Hydathode – Water Stomata (Structure and Functions of Hydathodes)
What are Hydathodes?
Hydathodes are specialized pores (openings) particularly present on the leaf margins, which exudes or secretes drops of water. The exudation of water as drops from the tip or margin of the leaves is called guttation. The process of guttation is facilitated by the hydathodes.
Ø They are also called as Water Stomata because they structurally resemble stomataand they facilitate guttation (secretion of droplets of water from the pores of plants).
Ø Hydathodes are commonly found in Angiosperms, especially in grasses.
Ø They are also present in some other plants such as water hyacinth, balsam, roses, Hibiscus and rarely in some non-angiospermic plants.
Phloem Structure, Composition & Classification of Primary and Secondary Phloem
What is phloem?
Phloem is a complex tissue system in plants. It is the food conducting tissue of vascular plants. Together with xylem, they form the vascular tissue system. The phloem composed of several types of cells among which some are living cells and some are dead. The term ‘phloem’ was introduced Nageli (1853) from a Greek word ‘phloios’ meaning ‘bark’. The ‘bark’ is a non-technical term describing all tissue outside the secondary xylem of the plant. Botanically the bark includes secondary phloem, cortex, primary phloem and periderm. The current post describes the structure, composition and classification of phloem.
Location of phloem in plants:
Ø Usually, the phloem is situated external to xylem.
Ø In leaves, the phloem is located on the abaxial side (lower side).
Ø In some plants (members of Cucurbitaceae and Convolvulaceae), the phloem is present on both external and internal to xylem. Such a vascular bundle is called bicollateral vascular bundle.
Ø Phloem present internal to the xylem is called ‘internal phloem’ or intra-xylary phloem.
Ø Phloem located external to the xylem is called ‘external phloem’.
Tyloses (singular – tylosis) are the balloon like outgrowth of parenchymatous cells to the lumen of tracheids or vessels of the secondary xylem.
The axial and ray parenchyma of the xylem develops protrusion and these protrusions enter or invade into the lumen of tracheids or vessels through pits. This invasion of parenchyma occurs when tracheids or vessels become inactive or damaged.
Who discovered Tyloses?
Malpighi (1686) for the first time reported the ‘balloon-shaped sacs’ in the lumen of vessels of heartwood and he named it as tyloses based on the Greek word ‘Thyllen’ meaning ‘bag’ or ‘container’.
Xylem (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.
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.