The conductive tissues of plants are responsible for orchestrating the long-distance passage of nutrients through the different structures of the plant organism. Plants that present conductive tissues are called vascular plants.
There are two classes of conductive tissues: xylem and phloem . The xylem is made up of tracheal elements (the tracheids and tracheas) and is responsible for the transport of water and minerals.
The phloem, the second type of conductive tissue, is formed mainly by sieve elements and is responsible for conducting the products of photosynthesis , redistributing water and other organic materials.
Both types of conductive cells are highly specialized for their function. The development pathways that allow the formation of the conductive tissue are well organized processes. In addition, they are flexible to environmental changes.
This conductive system has contributed significantly to the evolution of terrestrial plants, about a hundred million years ago.
The vascular tissue of plants
As in animals, plants are made up of tissues. A tissue is defined as an organized grouping of specific cells that fulfill specific functions. Plants are composed of the following main tissues: vascular or conductive, growth, protective, fundamental and support tissues.
Vascular tissue is similar to the circulatory system of animals; It is in charge of mediating the passage of substances, such as water and molecules dissolved in it, through the different organs of the plants.
Classification of xylem according to its origin
The xylem forms a continuous tissue system for all the organs of the plant. There are two types: the primary, which is derived from procambium. The latter is a type of meristematic tissue – this tissue is young, undifferentiated and is located in the regions of the plants that are destined for continuous plant growth.
The origin of the xylem can also be secondary when it is derived from the vascular cambium, another meristematic plant tissue.
Characteristics of the xylem
Conductive cells in the xylem
The main conducting cells that make up the xylem are the tracheal elements. These are classified into two main types: tracheids and tracheas.
In both cases, the morphology of the cells is characterized by: elongated shape, presence of secondary walls, lack of protoplast at maturity, and may have pits or alveoli in the walls.
When these elements mature, the cell dies and loses its membranes and organelles. The structural result of this cell death is a thick, lignified cell wall that forms hollow tubes through which water can flow.
Tracheids are long, thin cellular elements, shaped for use. They are located overlapping each other in vertical rows. The water passes through the elements through the pits.
In vascular plants lacking seeds and gymnosperms the only conductive elements of xylem are the tracheids.
Compared to tracheids, tracheae are generally shorter and wider, and like tracheids they have pits.
In the tracheas, there are holes in the walls (regions that lack both primary and secondary walls) called perforations.
These are located in the terminal zone, although they can also be in the lateral regions of the cell walls. The region of the wall where we find the perforation is called the perforated plate. The xylem vessels are formed by the union of several tracheae.
Angiosperms have vessels made up of both tracheas and tracheids. From an evolutionary perspective, tracheids are considered ancestral and primitive elements, while tracheas are derived, more specialized and more efficient plant characteristics.
It has been proposed that a possible origin of the tracheas could have occurred from an ancestral tracheid.
The xylem has two main functions. The first is related to the conduction of substances, specifically water and minerals throughout the body of vascular plants.
Second, thanks to its resistance and the presence of lignified walls, the xylem has support functions in vascular plants.
Xylem is not only useful for the plant, it has also been useful for humans for centuries. In some species, the xylem is wood, which has been an essential raw material for societies and has provided different types of structural material, fuel and fiber.
Classification of phloem according to its origin
Like xylem, phloem can be of primary or secondary origin. The primary, called the protofloem, is usually destroyed during the growth of the organ.
Conductive cells in the phloem
The main cells that make up the phloem are called sieve elements. These are classified into two types: sieve cells and the elements of the sieve tube. “Sieve” refers to the pores that these structures have to connect with adjacent protoplasms.
Sieve cells are found in pteridophytes and gymnosperms. Angiosperms, for their part, have the elements of the sieve tubes as conductive structures.
In addition to conductive elements, the phloem is made up of highly specialized cells, called companions and parenchyma.
Phloem is the type of conductive element responsible for the transport of the products of photosynthesis, sugars and other organic materials. The journey occurs from the mature leaves to the growth and nutrient storage areas. In addition, the phloem also participates in the distribution of water.
The phloem transport pattern occurs from the “source” to the “sink”. The source is the areas where the photoassimilates are produced, and the sinks include the areas where these products will be stored. The sources are generally leaves and the sinks are roots, fruits, unripe leaves, among others.
The correct terminology to describe the transport of sugars in and out of the sieve elements is loading and unloading of the sieve element. Metabolically, the discharge of the phloem requires energy.
Compared to the normal speed of diffusion, solute transport occurs at much higher speeds, with an average speed of 1 m / h.
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