The vascular tissue system in plants includes a complex network of conducting tissues that are vital for conducting nutrients, water, minerals and various organic compounds throughout the plant. The two primary components of this system are phloem and xylem.
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What are Vascular Tissues?
Vascular tissues are conducting tissues that consist of phloem and xylem. Both phloem and xylem are complex tissues composed of many different cell types. The former is primarily concerned with food transport and the latter is concerned with water transport. Also, the distribution of vascular tissue varies considerably between different taxa as well as different organs.
The vascular tissues can also be primary or secondary in nature. The primary vascular tissue is derived from procambium which itself is produced by the apical meristems and also by the primary thickening of meristems in monocot stems. The secondary vascular tissue is derived from the vascular cambium in dicots and from the secondary thickening of meristems in a few monocots.
Phloem
Phloem has a complex role in translocating soluble organic compounds that are produced during photosynthesis. The primary phloem is formed by the apical meristem and the secondary phloem is formed by the vascular cambium. Phloem may develop precociously in regions that require a copious supply of nutrients, such as developing sporogenous tissue.
This complex tissue consists of conducting cells called sieve elements and associated specialised parenchyma cells called the albuminous cells or companion cells. Albuminous cells are mostly found in gymnosperms and seedless vascular plants. These two closely interdependent cells are produced from a common parent cell, but differ in their development. The sieve elements are linked axially to form sieve tubes. The two basic types of sieve elements are – sieve cells and sieve-tube elements. They are primarily differentiated by their pore structure.
The phloem parenchyma cells include the specialised companion or albuminous cells as well as unspecialized cells, sclereids and fibres. The companion cells act as companions for sieve-tube elements in carrying out the cellular functions. The cytoplasm of the companion cell is connected to sieve tube elements via plasmodesmata. The albuminous cell also has a similar functioning but is associated with the sieve cells. The further supportive cells include irregularly shaped sclereids and narrow, long fibres.
The walls of sieve elements are thin and possess characteristic regions called the sieve areas. They connect adjacent sieve elements. Also, the sieve areas consist of groups of pores and associated callose. In sieve cells, the sieve areas are distributed throughout the cell wall, but in sieve tube elements they are primarily localised on the adjoining end walls. Thus they form a sieve plate that links two axially linked elements of a sieve vessel.
Xylem
The primary function of the xylem is to act as a water conducting tissue. The water conducting cells are called tracheary elements and are typically linked to form vessels. They have thickened lignified cell walls and lack contents at maturity. The two basic types of tracheary elements are – tracheids and vessel elements.
Vessel elements possess large perforations in their end walls adjoining other vessel elements, whereas tracheids lack these perforations. These perforations can be of three types:
- Simple perforation plate (one opening)
- Scalariform perforation plate (several openings distinguished by a series of parallel bars)
- Reticulate perforation plate (reticulate mesh-like opening)
Helical and annular thickenings are typically seen in the first-formed elements or protoxylem. The later-formed primary tracheary elements (metaxylem) and also secondary tracheary elements typically possess bordered pits in their lateral walls. These pits vary considerably in shape, size and arrangement. The pits can be polygonal, oval or elongated (scalariform pitting), arranged in transverse rows (opposite pitting) or in tightly packed arrangement (alternate pitting).
Similar to phloem, the xylem also contains fibres and parenchyma cells. The xylem parenchyma are thin-walled unspecialised cells that serve the purpose of storage.
See more:What are Vascular Plants?
Difference between Xylem and Phloem
Xylem is a water-conducting tissue consisting of vessel elements, tracheids, xylem parenchyma and fibres. Whereas phloem is a food-conducting vascular tissue consisting of sieve tube elements, sieve cells, phloem parenchyma and phloem fibres.
Xylem plays a vital role in replacing water that is lost through photosynthesis and transpiration. Phloem accounts for protein transportation throughout the plant. Also, the movement of minerals and water in xylem is supported by a negative pressure whereas the movement in the phloem is supported by positive hydrostatic pressure.
Vascular and Cork Cambium
In some plants, the secondary vascular tissue is produced by vascular cambium, which usually becomes active at a short distance behind the stem apex. These unspecialised meristematic cells are initiated between the primary phloem and primary xylem. In woody plants, it typically generates secondary phloem (on the outer edge) and secondary xylem (on the inner edge).
Cork cambium is another meristem that accounts for secondary growth in the stems and roots of plants. Their main function is to produce a tough protective sheath called cork for the plants.
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