MERISTEM: CHARACTERISTICS, FUNCTIONS, HISTOLOGY AND TYPES - BIOLOGY - 2024

The meristems are specialized tissue in plant cell division. From the activity of these tissues, all the adult tissues of the plant originate. Meristematic tissue is located both in the root and in the stem (aerial part of the plant). They make up the so-called growth points.

They have been classified using various criteria. One of these is the moment of appearance in the development of the plant. They can also be differentiated by the position they occupy in the body of the plant.

Histological arrangement of the apical stem meristem. Fernandacastillo1, from Wikimedia Commons

Meristems are also known as buds. These tissues are active throughout the life of the plant, and are responsible for the indeterminate growth of these organisms.

characteristics

Origin

All plants originate from the activity of a single cell (zygote). Later, when the embryo differentiates, there are certain areas that maintain the ability to divide.

In the radicle of the embryo, cells in the subapical position continue to divide. While in the stem, the plumule, constitutes the first bud (meristematic zone) of the plant.

Also, in some cases already differentiated cells of the plant body can divide again. This can occur when some damage is generated to the plant or by endogenous control.

Cytology

Meristematic cells are undifferentiated. Their size ranges from 10-20 µm and they are isodiametric (with all their sides equal). They are characterized by presenting a thin primary cell wall made up of pectins, hemicellulose and cellulose.

The nucleus occupies the largest cell volume. It is considered that it can occupy up to 50% of the cytoplasm. ´

There is a great abundance of ribosomes. There are also numerous dictyosomes forming the Golgi apparatus. The endoplasmic reticulum is sparse.

In general, there are many small vacuoles, which are dispersed throughout the cytosol.

The plastids are not differentiated, which is why they are called proplastidia. Mitochondria are very small with few mitochondrial ridges.

Hormonal activity

The activity of meristematic cells is regulated by the presence of substances produced by the plant. These are hormones (substances that regulate the activity of a tissue).

The hormones that intervene most directly in the activity of meristems are auxins and gibberellins.

Auxins stimulate the formation and growth of roots. Likewise, in high concentrations they can inhibit division in stem meristems.

Gibberellins are capable of stimulating cell division in dormant meristems. These buds have generally stopped their growth due to the influence of environmental factors. The movement of gibberellins to these areas breaks the latency and the meristem begins its activity.

Features

The function of the meristem is the formation of new cells. This tissue is in constant mitotic division and will give rise to all the adult tissues of the plant.

These tissues are responsible for the growth in length and thickness of stems and roots. They also determine the pattern of development of the plant's organs.

Histology

The meristems that are located at the apex of the stem and the root tend to be conical in shape. Its diameter can range from 80-150 µm.

In the stem, this tissue is located at the apical end. In the root, the meristematic cells are located just above the caliptra, which protects them.

The meristems of both the stem and the root have a particular histological organization. This determines the type and position of the adult tissues to which they will give rise.

Apical stem meristem (AVM)

The meristematic zone in the aerial part of the plant forms the buds. In the most apical part of the meristem, the least differentiated cells are located. It is known as a promerystem and has a particular configuration.

Two levels of organization can be recognized. At the first level, the dividing capacity of cells and the position they occupy in the promerystem are taken into account. Three zones are presented:

Central zone

It is made up of cells that are elongated and highly vacuolated. These cells have a low rate of division compared to other areas of the promerystem. They are pluripotent, so they have the ability to originate any tissue.

Peripheral zone

It is located around the cells of the central zone. The cells are small and highly stained. They are divided frequently.

Medullary or rib area

It occurs just below the central zone. The cells are vacuolated and are arranged in columns. They make up the medulla of the meristem and are the ones that give rise to the largest amount of stem tissues.

When the cells in the central zone divide, some of the daughter cells move to the sides. These will form the peripheral zone and will give rise to the foliar primordia.

The cells that form towards the lower part of the central zone, join the medullary zone.

At the second level of organization of the AVM, the configuration and division planes of cells are taken into account. It is known as the tunic-body configuration.

Tunica

It is made up of the two outermost layers of the AVM (L1 and L2). They divide only in the anticline plane (perpendicular to the surface).

The outermost layer of the tunic will originate the protodermis. This primary meristem will differentiate into the epidermal tissue. The second layer participates in the formation of the fundamental meristem.

Body

It is located under the tunic. It is made up of several layers of cells (L ₃). They are divided both anticline and pericline (parallel to the surface).

The fundamental meristem and the procambium are formed from the cells of the body. The first will form the tissues of the cortex and medulla of the stem. Likewise the mesophyll tissues in the leaf. The procambium will give rise to the primary vascular tissues.

Subapical root meristem (MSR)

The meristematic zone of the root has a subapical position, because it is protected by the caliptra, which prevents cell damage when penetrating the soil.

The organization of the MSR is much simpler than that of the MAV. Below caliptra, there is a group of cells with a low rate of division. These form the quiescent center, which is considered a reserve center for meristematic cells.

Lateral to the quiescent center, one to several initial cell layers are present, depending on the plant group.

The outermost layer of initial cells will give rise to the cells of the caliptra. It will also form the protodermis. The innermost layers divide to give rise to the fundamental meristem and the procambium.

Types

Various criteria have been used to classify meristems. The most used are the position and moment of appearance on the plant.

-By position

Depending on where they are located in the plant, we have:

Apical

They are located in the terminal areas of the roots and stems. In the stem they are located in apical position and in the terminal part of the branches. They form the apical and lateral buds.

In each root there is only one meristem, which has a subapical position.

Side

They occupy a peripheral position to roots and stems of gymnosperms and dicotyledons. They are responsible for the growth in thickness in these plants.

Interlayer

They are located on the stem. They are distant from the apical meristems and interspersed with adult tissues. They are responsible for increasing the length of internodes and leaf sheaths in some plants.

Adventitious

They are formed in different positions in the body of the plant. This occurs in response to various stimuli. Adult cells are capable of resuming meristematic activity.

Among these we have the scarring meristems originated when mechanical damage is caused to the plant. Likewise, meristemoids, which are isolated meristematic cells that can originate structures such as stomata or trichomes.

-By appearance time

In all plants there is a primary growth that basically consists of the growth in length of the stem and roots. The tissues that are formed originate from the so-called primary meristems.

In some groups, such as gymnosperms and many dicots, secondary growth occurs. This consists of an increase in the diameter of stems and roots.

It occurs mainly in trees and shrubs. The tissues that give rise to this growth originate from the activity of secondary meristems.

Primary

These are the protodermis, the fundamental meristem, and the procambium.

The protodermis is going to give rise to the epidermal tissues in roots and stem. It is always located in the outermost part of the plant.

From the activity of the fundamental meristem, the different types of parenchyma originate. Likewise, mechanical tissues (collenchyma and sclerenchyma) are formed.

On the stem it is located outside and inside the procambium. The internal tissues will form the medulla and the external tissues the primary cortex. At the root it is between the protodermis and the procambium. The tissues that it originates constitute the radical cortex.

The procambium forms the primary vascular tissues (xylem and primary phloem). The cells of this meristem are elongated and vacuolated. In the stem it is located in a lateral position, while in the roots it occurs in the center of the organ.

Secondary

They are the phelogen or suberous cambium and the vascular cambium.

Phellogen is formed by the dedifferentiation of adult stem or root cells. In the stem it can originate from any tissue of the primary bark. At the root it is formed from the activity of the pericycle.

This meristem forms the suber or cork towards the external part of the organ. Towards the internal part the felodermis originates. The set of suber, felogen and felodermis constitutes the secondary cortex.

Secondary vascular tissues are formed from the division of the vascular cambium. This meristem originates from procambium remains that lie dormant in stems and roots.

Likewise, in the roots the pericycle also participates in its formation. In the stem, parenchymal cells can participate in the formation of the vascular cambium.

The meristem originates externally secondary phloem and internally secondary xylem. In all cases, more secondary xylem is formed, which constitutes wood.

References

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