Difference Between Xylem and Phloem, Practice Problems and FAQs

The nature around us presents abundant resources from which we can obtain food and water. If these resources are not accessible, then we buy it from the market or order food online. Imagine being locked in your apartment for a few days with no food and water and no access to the internet for ordering online. Even the thought of it drives us crazy because we cannot survive without food and water. 

Same is the scenario with plants. But how do plants get enough food and water? Unlike us, plants do not have to depend on others for food as they prepare their own food by the process of photosynthesis. But they need water for this process which is absorbed from soil through the roots along with the minerals that are needed for their growth and development. 

The food is prepared in the leaves and other areas where chlorophyll is present. But this should reach every part of the plant. The water absorbed by the roots also has to reach all parts of the plants. But how is this achieved? Unlike us, plants lack circulating fluids that can help in transportation of nutrients and water. Instead, they have specialised tissues for this purpose. 

If you dip a plant in coloured water and then cut a thin section of the stem and observe it under the microscope, you can find the path of the coloured water. These tissues which resemble cylindrical pipes and carry the coloured water are known as xylem tissues. Another set of tissues known as the phloem tissues are responsible for the transport of food in the plants.

                             Fig: Cross section of stem showing the path 
                              which the coloured water travelled

Do you think xylem and phloem can help in transportation of water and food in very tall trees too? Hyperion is the tallest tree in the world found in Redwood National Park, California. It is a coastal redwood and is 115.54 m tall. This is as tall as a 25 storey building. What is interesting is that xylem and phloem tissues help in conduction of water and food even in trees as tall as the Hyperion. Imagine how efficient these tissues must be in order to transport water from the roots in the ground to a leaf that lies hundreds of metres above the ground!

                               Fig: Hyperion

Since the functions of these tissues are extremely different, they vary a lot in structure and other characteristics as well. So in this article, we are going to discuss the differences between xylem and phloem tissues. 

Table of contents:

• Complex permanent tissues
• Xylem
• Phloem
• Difference between xylem and phloem
• Practice Problems
• FAQs

Complex permanent tissues

In plants, the tissues which continuously divide to help in plant growth are known as the meristematic tissues. Permanent tissues arise from meristematic tissues once they lose their ability to divide and perform specialised functions. These tissues can be made up of single types of cells or more than one type of cells. Complex permanent tissues are those which are made up of more than one type of cell and function together as a unit. There are two types of complex permanent tissues: 

• Xylem 
• Phloem 

Xylem and phloem are also called the vascular tissue or conductive tissue. The major function of them is to transport the water, minerals and food. 

                                                  Fig: Xylem and phloem

Xylem

The complex permanent tissue which helps in the transport of sap (containing water and minerals) from root to different parts of the plant is called xylem. The transportation is unidirectional. Both living and non living cells are present in xylem tissues. Another function of xylem is to provide mechanical support to the plants. There are two types of xylem tissues on the basis of their origin - primary (arises from procambium during primary growth) and secondary xylem (arises from vascular cambium during secondary growth). Primary xylem can be of two types - protoxylem that is formed earlier and metaxylem that is formed later. On the basis of the relative position of protoxylem and metaxylem, xylem can be classified as endarch (protoxylem towards centre and metaxylem towards periphery), exarch (protoxylem towards periphery and metaxylem towards centre) and mesarch (metaxylem in the centre and surrounded by protoxylem on all sides). 

                         Fig: Protoxylem and metaxylem in vascular bundle

Xylem is composed of the following four components: 

Tracheids: The elongated, dead xylem cells with central cavity and tapering ends that help in conduction of water and minerals and also provide mechanical support.

• Vessels: The long and cylindrical, tube-like cells of xylem with lignified walls. The vessels along with the tracheids help in conduction of water and minerals. They also provide mechanical support. Vessels are absent in pteridophytes and gymnosperms.

• Xylem parenchyma: The thin walled and living xylem cells that store food materials.

• Fibres: The sclerenchymatous and highly thick walled components of xylem for mechanical support.

Except parenchyma all the components of xylem are dead at maturity. 

                                     Fig: Components of xylem

Phloem

The complex permanent tissue which helps in the transport of food materials from the leaves to several other plant parts is called phloem. There are two types of phloem based on their origin. They are primary phloem (arise from procambium during primary growth) and secondary phloem (arise from vascular cambium during secondary growth). Primary phloem are of two types - protophloem (formed earlier) and metaphloem (formed later). 

                                 Fig: Phloem in vascular bundle

There are different components that comprise phloem and these components vary in gymnosperms and angiosperms. 

There are major four components of phloem in the angiosperms. They are: 

• Sieve tube elements: The long, longitudinally placed tube-like structures, associated with the companion cells. Sieve plates are the perforated end walls of sieve tube elements. 

• Companion cells: The living parenchymatous cells which lie parallel to the sieve tube elements and assist them in translocation of food. Companion cells are absent in pteridophytes and gymnosperms.

• Phloem parenchyma: The elongated and cylindrical cells in phloem with tapering ends. They help in storage of food and lateral conduction of organic materials. They are usually absent in monocots.

• Phloem fibres: The elongated and unbranched sclerenchymatous phloem cells with needle-like apices. These are meant for providing mechanical support and rigidity to the organ.

All the phloem components except phloem fibres are living. 

                            Fig: Phloem components of angiosperms

The phloem of gymnosperms lack companion cells and sieve tubes. The components of phloem in a gymnosperm includes:

• Albuminous cells
• Sieve cells 
• Phloem parenchyma 
• Phloem fibres

Difference between xylem and phloem 

Now we know that xylem and phloem are two major tissues present in the vascular bundles. The major function of these tissues is the conduction and transportation of materials inside the plant. But the type of materials transported by xylem and phloem are different. This is one of the important differences between them. Now let’s see, in what all other aspects the xylem differ from phloem tissues. 

Practice Problems

Q 1. Which of the following statements is true about the xylem tissues?\

1. Xylem transports sap from root to different parts of the plant.
2. Xylem transport is bidirectional.
3. On the basis arrangement of xylem in the vascular bundle, they can be classified as endarch, exarch and mesarch.
4. Xylem fibres are sclerenchymatous and highly thick walled components of xylem for mechanical support.

a. A, B, C, D
b. A, C, D
c. A and C
d. B only

Answer: The complex permanent tissue which helps in the transport of sap from root to different parts of the plant is called xylem. The transportation is unidirectional. Both living and non living cells are present in xylem tissues. Another function of xylem is to provide mechanical support to the plants. There are two types of xylem tissues on the basis of their origin. They are primary and secondary xylem. On the basis of their arrangement in the vascular bundle, they can be classified as endarch, exarch and mesarch. There are four components for xylem. They are tracheids, vessels, xylem parenchyma and fibres. Xylem fibres are the sclerenchymatous and highly thick walled components of xylem for mechanical support. Hence the correct option is b.

Q 2. Match the following column I with column II. 

a. A- I, B-II, C-III, D-IV
b. A- II, B-I, C-III, D-IV
c. A- IV, B-II, C-III, D-I
d. A- II, B-III, C-IV, D-I

Answer: Complex permanent tissues are those which are made up of more than one type of cell and function together as a unit. The two types of complex permanent tissues are xylem and phloem. The complex permanent tissue which helps in the transport of sap from root to different parts of the plant is called xylem. There are four components for xylem and they are tracheids, vessels, xylem parenchyma and fibres. Tracheids are the elongated, tube-like, dead xylem cells. Xylem parenchyma is the thin walled and living xylem cells that store food materials. The complex permanent tissue which helps in the transport of food materials from the leaves to several other plant parts is called phloem. The major four components of phloem in the angiosperms are companion cells, sieve tube elements, phloem fibres and phloem parenchyma. Companion cells are the non-conducting phloem cells connected to sieve tube elements. Phloem fibres are the elongated and unbranched sclerenchymatous phloem cells with needle-like apices. Hence the correct option is d. 

Q 3. What is the difference between the fibres of xylem and phloem?

Answer: The highly thick walled and sclerenchymatous components of xylem are called xylem fibres. It has an obliterated central lumen and they also provide mechanical support. Xylem fibres can be septate or aseptate. The phloem cells that are sclerenchymatous are called phloem fibres or bast fibres. They are unbranched and elongated cells and their apices are needle-like. Their major function of phloem fibres is to provide mechanical strength to the plants, because they have thick walls. Phloem fibres lose protoplasm at maturity and they become dead cells.

Q 4. What are the differences between the components of xylem and phloem?

Answer: The major differences of the xylem components and phloem components are as follows:

FAQs

Q 1. What are the different types of companion cells?
Answer: There are three types of companion cells. They are intermediary cells, transfer cells and ordinary companion cells. The intermediary companion cells have extensive plasmodesmatal connections to the bundle sheath cells. Transfer cells have walls with ingrowths and they have a few plasmodesmata present in them. The ordinary companion cells also have few plasmodesmatal connections and they have no ingrowths in the wall. 

Q 2. What do you mean by Strasburger cell?
Answer: Albuminous cell is also known as Strasburger cell since it was first detected by Strasburger in 1891. It is seen in the gymnosperms, analogous to the functions of companion cells that are found in angiosperms. But ontogenetically the albuminous cells and companion cells are different. Albuminous cells are found in association with sieve cells. These cells contain albumin which is a protein. 

Q 3. Why are tracheid cells dead?
Answer: Due to the deposition of lignin and excess cell wall thickening, the tracheids are dead cells. This is the reason for the hard cell wall which makes the water and other components impermeable to the cell. Those materials are essential for the metabolic processes of the cell.

Q 4. What are the organelles that are present in the companion cells?
Answer: Typical organelles of a plant cell including the chloroplast and ribosomes are present in the companion cells. They also have dense protoplasm in them. There will be numerous branched plasmodesmata which interconnects sieve elements and companion cells. The degeneration of companion cells occurs when the associated sieve elements cease to function.

YOUTUBE LINK: https://www.youtube.com/watch?v=ISN_MHEe4Yc 

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