Complex Permanent tissue - Xylem and its components

 

• The complex permanent tissue is a heterogeneous tissue and made up of more than one type of cell.

• Xylem and phloem are examples of complex permanent tissue because they have more than one type of cell in their constitution. 

• Xylem is the conducting tissue that first appeared in the pteridophytes and was involved in  transportation of the water from root  to the aerial part of the plant.

• The term was coined by Nageli.

• The main components of xylem are tracheids, vessels, xylem fiber and xylem parenchyma.

• Tracheids and vessels are tracheary elements because they are principally involved in conduction. 

• Tracheids are elongated dead cells and have lignified walls with a wide lumen which is involved in conduction of water.

• Tracheids may be  circular and polygonal when we observe the transverse section of tracheids. The area of the primary wall and Middle lamella form the pit membrane.

• Depending upon the morphology of its chamber or cavity, a pit is called simple or border. In surface view, the pit may appear circular, elliptical or angular. 

• Usually pits occur in pairs and each are present at  the two adjacent cells. When a pit is single, they are called blind pits. 

• The unthickened area in the Wall of tracheids is permeable to water. The water passes from one tracheid  to another quite rapidly.

• Tracheids are the only conducting or tracheary element in the non flowering plant.

• 95% of Gymnosperm wood is formed of the tracheids and In angiosperm hardly 5% of wood consist of the tracheids.

Vessels -

• These are elongated water conducting channels formed by a large number of empty lignified cells called vessels element.

• Due to their formation from a number of cells, vessels are syncytes. In the vessel, the end plates of individual vessel elements are perforated  to make  continuation for pipe or tube.

• The perforation of the two types is simple and multiple. In simple perforation, only a single pore is present. It is a common type of perforation. In multiple perforation,there are multiple pore.

Remember - 👌🏽👌🏽Tracheids are the only components of xylem that are dead while the vessel, xylem fiber and xylem parenchyma are living. 

• The transfer section shows that the vessels in xylem are circular or oval in monocot but these are polygonal in Dicot.

• Vessels are characteristic of angiosperms but  they occur sometimes in vascular plants and plates like Equisetum, Selaginella etc.

• The plate degeneration of  vessels provide an uninterrupted flow of water and flow the  mineral to long distance.

• They are therefore more efficient as compared to tracheids. Like tracheids, vessels  provide mechanical support to the plant body. 

Xylem fiber or woody fiber -

• These are made up of sclerenchyma and present inside the xylem.

• Xylem fiber possesses thick lignified walls, narrow lumen and simple pits. They are often longer than tracheids. xylem fiber are of two types - libriform fiber and fiber tracheids.

• Xylem fiber provides mechanical strength. Their quantity makes the wood hard or soft. 

• Xylem Parenchyma is a parenchyma present inside xylem.

• Primary xylem parenchyma is usually made up of non-lignified cells. Cells become thick walls and even lignified in secondary xylem.

• They Store food and perform photosynthesis. They stored food in the form of proteín, fat, tannin, and crystal.