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Permanent Tissues: Phloem, its Components, Types and Differences

Drinking a juice from a long glass is really fascinating.

Do you believe that it is the same kind of mechanism which is happening inside the plants?

Plants need food and water for their survival. The transport of essential nutrients is happening through the complex permanent tissues. It is phloem tissue which helps in the transport of food materials.

Just like we order food through different food applications, phloem transports the food from the leaf to different parts of the plant.

transportation food meterials

Since phloem is a complex tissue, it will obviously have many kinds of cells present in it. Let's understand more about phloem.

Table of Contents

Phloem

Phloem is a complex permanent tissue that helps in the transport of food materials. It helps in the transport of food materials from leaves to other parts of the plant. It is made up of phloem components. In angiosperms and gymnosperms, they have different phloem components. Phloem is also called bast. 

Components of Phloem in Angiosperms

In angiosperms it possesses four components as follows:

  • Sieve tube elements
  • Companion cells
  • Phloem parenchyma
  • Phloem fibres

phloem components of angiosperms

Sieve tubes

Sieve tubes are long, tube-like structures arranged longitudinally in the form of channels. They are associated with companion cells. The end walls have large pores or sieve pits and are known as sieve plates. Mature sieve tube elements have peripheral cytoplasm, large vacuoles and they lack nuclei. Sieve pores become impregnated with callose at maturity. Functions of sieve tube elements are controlled by the nucleus of companion cells. 

sieve tubes

Companion cells

Companion cells are specialised parenchymatous cells. They are closely associated with sieve tube elements. They are non-conducting cells. Longitudinal walls between companion cells and sieve tubes are connected by pit fields using plasmodesmata. They help in maintaining the pressure gradient in sieve tubes. 

companion cell

Phloem Parenchyma

Phloem parenchyma are elongated cells, tapering and cylindrical cells. They have dense cytoplasm and nucleus. The cell wall is composed of cellulose and has pits. They store food materials like resins, latex and mucilage. They are absent in most monocots. 

phloem parenchyma

Phloem Fibres

Phloem fibres are sclerenchymatous cells. They are also called bast fibres. They are absent in primary phloem and present in secondary phloem. They are elongated and unbranched cells. They have pointed needle-like apices. They have thick cell walls and provide mechanical strength. At maturity phloem fibres lose protoplasm and become dead. Commercially important phloem fibres include jute, flax and hemp. 

phloem fibre

Components of Phloem in Gymnosperms

In gymnosperms the phloem consists of albuminous cells, sieve cells, phloem parenchyma and phloem fibres. They lack companion cells and sieve tubes. Sieve cells are elongated cells and they have sieve areas. Albuminous cells are modified parenchyma cells. 

Types of Phloem

Based on their origin phloem can be classified into two types as follows:

  • Primary phloem
  • Secondary phloem

Primary Phloem

It is the phloem produced from the apical meristem. 

Types of Primary Phloem 

Primary phloem is of two types based on period of formation.

  • Protophloem - It is the outer portion of the phloem consisting of narrow tube elements.
  • Metaphloem - It is the inner portion of the phloem. It is normally formed after the plant organs reach maturity. 

Secondary Phloem

It is the phloem produced from vascular cambium.

Differences between Meristematic tissues and Permanent tissues

Meristematic tissues

Permanent tissues

It is a simple tissue

It may be simple or complex tissue

The cells are small

The component cells are large

They possess isodiametric cells

Shape of the cells could vary according to the tissue

They possess thin cell wall

Cell wall can be thin or thick

Cells undergo repeated divisions

Cells don't divide normally

Intercellular spaces are absent

Intercellular spaces are present

They show high metabolic activities

Metabolic activities are comparatively less

Practice Problems of Phloem

Q1. Identify the plant cell without a nucleus?

  • Cambium cells
  • Sieve tube elements 
  • Root hairs 
  • Companion cells 

Solution: Sieve tube elements are one of the components of phloem tissue which on maturation lose the nucleus and contain large vacuoles with peripheral cytoplasm. Hence the correct option is b.

Q2. There is a common feature in between vessel elements (Xylem) and (pholem) sieve tube elements. Which is it?

  • Enucleate condition
  • Non-living nature
  • Thick secondary wall
  • Pores on lateral walls

Solution: The xylem vessel elements and sieve tube elements of phloem (at maturity) both lack nuclei. Hence the correct option is a.

Q3. Anatomically jute fibres are __________________.

  • xylem fibres
  • cortical fibres
  • pith fibres
  • secondary bast or phloem fibres

Solution: Jute fibres are considered as phloem fibres or bast fibres anatomically. During the primary growth phloem fibres are absent. Hence the correct option is d.

Q4. Assertion: In plants the long-distance transportation of photoassimilates occurs through sieve tubes.

Reason: Mature sieve tubes contain parietal cytoplasm and perforated sieve plates.

  • Both assertion and reason are true. But the reason is not the correct explanation for the assertion
  • Both assertion and reason are true and the reason is a correct explanation of the assertion 
  • The assertion is true. But the reason is false
  • Both assertion and reason are false 

Solution: Photoassimilates are the carbohydrates formed in the leaves of the plants by the process of photosynthesis. They are normally transported from the leaves to the other parts of the plants like buds, fruits, seeds etc. The main components that transport the food materials are sieve tube elements. It is a long, tube-like structure and it has perforations in the end walls. Hence the correct option is b.

FAQs of Phloem

Question1.- What are companion cells?

Answer. Companion cells are modified parenchymatous cells. They are closely associated with sieve tube elements. Companion cell contains the nucleus and other cellular components. They are non-conducting cells. Longitudinal walls between companion cells and sieve tubes are connected by pit fields using plasmodesmata. The pressure gradient in sieve tubes is maintained by companion cells. 

Question2.- What are the differences between the components of phloem in angiosperms and gymnosperms?

Answer. In angiosperms it possesses four components of phloem. They are sieve tubes, phloem parenchyma, phloem fibres and companion cells. In gymnosperms the phloem consists of albuminous cells, sieve cells, phloem parenchyma and phloem fibres. They lack companion cells and sieve tubes. Sieve cells are elongated cells and they have sieve areas. Albuminous cells are modified parenchyma cells. 

Question3.- Explain about the commercially important component of phloem?

Answer. Phloem fibres are the commercially important component of phloem. They are also called bast fibres. Commercially important phloem fibres include jute, flax and hemp. They are sclerenchymatous cells which are absent in primary phloem and present in secondary phloem. They are elongated and unbranched cells with pointed needle-like apices. They provide mechanical strength. At maturity phloem fibres lose protoplasm and become dead. 

Question4.- What are the two types of phloem based on their origin?

AnswerBased on their origin phloem can be classified into two types. They are primary phloem and secondary phloem. Primary phloem is the phloem produced from the apical meristem. Primary phloem is of two types based on period of formation. They are the protophloem and the metaphloem. Secondary phloem is the phloem produced from vascular cambium.

Related Concepts to Phloem in Biology

NCERT Class 11 Biology Chapters

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

 

 

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