(Structure, Composition & Classification of Primary and Secondary Xylem)
What is vascular tissue?
The tissue associated with conduction of water, minerals and food materials in plants are called vascular tissue. Plants with a well-developed conductive system are thus called as ‘vascular plants’. Vascular plants are also known as ‘Tracheophyta’ (‘trachaea’ = vessels, a component of xylem, ‘phyta’= plants).
The vascular systems in plants composed of two types of tissues
(1). Xylem: Tissue for the conduction of water and minerals.
(2). Phloem: Tissue for the conduction of food materials.
Learn more: Difference between Xylem and Phloem
Both the xylem and phloem are complex tissues composed of more than one types of cells. Xylem and phloem are closely organized in plants. The vascular bundles found in the primary structures of plants are formed by the association of xylem and phloem.
What is xylem?
The xylem is one of the conductive tissues in plants. It is a complex tissue composed of many types of cells. The term xylem was proposed by Nageli (1858) and he derived the word from a Greek word ‘xylos’ meaning wood. The main function of xylem is to conduct water and minerals from roots to leaves. The secondary xylem also provides mechanical support due to the presence of thick lignified cell wall.
What are the components or elements of xylem?
The xylem composed of four types of cells. Among these cells, some cells are living and some are dead.
The four elements of xylem are:
(3). Xylem Fibres
(4). Xylem Parenchyma
Tracheids are the fundamental cell type in the xylem. They are elongated tube like cells with tapering ends and chisel like in appearance. The cells are non-living at their maturity and the mature cells are empty without protoplast. They have highly lignified secondary cell wall and the cells angular and polygonal in cross section. The average length of tracheid is 5 – 6 mm. Major portions of the cell wall of tracheids are perforated with pits. They also possess pit pairs between two adjacent tracheids at their common walls. Pits may be simple circular pits or advanced bordered pits.