Plants can be divided into two large groups, vascular or tracheophyte plants , also known as higher plants, and non-vascular plants. Non-vascular plants have no roots or vascular tissue. Tracheophytes, on the other hand, have roots and conducting tissues specialized in transporting water, nutrients and metabolites.
Within these vascular tissues two types, phloem and xylem , each with a different structure and a specific function are distinguished . It is thought to have appeared 450 million years ago and allowed plants to colonize terrestrial habitats.
Both the phloem and xylem have a tubular structure and in addition to transporting water, minerals and nutrients, they also provide structural support and carry chemical signals that allow communication between different parts of the plant.
Together they form the vascular bundles (stelas) with different arrangement between different parts of the plant and between different types of plants. Generally the phloem is located in the external part of the vascular bundles and the xylem in the inner part .
The process of formation of vascular bundles occurs in several steps. First protoprolloem and protoxilema are formed from the procámbium, a tissue of the meristem that appears in the embryo and in the apical meristems of adult plants. Then, from the fascicular cambium, the metaxilema and metafloema are formed, and later, if the plant has secondary growth, the secondary xylem and phloem will form from the vascular cambium.
The phloem, also called sifting tissue , is the conductive tissue responsible for transporting what is known as processed sap , which is composed mainly of water, sugars, minerals and other organic substances (hormones, amino acids, etc.).
The transport in the phloem is known as translocation and is multidirectional . It is produced by the positive hydrostatic pressure created by the concentration gradient of the organic substances within the phloem, mainly sugars; In this way the flow is produced from the sources of sugars towards the zones of use .
During periods of growth, usually during spring, the storage organs, for example the roots, are the source of sugars that are transported through the phloem to the growth meristems. When leaves and photosynthetic parts are the source of sugars, transport is given in the opposite direction to storage organs, such as roots and fruits.
The phloem is located in the external part of the vascular bundles in the majority of plants, being able to appear also in the internal part in some dispositions, for example in the antifloica disposition. In the phloem we can find two types of cells:
- Conductive or sieve elements : sieve tubes and sieve cells.
- Non-conductive elements : cells of the parenchyma (accompanying cells) and sclerenchyma (fibers and sclereids).
The sieve tubes are formed by a longitudinal series of anucleated living cells , called sieve tube elements, which have a cell wall thickened with deposits of callus . The elements of the sieve tube are interconnected to each other by the sieve plates , located at the apical ends of each cell and which connect the cytoplasm of contiguous cells through their numerous pores.
The sieve tubes are typical angiosperm , gymnosperm while the conductive elements are called sieve cells. The cribous cells communicate with each other through fields of pores called cribous areas, similar but of different structure to the sieve plates.
The metabolism of the conductive elements of the phloem depends on several supporting cells that are located around it. Support cells are specialized parenchyma cells; Among them the accompanying cells, the transfer cells and the intermediary cells, whose occurrence and arrangement can be very variable. Sclerenchyma and sclerid fibers can also be observed that provide protection and support.
The xylem is a lignified plant tissue that is responsible for transporting the so-called raw sap , composed of water and mineral salts from the root. Wood, the main mechanical support fabric in most plants, is mainly composed of xylem, which is why it is also known as wood , although it is present throughout the plant.
Transport in the xylem, unlike the phloem, is unidirectional and the movement occurs mainly by negative pressure . The conductive elements of the xylem are known as tracheal elements and can be of two types, tracheids and vessel elements . Both are dead cells in their functional maturity and their cytoplasm disappears.
The elements of the vessels are joined longitudinally and form the vessels of the xylem. There may be lateral flow through pits on the cell walls but the main flow is longitudinal through porous zones known as perforation plates . The tracheids are thinner, with less internal volume, and do not have perforation plates, only the walls dotted, reason why in general they have less capacity of conduction.
The tracheids are the only tracheal elements of the gymnosperms, in the angiosperms appear of the two types. Both have a thickened and lignified wall with thickenings in characteristic ring, spiral, reticular or dotted arrangements believed to facilitate the flow of water by capillarity .
In addition to the tracheal elements, in the xylem there are also parenchyma cells whose main function is the storage of water, starch and nitrogen . Sclerenchyma fibers also appear and sclereids that give protection and support.
|Principal function||Translocation: transport of nutrients, such as sugar and amino acids, from leaves to storage organs, and from leaves and storage organs to growing areas.||Transport of water and mineral salts from the roots to the aerial part of the plant.|
|Other Functions||It forms the vascular bundles next to the xylem.||It forms vascular bundles and forms the main mechanical support fabric of the plant.|
|Elements||They are living cells, although the conducting elements (sieve tubes and sieve cells) have no nucleus and their metabolism depends on support cells.||The conducting elements are dead cells at maturity (tracheids and vessel elements).|
|Location||On the outside of the vascular bundles.||In the center of the vascular bundles.|