Vascular Tissue System
- It is a characteristic of the higher cryptogams ie pteridophytes and all groups of the spermatophytes (Angiosperms and Gymnosperms).
- The vascular tissue system consists of a number of vascular bundles which are found to be distributed in the stele.
- The stele is a central cylindrical portion of the stem and the root, commonly surrounded by the endodermis and consists of vascular bundles, pericycle, pith and medullary rays.
- Each vascular bundle consists of xylem and phloem tissues with or without cambium.
- The function of this system is to conduct water and nutrients from roots to leaves through the xylem and translocation of prepared carbohydrates from leaves to other storage organs and growing regions of the plant body through the phloem.
- Structure of xylem It is a complex tissue.
- Most of its components are lignified and dead. Only xylem parenchyma is living with the Xylem fibres component.
Xylem consists of following components,
- The tracheid is a fundamental cell type in the xylem.
- It is an elongated tube-like cell having tapering, rounded or oval ends and hard and lignified walls.
- The walls are not much thickened.
- At maturity, it is non-living ie without protoplast.
- In the transverse section, the tracheid is typically angular, though more or less rounded forms occur.
- The smallest tracheids form protoxylem and the bigger ones form metaxylem.
- Tracheids have different types of bursts in them and can be classified as annular, spiral, scalariform, reticulate or pitted tracheids.
- The cell cavity or lumen of a tracheid is large and without any contents. Tracheids alone make the xylem of ferns and gymnosperms, while in the xylem of angiosperms they occur associated with the vessels and other xylary elements.
- The function of the tracheids in the conduction of water and mineral salts.
- The thick and firm walls of the tracheids also aid in support.
- The organs in which fibres are absent, the tracheids play a prominent role in providing support and giving mechanical strength.
Fibres and fibre tracheids –
- In the phylogenetic growth of the fibre, the wall thickness is increased while the diameter of the lumen going to decreases.
- In most types, the length of the cell also decreases and the number and size of the pits found on the walls also decreases.
- Sometimes the lumen of the cell becomes too much narrow or altogether obliterated and simultaneously pits become quite small in size.
- At this stage it is assumed that very little or no fluid conduction through such cell types, normal fibers form.
- Between such cells (i.e. fibres) and normal tracheids, there are many transitional forms that are neither typical fibres nor typical tracheids. These transitional types are designated as fibre tracheids.
- In the phylogenetic development of the tracheid, the diameter of the cell has increased and the wall has become perforated by large openings.
- Because of the presence of perforations in the partition walls, they form long tubes through which water is being conducted from root to leaf i.e. vessels are like pipes. They do not have any cross walls.
- They are lignified.
- The secondary walls of vessel elements develop in a wide variety of patterns.
- The secondary thickenings are deposited in the vessels as rings, continuous spirals or helices, with the individual coils of helix here and there, interconnected with each other, giving the wall a ladder-like appearance.
- Such secondary thickenings are called annular, spiral or helical and scalariform respectively.
- Vessels are characteristic of the angiosperms.
Xylem parenchyma –
- It is a living component of the vascular bundle.
- Cells are thin-walled.
- In secondary xylem, such cells occur vertically more or less elongated and placed end to end known as wood parenchyma/ xylem Nucleus parenchyma.
- The xylem parenchyma cells are noted for the storage of food in the form of starch or fat. Tannins, crystals and various other substances also occur in xylem parenchyma cells.
- These cells assist directly or indirectly in the conduction of water upward through the vessels and tracheids.
- The xylem as a whole is the water-conducting tissue of the plants.
- The tracheids and the vessels are chiefly concerned with water conduction.
- The wood parenchyma and the xylem ray parenchyma serve secondarily as storage of food like Starch, oils etc and also secondary metabolites like crystals, gums, resins etc.
- Xylem also serves as mechanical tissue holding the plant body upright.
The phloem is the principal conducting tissue of the elaborated food substances in the vascular plants.
Phloem like xylem is a complex tissue, and consists of the following elements,
Sieve element –
- The sieve elements constitute the chief conducting elements of the phloem.
- The sieve elements are classified into two types-sieve cells and sieve tubes.
- In the tissue, they overlap each other, and the sieve areas are usually numerous on these ends.
- The sieve cells are present in the phloem of the lower vascular plants-the pteridophytes and the gymnosperms, where they form the sole conducting elements in the phloem.
These are single cells that do not unite to form multicellular tubes.
They are commonly long and slender and they are tapering at their ends.
In the tissue, they overlap each other, and the sieve areas are usually numerous on these ends.
Companion cells –
- The companion cell is a specialized type of parenchyma cell that is closely associated in origin, position and function with sieve tube elements.
- When seen in the transverse section, the companion cell is usually a small triangular, rounded or rectangular cell beside sieve tube elements.
- These cells are living having abundant granular cytoplasm and a prominent elongated nucleus which is retained throughout the life of the cell.
- The companion cells do not contain starch. They live only as long as the sieve tube elements with which they are associated and they are crushed with those cells.
Phloem fibres –
- In many flowering plants, fibres form a prominent part of both Phloem primary and secondary phloem.
- Only single pits are found on the walls of phloem fibres. The walls of Lumen Lacuna may be lignified or non lignified (cellulose).
- Because of the strength of strands of phloem fibres, they have been used for a long time in the manufacture of cards, ropes, mats and fibre (bast fibre) cloth.
- The fibre used in this way has been known since early times as bass or bast and this way the phloem fibres are also known as bast fibres.
Phloem parenchyma –
- The phloem contains parenchyma cells that are concerned with many activities characteristic of living parenchyma cells such as storage of starch, fat and other organic substances. eg tannins, resins etc.
- The parenchyma cells of the primary phloem are elongated.
- There are two systems of parenchyma found in the secondary phloem. These systems are vertical and horizontal.
- Vertical – The parenchyma of the vertical system is known as phloem parenchyma.
- Horizontal – The horizontal parenchyma is composed of phloem rays.
- The phloem parenchyma occurs as a component of the phloem tissue of gymnosperms and the dicotyledons angiosperms.
- It is absent in the phloem of monocotyledons.
Types of Vascular Bundle
A vascular bundle consists of a strand-like portion having xylem and phloem of the primary vascular system.
According to the arrangement of xylem and phloem in the vascular bundles, they are being arranged in the following main types,
Function Of Vascular Bundle –
- The main function of this tissue system is to conduct water and other nutrients from roots to leaves through the xylem.
- While translocation of prepared carbohydrates from leaves to other storage organs and growing regions of plant body through the phloem.
- Vascular bundles form continuous and inter-connected system in the different organs of the plants.
- Xylem also gives mechanical strength and support to the plant body.
- Cambium is lateral meristematic tissue that causes secondary wood formation in dicots and conifers.
Radial Vascular Bundle
- In this type, the xylem and phloem are found as separate patches on alternate radia.
- This type is characteristic of roots.
- The radial bundle is the most primitive type of vascular bundle.
Conjoint Vascular Bundle
In this type, xylem and phloem occur side by side on the same radius.
According to the position of phloem, these are recognized into two types,
Collateral Conjoint Vascular Bundle
- The xylem and phloem lie together on the same radius in the position that the xylem lies inwards and the phloem outwards.
- Here the phloem occurs on one side of the xylem strand.
- It is further divided into two sub-types. ‘Conjoint collateral open’ and ‘conjoint collateral closed’.
Conjoint collateral open
- If a strip of cambium is present between the xylem and phloem, such bundles are called open.
- It is a characteristic of dicotyledons.
Conjoint collateral closed
- In this type of vascular bundle, the strip of cambium is absent between the xylem and phloem.
- Such bundles are called closed. It is a characteristic of monocotyledons.
Bicollateral Conjoint Vascular Bundle
- In such bundles, the phloem is found to be present on both sides of the xylem.
- Simultaneously two cambium strips also occur.
- Various elements are arranged in the following sequence- outer phloem, outer cambium, xylem, inner cambium and inner phloem.
- Such bundles are commonly found in the members of Cucurbitaceae.
- Such bundles are always open.
Concentric Vascular Bundle
- In this type, one kind of vascular tissue (xylem or phloem) is surrounded by other vascular tissue.
- Such bundles are always closed.
Amphivasal Concentric Vascular Bundle –
- These are also known as centrophloeic orleptocentric Concentric Vascular Bundle.
- If the xylem surrounds the phloem it is called the amphiversal bundle as found in Dracaena, Yucca and other monocots and some dicots.
Amphicribal Concentric Vascular Bundle –
- These are also known as centroxylic orhydrocentric Concentric Vascular Bundle.
- If the phloem surrounds the xylem, it is called amphicribal bundle as found in many ferns.