next section, we will compare xylem and phloem tissue.
Comparison of Xylem and Phloem Tissue
Mature living cells
Xylem cells do not have mature living cells whereas phloem cells have.
Substances transported
Xylem tissue transports water and mineral ions whereas the phloem tissue transports organic compounds or assimilates
Transportation Process
Xylem tissue transports water and mineral ions through transpiration while the phloem tissue transports organic substances through active translocation
Direction of Flow
The xylem tissue transports substances upwards whereas the phloem tissue transports substances two ways
Presence of End Walls
No end walls are present in xylem tissue while phloem tissue has end walls
Cell Wall Material
The cell wall in xylem tissue is composed of lignin and cellulose, whereas in phloem tissue the cell wall is made up of cellulose only.
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Source: https://www.superprof.co.uk/resources/academic/academic-science/biology-science/biology-a-level/plant-transverse-section.html
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