Xylem and phloem

The xylem and phloem make up the conductive system of plants called vascular. They are present in almost all plants, the only exception being the group of bryophytes, which is avascular.

Conductive tissues are present in all plant organs, they transport water, mineral salts and also organic substances from photosynthesis.

In addition, they can reserve substances in their cells.

Index [ hide ]

  • What is xylem?
  • Cell types
  • Primary xylem
  • Secondary xylem
  • What is phloem?
  • Cell types
  • Primary phloem
  • Secondary phloem

What is xylem?

The xylem can also be called “wood”. E le  is responsible for the transport of inorganic substances, formerly called “crude sap.”

Inorganic substances are basically composed of water and mineral salts, they are transported upwards, from the root to the leaves.

Cell types

The xylem has four types of cells :  vessel elements,  tracheids, parenchyma and fibers.

The vessel elements  have secondary walls that cause them to die at maturity. The walls have perforation plates at the ends, which connect one cell to another, forming a column within the body of the plant.

The flow of water in the vessel elements is continuous and therefore more efficient. This is possible because the  perforation plates at  the ends of the cells prevent barriers.

Tracheids are living cells, therefore, without a secondary wall. They do not have the perforation plate at the end, but points along the entire primary cell wall.

The passage of water through the points prevents the formation of bubbles inside the plant, which can cause embolism and interrupt the flow.

When embolism occurs, pressure causes the points in that location to close and divert the flow of water to the vessel element on the side.

It is important to remember that pteridophytes and gymnosperms generally have only tracheids, parenchyma and fibers, that is, the vessel elements are structures present mainly in angiosperms.

Xylem cells

Primary xylem

The primary xylem originates from procambium, being formed  during primary growth,  while the plant grows in length.

As it is formed in the early stages of plant development, the cells of the primary xylem can still be alive, they are usually elongated, with dense cytoplasm, have a well-defined nucleus and the primary cell walls.

The first xylem that forms is called a protoxylem, after differentiation it becomes a metaxylem, and the deposit of lignin begins to be more intense on the walls causing the cells to die.

Primary Xylem – protoxylem and metaxylem

Secondary xylem

The  secondary xylem  comes from the vascular exchange, a secondary growth meristem, that is, it is at this moment that the plant grows laterally, increasing its thickness.

Did you know that the wood we know is nothing more than the secondary xylem of a tree? Because of this fact, we can see how resistant the xylem can be.

What is phloem?

The phloem  is also a complex tissue, its main function is to transport organic sap which consists of amino acids, lipids, hormones and proteins.

This transport occurs in the opposite direction to the xylem, starting at the leaves and going towards the rest of the body.

Cell types

The phloem consists of five types of cells : cells Sieve, sieve tube element, parenchyma (not specialized and companion cells), and sclereids fibers.

Sieved cells and sieved tube elements are called sieved elements, they have many pores that clump together at the ends and that is what gives them the name sieved.

All phloem cells are alive and it is the pores that allow protoplasts from two different cells to communicate.

The sieved cells have finer pores  than the sieved tube elements  and distributed throughout the wall. In the sieved tube elements the pores are concentrated forming the sieved areas and are also at the ends of the cells, forming the sieved plate.

Phloem cells

Primary phloem

The primary phloem originates from the procambium during the early development of the plant.

It is also differentiated into protofloem and metafloem, as well as the primary xylem.

Secondary phloem

The  secondary phloem  is from the cambium during secondary growth.

The vascular exchange produces cells unevenly and that is why, in secondary growth, the amount of secondary xylem is much higher than that of secondary phloem.

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