Introduction
Every morning one watches the sunlight light up the posters on the wall or the clothes on the chair or the books on the table depending on the kind of room one keeps. Regardless, the sun lights up the day indicating the start of another day, which forces one to eventually get out of bed to be productive. As much as one enjoys staying in bed, to be productive is the aim of life and this aim requires energy.
Energy is much like the currency one pays to get work done. In living creatures, energy is mostly in the form of food. We know that the food consumed by animals is either plants or other animals (meat). In plants, through photosynthesis food is stored as glucose and starch. Production and transport of food and water in plants takes place via special, complex tissues – xylem and phloem.
To understand this further, let's learn first about the process itself. Photosynthesis is an essential process that produces glucose and oxygen through the process of oxidation. From the air and soil, plants pick up water and carbon dioxide. Then, in the leaves, they undergo the process of oxidation to release oxygen and glucose. The glucose is stored in the plant and the oxygen is further liberated into the atmosphere.
The process of photosynthesis is crucial for survival and is made possible by the vascular tissues – xylem and phloem. Xylem and phloem tissues, together form a transport unit – the vascular bundle. Both, the xylem and phloem tissues are extremely valuable since they enable the production and storage of energy in the plant. And one would think, with similar functions, they might be rather similar in all attributes. But that would be a false assumption. There are many differences between these tissues.
Differences Between Xylem and Phloem in Tabular Form
| Parameter | Xylem | Phloem |
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Cell type
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In xylem tissue, the cells are mostly dead cells at maturity. Xylem cells – tracheids – are elongated and tubular. They are mostly hard and transport the xylem sap (water and minerals) through the plant. In addition to tracheids, there are also other fibers and parenchyma present.
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In phloem tissue, the cells are living. These cells for various components like the sieve elements, sieve plates, companion cells, parenchyma, and sclerenchyma.
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Structure
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The tracheids are tubular and thin. They form a continuous column. There is a thin modified primitive cell membrane present between each tracheid that is called the pit membrane. It prevents the passing of air bubbles so the transport remains undisturbed.
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These have sieve tube cells that comprise a sieve in their lateral or end walls. The food or organic matter flows longitudinally through the sieves and microtubules.
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Location
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While both tissues are located in the roots, stems, and leaves of a plant, xylem tissue is usually found deep in the plant's vascular bundle.
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Phloem tissue lies usually in the outer part of the vascular bundle.
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| Mechanical Support |
Since there is the presence of lignin in the xylem tissue, in addition to transporting water, it also provides mechanical support to the plant.
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Phloem does not offer mechanical support to the plant.
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| Movement |
For the xylem, the flow of nutrition is unidirectional. It carries the sap upward only.
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Phloem has a bidirectional movement of nutrition. The food flows from the source to the area of requirement.
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| Conduction |
Xylem tissue conducts the xylem sap through negative pressure.
It is a passive process and does not require energy.
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Phloem tissue conducts the phloem sap through positive hydrostatic pressure.
It is an active process and requires energy.
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| Proportion |
Xylem forms most of the bulk of the body of the plant.
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Phloem forms a rather small part of the body of the plant.
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Conducting cells
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The conducting cells in the xylem tissue are dead.
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The conducting cells in the phloem tissue are living cells.
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| Function |
As afore-mentioned, the prime function of the xylem tissue is to transport water and minerals from the roots to the various parts of the plant.
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The phloem tissue transports the food (sugar like glucose and amino acids) from leaves to the storage and growing parts of the plant. This function of phloem is also known as translocation.
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| Sap | Xylem sap consists of mainly inorganic ions, a few organic solutes, and water. |
Phloem sap consists of organic sugars and water.
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Shape
| Xylem tissue has a star-like shape. | Phloem tissues don't have a particular shape. |
What Is Xylem?
It is the vital, permanent vascular tissue in a plant that is given the job of transporting water. It carries the water from the roots up to the rest of the plant. This action of the xylem tissue ensures that the water that is lost during the processes of photosynthesis and transpiration is replaced. In addition to water, it also tugs along essential nutrients, inorganic ions, and minerals. The activity of xylem tissue is rather passive. It does not require energy. The way this comes about is via capillary transport. Capillary transport is when the xylem sap travels upward against gravity. This happens when the water cells move up due to surface tension and negative pressure. Another contributing factor is the affinity of water cells to the xylem cells. This aids the upward transport of water from the roots. The flow of water in xylem tissues is more due to bulk flow rather than the cell diffusion process.
The xylem tissue is composed of specialized cells. It has components that convey water. These cells are called tracheary elements. The tracheary elements have tracheids and vessel members. The tracheids are water-conducting cells present in most plants (seedless plants and gymnosperms – plants where the seed is not enclosed by the ovary). Between the tracheids is a modified primary cell wall called the pit membrane. This pit membrane attempts to keep the air bubbles from causing disruptions in the flow of water. Like the tracheids, the vessel members are water-conducting cells present in the angiosperms – flowering plants. Between the vessel members are gaps where primary and secondary walls are absent. These areas are called perforations and work similarly to the pit membranes. They also attempt to keep the air bubbles from impeding the water flow. But fractures from the air bubbles to the water flow are not uncommon.
Additionally, xylem tissues also have fibers cells and parenchyma, which help in giving the plant structure more support. The lignin in the secondary wall of the xylem tissue is especially useful for maintaining structure under high pressures and making the xylem tissue water-proof.
When it comes to their development, xylem tissue develops into primary xylem from the tips of the roots and stems (apical meristems). In larger trees, the xylem tissue gives rise to an outer layer as it expands further in grith. This layer is called the secondary xylem or the sapwood. It surrounds the primary xylem (heartwood) which over time dies and contributes mainly to the strength and structure of the tree. Therefore, in older trees, it is only the trunk that performs the function of water conduction. Another interesting fact is that when these older trees are cut across the girth, their age can be determined by counting the number of annual xylem rings formed.
What Is Phloem?
Like xylem, phloem is a vascular tissue also tasked with transportation. But unlike xylem, phloem carries food. It is the complex tissue that carries food and organic compounds that are formed due to the process of photosynthesis to the various storage parts of the plant. Thus, it carries food primarily from the leaves to the roots and tubers, etc.
To transport food, the phloem tissue uses energy and turgor pressure. This happens because an osmotic gradient is created by the high concentration of the organic solutes inside the plant cells. In addition to the high concentration of organic solutes, there is passive osmosis of water through the adjacent xylem tissue. Both these actions create the osmotic gradient and a high turgor pressure which enables the transport of food amidst the various parts of the plant from the leaves. Hence, it is positive hydrostatic pressure that brings about the movement of food in the phloem tissue.
For its purpose of conduction of food, the phloem tissue has specialized cells. These specialized cells are called sieve elements. Sieve tubes are elongated tubes that have sieve-like porous plates between the cells through which food travels. The parenchyma cells in the phloem tissue are the transfer cells and border parenchyma cells. The border parenchyma cells are present at the veinlets of the leaves and the edges of the sieve tubes to enforce better transport of the food.
In non-flowering plants, the parenchyma cells are called companion cells or albuminous cells and have a larger number of mitochondria and ribosomes. They carry out the cellular functions of the sieve elements. The sclerenchyma cells structure the phloem fibers. They are long and flexible and serve to form soft fibers like hemp which can be used commercially. In addition to the transport of food, these phloem cells provide some strength and framework to the structure of the plant.
The development of phloem tissue is as follows. Like the xylem tissue, the phloem tissue also develops from the tips of roots and shoots. This phloem is of two types – protophloem and metaphloem. Protophloem consists of cells that mature during the growth of the area where they lie. In contrast, metaphloem are cells that mature after growth or elongation. In the earlier maturing protophloem, the sieve tubes are unable to stretch and as a consequence are torn and destroyed. The other cell types in the protophloem are converted into fibers. The later maturing metaphloem continues to grow and functions normally for the rest of the plant's life.
Differences Between Xylem and Phloem Tissues in Points
The following are the main differences between xylem and phloem tissues –
Xylem tissue in plants is responsible for the transport of water while phloem tissue is responsible for the transport of food.
- Xylem tissue is mostly composed of dead cells but phloem tissue consists of only live cells.
- Xylem tissue can conduct water in only one direction – upwards from the root to the other parts of the plant whereas phloem tissue can conduct food bidirectionally.
- Xylem tissue forms the bulk of the plant body and the phloem tissue forms a minimal part of the plant body.
- In the vascular bundle, the xylem tissues are present in the center of the bundle while the phloem tissues are present in the outer part of the bundle.
- Xylem tissue gives the plant more structural integrity due to the presence of lignin whereas phloem tissue does not contribute as much to the structural integrity of the plant.
- The xylem tissue is made up of dead cells – tracheids and vessel members. The live cells are the parenchyma cells that help in storage and support. The phloem tissue has various specialized live cells – the sieve tubes, specialized parenchyma, and sclerenchyma cells.
- The xylem cells have a star-like shape. The phloem tissue cells, on the other hand, have no specific shape.
- The action of the xylem tissue requires no energy and is thus a passive process. In comparison, the action of the phloem tissue requires energy and is thus an active process.
- The conduction of xylem sap is through negative pressure while the conduction of phloem sap is through positive pressure.
Conclusion
While both xylem and phloem vascular tissues have varied functions, their common function of transport deserves acknowledgment. It is this function that lets the plant produce and also distribute energy within itself and since plants are the primary source of food for most creatures, the function of xylem and phloem tissues, thus, is necessary for the survival of not just plants but also those dependent on them.
Therefore, xylem and phloem tissues are responsible for more than mere transport of water and food. They are extremely important and necessary for most actions of most living beings residing on the planet. They are necessary for activities like walking, talking, breathing, etc. They are needed even for activities like watching the sunrise, cleaning up one's room, and maintaining the study table.
References
- https://www.worldatlas.com/articles/where-do-plants-store-their-food.html
- https://www.nationalgeographic.org/encyclopedia/photosynthesis/
- https://www.britannica.com/science/xylem
- https://www.britannica.com/science/phloem
- https://www.toppr.com/guides/biology/difference-between-xylem-and-phloem/
