Simple Permanent tissue

Introduction:

• Simple permanent tissues are the type of tissues that are made of similar types of cells that perform the same function.
• This type of tissue is majorly found in the construction of organs, epidermal layer, and mechanical support.
• This tissue forms the basis of the plant body.
• Simple permanent tissue is differentiated into three further categories based on its function and cellular structure.
• Parenchyma- Living tissue with circular, isodiametric cells with thin cellulosic walls.
• Collenchyma- Living mechanical tissue with oval or polygonal-shaped cells with no intercellular spaces and walls with thickening of cellulose, hemicellulose and pectin.
• Sclerenchyma- Dead mechanical tissue, with long tapered cells with highly thickened lignified walls.

Topic covered:

• Parenchyma
• Collenchyma
• Sclerenchyma

1. Parenchyma
 

Introduction:

• It is a primitive and first evolved tissue. It is also called the fundamental tissue as remaining all different types of tissues are derived from it.
• Parenchyma is a living tissue with thin cellulosic walls.
• Parenchyma forms the major component within the organs.
• Generally, the cells are isodiametric in shape.

Detailed explanation:
 

A. General characteristics:

• Parenchyma cells may be spherical, oval, round, polygonal or elongated in shape.
• These cells have thin cellulosic cell walls.
• These cells may have small intercellular spaces or are closely packed.
• Parenchyma cells possess a large central vacuole and peripherally placed cytoplasm with a prominent nucleus.
• Parenchyma cells use the plasmodesmata connections to interact with each other.
• Parenchyma is the basic fundamental tissue and is a part of ground tissue that helps form soft areas of leaves, stems, roots, flowers and fruits.
• Parenchyma shows a wide range of modifications to perform special functions.

B. Modifications of Parenchyma-
 

           
          i) Prosenchyma-

• Structure- It consists of thick-walled, elongated cells with pointed ends.
• Function- It gives mechanical support to the plant organ.
• Occurrence- Found in the pericycle of the root.
  
  ii) Stellate Parenchyma-
• Structure- Stellate or star-shaped cells with limited air spaces.
• Function- Provides mechanical support
• Occurrence- Found in leaf bases or pseudostem of banana.
  
  iii) Chlorenchyma-
• Structure - It is the parenchyma with chloroplasts.
• Function- It performs photosynthesis.
• Occurrence- These are found in the mesophyll of leaves. Mesophyll tissue of dorsiventral leaves is differentiated into -

(a) Palisade parenchyma:

i) Their cells are rectangular, tightly fitted together & intercellular spaces are absent.
ii) They are present towards the adaxial/ventral / upper side of the leaf.
iii) Number of chloroplasts are more in palisade tissue as compared to spongy parenchyma
iv) So the upper surface of a leaf appears greener as compared to the lower surface.

(b) Spongy parenchyma:

i) Large intercellular spaces are present between these cells.
ii) So they facilitate transpiration and gaseous exchange.
iii) They are present towards the abaxial/dorsal / lower side of the leaf.

While in the case of isobilateral leaf mesophyll tissue shows no such differentiation.
 

iv) Aerenchyma-

• Structure- Rounded cells with large air cavities which are lysigenous in origin.
• Function- These help the aquatic plants to float because of buoyancy.
• Occurrence- Aquatic plants or hydrophytes possess this special type of parenchyma.

v) Mucilage parenchyma-

• Structure- It consists of mucilage and large vacuoles.
• Function- It stores water in these plants.
• Occurrence- Leaves of the succulent (fleshy) plants like Aloe vera, Opuntia, etc.

Functions of parenchyma:

(i) Storage of food & water.
(ii) To perform gaseous exchange & provide buoyancy to hydrophytes.
(iii) Fibre-like elongated parenchyma is called prosenchyma. It provides rigidity and strength.
(iv) To maintain the shape of the plant body.
(v) They perform all the vital activities of plants.

2. Collenchyma
 

Introduction:

• Collenchyma is often known as the living mechanical tissue which possesses pectocellulosic thickenings and thus provides flexibility and mechanical support to the plant parts.

Detailed explanation:
 

A. General characteristics:

• Collenchyma tissue is made up of cells that are elongated and in the transverse section appear oval, spherical or polygonal in shape.
• The cells of collenchyma are living and thick-walled due to the deposition of cellulose, pectin and hemicellulose mainly at the corners.
• They lack intercellular spaces.
• Collenchyma may also exhibit the presence of chloroplasts.

B. Types of Collenchyma-

• Collenchyma can be of three basic types depending on the position of the thickening and arrangement of cells- Angular, lamellate and lacunar collenchyma

i) Angular Collenchyma-

It is the most common type of collenchyma with deposition occurring at the angles of the cell walls.

• Example: Stem of Datura, Solanum and Tomato.

ii) Lamellate Collenchyma-

• The cells of this type of collenchyma are arranged in lamellar or plate forms as the name suggests. The thickenings are present on the tangential walls(walls parallel to the cell rows).
• Example: Stem of sunflower.

iii) Lacunate Collenchyma-

• This collenchyma has large intercellular spaces and has thickenings on the walls that border the intercellular spaces.
• Example- Cucurbita stem

Occurrence:

• They are mainly found in the hypodermis of the young dicot stem, petiole and the margins of the leaf and also in the pedicel of the flower.
• They are absent in monocots, roots and woody parts of the plant.

Functions of Collenchyma-

• This tissue acts as a protective measure for the plants to protect them against the tearing effect of wind and save the leaves.
• Collenchyma can provide plant parts with mechanical strength as well as elasticity.
• Collenchyma with chloroplasts also performs the function of photosynthesis.

3. Sclerenchyma
 

Introduction:

• Sclerenchyma is the main mechanical tissue and often known as the dead mechanical tissue whose cells lack protoplast and have thick lignified walls. It provides mechanical strength to the plant parts.

Detailed explanation:
 

A. General characteristics:

• Sclerenchyma is majorly a dead tissue with the absence of protoplast.
• These cells are long, narrow, with thick walls due to lignin deposition.
• These thickenings are because of the deposition of lignin, cellulose or both.
• Sclerenchyma is of two different types, Sclerenchyma fibres, Sclereids.
  
  
  

B. Types of sclerenchyma:

On the basis of structure, origin and development, sclerenchyma can be of two types-

1. Sclerenchyma fibres-

Structure-

• These are elongated fibres with pointed ends and narrow lumen.
• These fibres occur in longitudinal bundles, where the pointed ends of adjoining fibres together get interlocked, finally strengthening the overall tissue.
• The fibres that are adjacent to each other exhibit pits which are basically unthickened areas with a common membrane.

Occurrence-

• They are found in all parts of the plant that need mechanical support. e.g leaves, petiole, pericycle, cortex, xylem and phloem, etc.
• Commercial fibres obtained from plants are usually sclerenchyma fibres, e.g., jute, hemp, flax.

Function-

• Sclerenchyma fibres are the main mechanical tissue for plants.
• These fibres can withstand all sorts of physical force such as bending, compression, pull and shearing keeping the plant steady and intact.

B) Sclereids-

• These cells are highly thickened with an almost obliterated lumen within them.
• These cells are spherical, oval or cylindrical.
• Generally, they are isodiametric and broader when compared to the fibres.
• The thick walls can have pits in them that might be branched or unbranched pits.
• The sclereids can have elongated pit canals.

Types of sclereids-

• The cells might occur individually or in aggregates.
• There are many types of sclereids, based on their shapes.
  
  
  

i) Brachysclereids

• Stone cells or brachysclereids are spherical cells with branched pits in them. e.g. Pear, guava, sapota, etc.

ii) Macrosclereids

• These sclereids are column-like or rod-shaped cells. e.g. Epidermal covering of legume seeds.

iii) Osteosclereids

• These sclereids as the name suggests are bone-like or a column with two bulged ends, e.g., monocot seed coat.

iv) Astrosclereids

• These are sclereid cells in the shape of a star. e.g. Petiole of lotus, tea leaves.

v) Trichosclereids/ Filiform sclereids

• Long, hair like branched sclereids. Branching project intercellular spaces. e.g. hydrophytes, Olea.

Occurrence-

• These cells are located mostly in the hard parts of the plant,e.g., Endocarp of coconut and Almond, Tea leaves, fruit wall of the nuts, seed coat of legumes.
• The grittiness of the fruits like guava and pears is due to the occurrence of stone cells in their pulp.

Function of Sclereids-

• These cells provide stiffness to the parts of plants wherever they are found.
• But they can also be found in the pulp of fruits and are responsible for the grittiness(crispy) texture in fruits like guava, sapota etc.

Frequently Asked Questions - FAQs

1. What do you mean by palisade parenchyma?
Ans: Mesophyll of the dicot or dorsiventral leaves consists of chloroplast containing compactly placed columnar cells that are present towards the adaxial / ventral / upper side of the leaf.
 

2. Characteristic features of collenchyma.
Ans:

• Cells are thickened at corners due to cellulose, hemicellulose deposition.
• Intercellular spaces are absent.
• Cells are oval or polygonal and often contain chloroplast.
• These cells provide flexibility and mechanical support to young growing parts.

3. What do you mean by Asterosclereids?
Ans; Astrosclereids are star-shaped sclereid cells(type of sclerenchyma). E.g., Tea leaves

4. Name the tissue represented by jute fibres that are used to make ropes?
Ans; Phloem/bast fibres are used for making ropes.

5. Which tissues are specifically responsible for the mechanical strength of a plant?
Ans: Sclerenchyma fibres provide mechanical support to the plant protecting it from various physical stresses. Sclerenchyma can protect the plant against forces like compression, bending, shearing etc.

6. Define aerenchyma and its function.
Ans: Aerenchyma tissues are usually found in hydrophytes. These cells exhibit large air cavities within them, which helps the aquatic plants to float because of buoyancy.

7. Name the parts of the plant which show the occurrence of collenchyma?
Ans: Collenchyma is found in the hypodermis of the young stem, petiole and margins of the leaf and pedicel of the flower.
 

8. What is the function of collenchyma?
Ans:

• This tissue acts as a protective measure for the plants to protect them against the tearing effect of wind and save the leaves.
• Collenchyma can provide plant parts with mechanical strength as well as elasticity.
• Collenchyma with chloroplasts in them also perform photosynthesis.

9. Name the different types of modification of parenchyma
Ans: The various types of modifications of parenchyma are- Prosenchyma, stellate parenchyma, chlorenchyma, aerenchyma and mucilage parenchyma.

10. Which type of simple tissue is responsible for the grittiness of the fruit?
Ans: Sclereids, a type of sclerenchymatous tissue is responsible for the grittiness of fruits. Ex: guava, pear, sapota

11. Which is the living mechanical tissue and why is it called so?
Ans: The cells of collenchyma are living and thick-walled mainly at the corners because of the deposition of cellulose, pectin and hemicellulose. These pectocellulosic thickenings help collenchyma provide mechanical support and flexibility to the plant parts.