Biology

Xylem and phloem: the circulatory system of plants

Animals have a system of veins, arteries and capillaries to transport oxygen and nutrients through the body through the blood. This system works thanks to the fact that we have an organ that compresses the blood to push it through the circuit, the heart. For their part, plants have similar needs to transport substances, but no organ that mechanically pushes liquid through the stem. In this article we will talk about the mechanism that plants use to transport resources, composed of xylem and phloem .

Transport of substances in plants: xylem and phloem

Xylem and phloem
Xylem and phloem

Plants need a series of resources for their development: water, nutrients, oxygen, carbon dioxide and sunlight. Through the roots they absorb water and mineral nutrients from the soil (except those plants with aerial roots, such as orchids). On the other hand, photosynthesis and respiration take place in leaves, the former requires carbon dioxide and lz while the latter requires oxygen.

Thus, plants require a system that transports water and nutrients from the soil to the top of the leaves, while a different system must transport the products of photosynthesis through the stem to the roots, or to other organs. where they accumulate. The first of these systems is called xylem, while the second is known as phloem. The set of xylem and phloem is known as the vascular tissue system.

From the ground to the sky: the xylem

The name xylem comes from a Greek word that means “wood . ” The part of the trunk of the plants corresponding to the xylem is commonly called wood or firewood. Its function is the transport of water and mineral nutrients from the soil to the leaves of the plant. The mixture of substances transported by the xylem is called crude sap .

The xylem of angiosperms (flowering plants) is composed of lignified tubular cells, which form ducts called tracheids . These cells are joined by a series of perforated sheets.

In ferns and gymnosperms (plants without flowers), the xylem is made up of tracheae , made up of dead cells that retain their structure thanks to the accumulation of lignin, which is the molecule that gives hardness to wood. 

How does water rise through the xylem?

Xylem and phloem
Xylem and phloem

The xylem is found in the central part of the tree trunk. Water moves through the xylem by two different mechanisms: osmosis and suction. The osmosis causes displacement of water from the roots to the leaves, where the concentration of dissolved substances is higher.

The suction , in turn, powered by water evaporation in the plant leaves, which generates a pressure within the vascular system to force water rising from the leaves. This force allows the displacement of water long distances, let’s think that the trunk of a redwood can measure more than 100 meters in height and that the leaves are in the highest part, so the raw sap must travel that distance.

From the leaves to the ground: phloem

The word phloem comes from the Greek and means “bark” . Its function is to distribute the elaborated sap , which contains the products of photosynthesis (sugars) through the different structures of the plant. The fundamental parts of the phloem are the sieve cells and the sieve tubes , the latter are found in angiosperms and are formed by longitudinal series of sieve cells.

What distinguishes sieve cells from other plant cells is the type of union that exists between them, and that facilitates the passage of processed sap. The pores that exist between these cells are larger than those found in other plant cells .

The phloem is located on the outside of the tree trunk, outside the xylem, under the bark. The transport of substances through the phloem is slow , it is estimated that sucrose moves 2.5 centimeters per minute through the conducting vessels. The final point of transport of the elaborated sap are specialized organs with various functions, such as fruits or seeds , and reserve organs such as bulbs or tubers.

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