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.