Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. The xylem transports water and nutrients absorbed from the roots to the rest of the plant. The phloem transports energy in the form of glucose and other elements produced by photosynthesis to the rest of the organism.
Between the xylem and phloem is a meristem called the vascular cambium. This tissue divides off cells that will become additional xylem and phloem. In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth. Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. Both the production of wood and the production of cork are forms of secondary growth.
Since the xylem and phloem function in the conduction of water, minerals, and nutrients throughout the plant, the cells in vascular tissue are typically long and slender. As the plant grows, new vascular tissue differentiates in the growing tips of the plant. The new tissue is aligned with existing vascular tissue, maintaining its connection throughout the plant. The vascular tissue in plants is arranged in long, discrete strands called vascular bundles. These bundles include both xylem and phloem, as well as supporting and protective cells.
There are three different types of vascular bundles.
- Radial Bundles
Xylem and phloem strands are located on alternate radii in radial vascular bundles. These are mainly found in roots.
- Conjoint bundles
Xylem and phloem combine together into one bundles, Xylem lies towards the centre and phloem towards the periphery. There are two types of conjoint bundles.
- Collateral: Xylem and phloem lie on the same radius, xylem towards the centre and phloem towards the periphery. When cambium is present in collateral bundles, such bundle is called open, e.g. in dicot stems and collateral bundle without cambium is called closed, e.g. in monocot stems.
- Bicollateral: In this type of bundle, the phloem strands are present on both outer and inner side of xylem.
- Concentric Bundles
In this type of vascular bundle, one tissue is completely surrounded by the other. These are of two types Amphivasal and Amphicribral.
- Amphivasal: In this type of vascular bundle xylem surrounds the phloem, e.g. Dracaena.
- Amphicribral: In this type, phloem surrounds the xylem, e.g. in Ferns.
The types of cells of vascular tissue are as follows:
The basic function of xylem is to transport water from roots to shoots and leaves, but it also transports some nutrients. There are two types of conducting cells in xylem, tracheids and vessel elements. Both have thick lignified secondary walls and are dead at maturity. These cells create hollow cylinders that have high tensile strength. Materials moving within the xylem are under tension. Therefor the high tensile strength of the xylem cells keeps them from collapsing. Transport in the xylem occurs in one direction : roots->stems->leaves.
Long, slender cells with overlapping, tapered ends. Water moves between tracheid cells via the bordered pits. Bordered pits are thin areas in the cell walls where only primary cell wall material has been deposited. Tracheids are the more primitive (less specialized) of the two xylem cells. They are found in most woody, nonflowering plants.
- Vessel Elements
Short, wide cells arranged end to end. Their end walls are partially or wholly dissolved allowing them to form long, hollow tubes up to 3 meters long. The larger diameter and lack of end walls allows vessel elements to transport water more rapidly. Vessel element are evolutionarily more advanced than tracheids. They are found in angiosperms and are one of the major reasons why angiosperms the dominant land plant.
- Xylem Fibers and Xylem Parenchyma
Fibers lend support to the woody tissues (especially in plants with tracheids) while the parenchyma cells function to store metabolites, or function in secretion (resin ducts and laticifers).
Phloem transports dissolved organic material throughout the plant. Transport within the phloem is from source to sink. This means that the direction of movement of materials within the phloem may change over time. This movement depends on the time of year and age of the plant. Phloem cells are alive at maturity, mainly because movement of materials within the phloem requires energy. Also, the materials moving within the phloem are under pressure, which means that the cell walls of the phloem cells do not have to have as great a tensile strength.
- Sieve Cells
More primitive phloem conducting cells of ferns and conifers. Sieve cells are long and tapered with overlapping ends. They have sieve areas, fields of pores scattered over their cell wall surface. These areas allow direct contact between the protoplasts of adjacent cells. The pores are surrounded by callose, a complex carbohydrate that can block the pore opening after injury. Associated with the sieve cells are albuminous cells that play a role in aiding the movement of materials within the phloem.
- Sieve Tube Members
More advance phloem conducting cells of angiosperms. Sieve tube members are short and wide, and arranged end to end into sieve tubes. The sieve pores are large and are concentrated along the end walls of adjacent sieve tube members. These specializations allow solutes to move more rapidly in sieve tube members and sieve cells. At maturity the nuclei in the sieve tube members disintegrates, the ribosomes disappear, and the tonoplast (vacuole membrane) breaks down. Mitochondria and plastids are still present. Sieve Tube Members are always associated with companion cells which control the metabolism of the cells. These two cells are connected by numerous plasmodesmata. The companion cells aid in the movement of materials into and out of the sieve tube members. Sieve tube members also contain P-protein, which stands for Phloem-protein. This protein is located along the longitudinal walls of the cells. Some sieve tube members also contain a glucose polymer called callose. Both P-protein and callose are responsible for sealing wounds in the sieve tubes.
All these vascular tissues within a particular plant together constitute the vascular tissue system of that plant.