Complex permanent tissues

Introduction

• Permanent tissues are those kinds of tissues that have lost their ability to divide.
• These tissues originate from the meristematic tissues and represent those cells that lose their dividing ability and differentiate to perform specific functions.
• Complex permanent tissues are made up of different types of cells, i.e. heterogeneous.
• These tissues function as conducting tissues and all different types of cells work together as a unit.
• These tissues are also known as vascular tissues as they’re formed of vascular bundles.
• These tissues also provide mechanical strength to the plant.
• There are two main types of complex permanent tissues, i.e. Xylem and Phloem.
• Xylem is water and mineral conducting tissues.
• Phloem is involved in the translocation of synthesized food in the leaves to every part of the plant.

Topics covered:

• Xylem
• Xylem types on the basis of Origin and Development
• Xylem Elements

1. Xylem

Introduction:

• The conducting tissue responsible for water transport throughout a plant body from the soil via roots up to the stem and leaves is called the xylem.
• Xylem also provides mechanical strength along with conduction.
• This type of tissue is made up of 4 different kinds of cells, i.e. Tracheids, vessels, xylem fibres and xylem parenchyma.
• Amongst these 4 types, only vessels and tracheids are utilised for sap transport and are together known as tracheary elements.
• In the case of angiosperms, vessels are the main tracheary elements.
• Pteridophytes and Gymnosperms lack vessels.
• All these separate cells have individual functions. And they work together to conduct water from the soil via roots to the stem and the leaves.
• Xylem can be divided into two types based on their origin, i.e. primary xylem and secondary xylem.
• Based on their development pattern, primary xylem can be divided into two types, i.e. Protoxylem (early formed xylem) and Metaxylem (later formed xylem).
  
  
  
  
   

2. Xylem types on the basis of Origin and Development

Introduction:

• Xylem is a conducting tissue that helps in the transport of water and minerals from roots up towards the stem and leaves.
• On the basis of the origin of xylem tissue, they can be divided into two types- Primary xylem and secondary xylem.
• Xylem can also be differentiated into two types based on their development, i.e. Protoxylem and Metaxylem.
• Primary xylem develops from procambium. It is the early formed xylem that is found in a primary plant body.
• Secondary xylem develops from vascular cambium. It is formed as a result of secondary growth.
• Protoxylem is the early formed or first formed xylem tissue in a growing plant.
• Metaxylem is the later formed xylem in a plant. Metaxylem exhibits large tracheids and vessels.

Detailed explanation:

A. On the basis of origin-

• On the basis of origin, xylem is divided into-
    1. Primary xylem
    2. Secondary xylem

 1. Primary xylem-

• It is the first-formed xylem in the primary plant body.
• This xylem develops from the procambium tissues.
• The primary xylem exists in every organ of the plant body.
• This xylem occurs towards the centre and is found in patches.
• The primary xylem is then differentiated into two groups based on their developing pattern, i.e. protoxylem and metaxylem.
• Tracheid and vessels are longer and walls are not very thick.
• There is no sign of heart or sapwood.
• Annual rings are absent.
• Tyloses are not yet formed.

 2. Secondary xylem-

• This xylem is formed much later in a mature plant body.
• The secondary xylem originates from the lateral meristems (vascular cambium) which are located in between the primary xylem and primary phloem.
• The secondary xylem is the resultant of secondary growth due to vascular cambium.
• Secondary growth is not observed in all plants, it is restricted amongst the dicots and gymnosperms.
• The secondary xylem is present outside the primary xylem and in the form of a cylinder.
• Sapwood and heartwood are found in the secondary xylem of woody plants.
• The vessels and tracheids are shorter in length and are much thickened at the walls compared to the primary xylem.
• It shows the presence of annular rings.
• Tyloses are formed within the secondary xylem tracheids which makes them dead and gets deposited with various organic substances.

B. On the basis of development, the primary xylem can be-

Protoxylem-

• Protoxylem or the early formed xylem starts developing before the organ completes its growth.
• Protoxylem is comparatively less distinct.
• The elements are much smaller and narrower in the protoxylem.
• The tracheids and vessels of the protoxylem exhibit annular or spiral thickenings.
• The lignification of the protoxylem begins before the plant's elongation is complete.
• Protoxylem elements can be stretched.
• Protoxylem in monocots often get crushed during growth and it leads to cavities.

Metaxylem-

• Metaxylem or the later formed xylem begins to develop once the plant organ growth is done.
• Metaxylem is quite prominent.
• The metaxylem elements are large and broad in nature.
• Metaxylem exhibits thickening different from protoxylem, i.e. reticulate, scalariform or pitted.
• The lignification of the metaxylem begins after the plant's elongation is complete.
• The metaxylem cannot be stretched.
• Metaxylem doesn't get crushed.

3. Xylem Elements

Introduction:

• Xylem elements are the different types of cells that constitute xylem tissue.
• The cells that compose the xylem are tracheids, vessels, xylem fibres, xylem parenchyma.
• All these elements need to function as a unit to deliver or conduct water from roots to the shoots.
• Tracheids are primitive conducting elements of xylem containing narrow lumen and are the key xylem elements responsible for water transportation.
• Vessels are elongated cylindrical tubes that work alongside tracheids to transport water or xylem sap in angiosperms.
• Xylem parenchyma are living cells that act mainly as storage cells in the xylem.
• Xylem fibres are long, narrow cells that are mostly dead by maturity and provide mechanical support to the xylem elements.

Detailed explanation:

1. Tracheids

• They are the primitive conducting elements.
• These are elongated, lignified cells with the tapering end and narrow lumen that transport water or sap in the xylem.
• Tracheids are dead cells that are devoid of any protoplasm.
• They are unicellular, imperforated with bordered pits at cross walls.
• Tracheids are the major wood constituent in the gymnosperms up to about 95%.
• On the basis of thickening, tracheids can be of 5 types-

I) Annular- The thickening in tracheids that appear as multiple rings stacked upon each other with some gaps in between.

II) Spiral- The thickening in tracheids when looking like a DNA helix or spiral-shaped. Spiral and annular thickening is a characteristic of the primary xylem as they allow it to stretch.

III) Reticulate- This kind of thickening looks like a spider web, highly interlinked. It is said to be formed because of multiple spiral thickenings crossing each other.

IV) Scalariform- The thickening looks like a ladder, there are bands that are arranged transversely.

V) Pitted- This thickening is of special type. The pitted tracheids are uniformly thickened except for small unthickened areas. A pit contains a pit chamber and a pit membrane.The pit membrane is composed of primary wall and middle lamella. Pits are two types-

• Simple pits: It bears a uniform width of the pit chamber.
• Bordered pits: Its chamber is flask-shaped due to the overarching secondary wall on its mouth. Its pit membrane has a thickening of suberin called Torus. In bordered pits, the diffusion is regulated by torus and it functions as a valve. These pits are found abundantly in tracheids of gymnosperms (have the maximum number of bordered pits) and in vessels of angiosperms.

 

2. Vessels

• They are the advanced conducting elements which work along with tracheids for the transportation of xylem sap.
• It consists of a long, cylindrical tube- like structure made up of many cells called vessel members.
• As these vessel members are arranged in a column forming a syncyte (cell formed by fusion of cells) and are devoid of protoplasm. Hence, they are called dead syncyte.
• These cells have a central cavity and are surrounded by thickened walls formed due to lignin deposition.
• Vessels also exhibit perforation plates which are present on the end walls of vessel cells.
• These cells exhibit simple pores on the lateral walls.
• Thickening much like tracheids are of multiple types, i.e. Scalariform, Spiral, Annular, etc.
• Vessels are known to be absent in the case of the gymnosperms and pteridophytes except for Selaginella and Gnetum.

3. Xylem fibres-

• Xylem fibres are sclerenchyma cells present in the xylem tissue.
• These are dead cells due to highly lignified walls.
• They may be septate or aseptate.
• They are characterised by highly thickened walls and obliterated lumen.
• These are responsible for all the mechanical strength and are abundantly found in the secondary xylem.
• They provide strength to the tracheids and vessels.
• They are absent in the wood of gymnosperms.
  
  
  
  
   

4. Xylem parenchyma-

• Xylem parenchyma is living cells with thin cellulosic walls.
• The function of xylem parenchyma is the storage of fat, starch and other substances like tannin.
• The lateral movement of water in the xylem elements is performed by these cells.
• Ray parenchyma cells are the ones that assist the radial water conduction.
• The primary xylem only exhibits the axial parenchyma, but the secondary xylem exhibits both axial and radial parenchyma.

Complex permanent tissues (FAQ)

1. What role does xylem play along with water conduction?
Ans- Xylem can also provide plants with mechanical strength because of the presence of sclerenchymatous xylem fibres and xylem tracheids which are dead tissues with thickening on their walls and in later stages due to formation of tyloses get blocked and deposition in them makes them very tough.

2. State 4 features of protoxylem.
Ans- i. Protoxylem is the early formed or first formed xylem.
  ii. The elements are much smaller and immature in the protoxylem.
  iii. The tracheids and vessels of the protoxylem exhibit annular or spiral thickenings.

3. What makes the metaxylem tougher than the protoxylem?
Ans- The metaxylem elements are large and broad in nature. Metaxylem exhibits thickening different from protoxylem, i.e. reticulate, scalariform or pitted. The lignification of the metaxylem begins after the plant's elongation is complete.

4. What do we mean by bordered pits?
Ans- Bordered pits are found in the pitted tracheids in the xylem, they have narrow openings and wide bases and are of flask like appearance.

5. What are the types of thickenings found amongst the xylem tracheids?
Ans- There are in total of 5 types of thickenings found in tracheids-
a) Annular
b) Spiral
c) Reticulate
d)Scalariform
e) Pitted

6. State the role of xylem parenchyma?
Ans- Xylem parenchyma functions as storage cells for the storage of fat, starch and other substances like tannins. It helps in radial conduction of water by ray parenchymatous cells.

7. Which part of the xylem is responsible for providing mechanical strength?
Ans- Xylem fibres are sclerenchyma cells present in the xylem tissue. These are responsible for the mechanical strength and are mostly found in the secondary xylem.

8. Why is Vessel considered a dead syncyte?
Ans- Vessels are formed by arranging multiple tube-like cells arranged in a column forming a syncyte (cell formed by fusion of cells) and as these cells are devoid of protoplasm. Hence, they are called dead syncyte.

9. What are primitive and advanced conducting elements?
Ans- ● Primitive conducting elements are tracheids - found in all vascular plants.
  ● Advanced conducting elements are vessels- found in angiosperms; helps in better conduction due to broader lumen.

10. Difference between tracheids and vessels.
Ans- Tracheids-
 ● These are elongated, lignified cells with the tapering end and narrow lumen that transport water or sap in the xylem.
 ● Tracheids are dead cells that are devoid of protoplasm.
 ● They are unicellular, imperforated with bordered pits at cross walls.
Vessels-
 ● Vessels work along with tracheids for the transportation of sap.
 ● Vessels are long, cylindrical tube- like structures formed by numerous cells arranged one upon the other (multicellular) known as vessel         members to form a dead syncyte.
 ● These cells have a central cavity and are surrounded by thickened walls formed due to lignin deposition.

11. Which elements of xylem are dead and living respectively?
Ans- Tracheids, vessels and fibres are the xylem elements that are devoid of any protoplast and hence are dead while xylem parenchyma is the only living xylem element with ample protoplast.