Ultra structure and function of phloem
Ultra structure and function of phloem:-
Definition of Phloem:-
> The complex tissue known as phloem is the transportation system for soluble organic molecules in vascular plants.
> The other component of the vascular plant transport system, the non-living xylem, transports water and minerals from the root. The phloem is composed of living tissue that actively transports sugars to plant organs such as the fruits, flowers, buds, and roots using turgor pressure and energy in the form of ATP. Translocation is the term for this movement process.
> Phloem, whose name comes from the Ancient Greek word phloiós, which means “bark,” is the innermost layer of the bark in trees. Carl Nägeli invented the term phloem in 1858.
Types of Phloem:- There are two types of phloem:
i. Primary Phloem:-
> It is the type of phloem produced by the primary meristem of a vascular plant. Phloem that originates from the procambium during primary growth is known as primary phloem. Protophloem or metaphloem are the components of the primary phloem.
> The primary phloem is present in the primary plant body. As opposed to the secondary phloem, which develops inside the primary phloem, it occurs in the periphery. The secondary phloem has a radial system, whereas the primary phloem does not. The primary phloem has less number of phloem fibres, sieve tubes, and phloem parenchyma compared to the secondary phloem in cellular components. Additionally, the primary phloem often lacks sclereids.
ii. Secondary Phloem:-
> The secondary meristems of a vascular plant produce the secondary phloem. The meristematic tissue, vascular cambium, is involved in this growth. The secondary phloem develops from the vascular cambium during secondary growth. The increase in plant width, mainly in trees, is caused by secondary growth.
> The stems and roots contain the secondary phloem, which develops inside the primary phloem. The secondary phloem has a radial arrangement of phloem rays. Phloem fibres (also known as bast fibres), sieve tubes, phloem parenchyma, and sclereids are all found in more significant numbers in secondary phloem than in primary phloem. The secondary phloem’s sieve tubes are broader but shorter. As a result, photosynthate moves more quickly through secondary phloem sieve tubes than primary phloem sieve tubes.
Structure of Phloem Tissue:-
i. Sieve Elements:-
> Sieve element is the most highly specialised plant cell type. They are different in that they lack a nucleus at maturity and other organelles, including cytosol, ribosomes, and Golgi apparatus, which maximises the amount of space that can be used for material translocation. The elongated, narrow cells that make up the sieve elements are connected to form the phloem’s sieve tube structure.
> The ‘sieve member,’ found in angiosperms, and the more primitive ‘sieve cells,’ which are connected to gymnosperms, are the two primary forms of sieve elements. Both are derived from a common form of “mother cell”.
> Sieve Plates:- Sieve plates, modified plasmodesmata, are found at the junctions between sieve member cells. Sieve plates are comparatively large, thin sections of pores that make it easier for materials to move between element cells. The sieve plates also serve as a barrier to stop sap loss when the phloem is cut or harmed, which is frequently done by an insect or herbivore.
> Gymnosperms contain more primitive sieve elements than angiosperms do. Instead of sieve plates at the tapering end of the cell walls, they have numerous pores that allow material to pass through freely.
ii. Companion Cells:-
> Each sieve element cell generally has a “companion cell” in angiosperms and an albuminous cell (“Strasburger cell”) in gymnosperms.
> Companion cells possess a nucleus, a thick cytoplasm, a large number of ribosomes, and a large number of mitochondria. This indicates that because the sieve element lacks the necessary organelles, the companion cells can carry out the metabolic processes and other cellular tasks.
> Thus, companion cells are responsible for supplying energy for the movement of materials throughout the plant and the sink tissues, and for the facilitation of loading sieve tubes with photosynthesis-related products and unloading at the sink tissues.
iii. Phloem Parenchyma:- A group of cells called the parenchyma serves as the “filler” in plant tissues. They contain cellulose walls that are thin but flexible. Starch, lipids, proteins, tannins and resins in some plants are stored primarily by the parenchyma within the phloem.
iv. Phloem Sclerenchyma:-
> Sclerenchymatous cells are found in phloem fibres. The phloem, which gives the plant stiffness and strength, is mainly supported by the sclerenchyma. Both fibres and sclereids, two types of sclerenchyma, have a strong secondary cell wall and are often dead when they reach maturity.
> The bast fibres are thin, elongated cells with thick cellulose, hemicellulose, and lignin walls and a limited lumen (inner cavity) that support the tension strength while allowing the phloem to be flexible.
Function of Phloem:-
i. Water-based sap contains a lot of carbohydrates produced during photosynthesis. These sugars are sent to storage organs like tubers or bulbs or non-photosynthetic plant sections like the roots by phloem. The phloem, which transports sap, comprises still-living cells compared to the mostly-dead xylem.
ii. Phloem is a class of complex permanent tissue that develops into a conductive or vascular system in the plant’s body. It transports the prepared nutrients from the leaves to the growing areas and storage organs. It is also considered that vascular plants’ phloem sap contributes to the transmission of informative signals.