Méristème

At the Embryophyte S (house plants), a Méristème is a biological Tissu consisted of cells undifferentiated (or little differentiated) forming a zone of growth where take place the cellular divisions (Mitose S). One distinguishes usually the primary méristèmes, which ensure the growth of the plant in length, on the level of the Tige, the Feuille S or the roots, and the secondary méristèmes, persons in charge of diameter the growth of the Organe S of certain plants, like the trunk.

Méristèmes and development

Only vegetable fabric able to create new cells, the méristème is essential with the development of the plants. It creates all the plant thus: it is at the origin of the Tige, the Feuille S, the roots, the Fleur S (at the plants with flower) or of the Branche S. It is moreover responsible for the specific measure of the sheets or the branches around the stem, what is called the Phyllotaxie.

Méristèmes in the vegetable world

According to the various groups, the méristèmes are different.

Foams and ptérydophytes

The Foam S and the Ptéridophyte S (of which the Fougère S) have a single apical cell of tetrahedral form , which thus presents four faces. With each cellular cycle, this cell divides and gives a new cell on one of its faces, then each new cell is produced by each face, and this in a cyclic way.

Gymnospermes

The Gymnosperme S (of which the Conifère S) have a fabric meristematic made up of two layers of cells (if one considers the organization in layers): a first surfacing whose cells only have a division anticline, and a layer in lower part whose divisions are anticlines and periclines.

Angiospermes

The plants with flower have a fabric meristematic similar to the gymnospermes, but with a transition course (L2) between the external layer (L1) and the internal layer (L3), whose cells have a division mainly anticline. (see further the organization from fabrics meristematic)

Indefinite growth

Although each plant crossed according to its own rules, each méristème can continue to grow as long as it is in life: at good number of plants the meristematic growth is potentially unspecified, which supposes that the complete shape of the plant is not given in advance, as it is the case at the animal Bilatérien S which have a definite Plan of organization.

Histogenesis and organogenesis

The méristèmes form fabric S (histogenesis) such as conducting fabrics, Xylème S and Phloème S, and Organe S (organogenesis), like the Feuille S, them - even made up of fabrics with a particular organization.

The méristème racinaire and the apical méristèmes of Tige S produce uninterrupted fabrics, which allow the root and the stem to grow without to create new bodies. In the same way, the secondary méristèmes produce fabrics, in particular for the growth in thickness.

The formation of bodies implies the formation on the level of the méristèmes of outline S (of the foliar outlines for example), i.e. solid masses of cell S programmed to give a particular body, which will grow until forming the body in question.

The growth of fabrics is done by lengthening of the cells: thanks to the pressure generated by water (pressure of turgescence), the cells and their wall lengthen.

One can thus distinguish in a general way:

the méristèmes racinaires and secondaries are only histogenic;
the apical méristèmes are at the same time histogenic and organogenic.

Different the méristèmes

There exist various types of méristèmes related to the various parts which the méristèmes during the development play, and especially with the type of specialized fabrics that they generate.

Méristèmes primary educations

The primary méristèmes are the first to be set up; they form all primary education fabrics of the plant. They allow a growth in length.

Méristèmes secondaries

The herbaceous plants (of which the Monocotylédone S) do not make secondary growth. This one allows the diameter growth of the stem, which can go until the formation of a trunk. The secondary méristèmes create indeed in particular Bois on the one hand, cork on the other hand. However, this secondary growth takes place only after the installation of the primary structure of the plant.

The méristème the secondary main thing is the Cambium. Present only at the Dicotylédone S, it also is called sat generating wood-and-bast (AGLL). In the stems, this fabric is located between the primary Xylème has centrifugal differentiation in the stem, centripetal in the root on the one hand, and the primary Phloème has differentiation centrifuges on the other hand. It thus will create secondary conducting fabrics; the cells which constitute it carry out radial divisions so as to create:

towards the interior: secondary xylème, whose characteristic is to be lignified. They thus constitute in addition to the vessels a fabric of support forming wood.
towards outside: secondary phloem, which one calls fabric Liber (of support, but so conducting: it leads the falling sap, known as " Sap élaborée" who goes from the sheets to the roots).

The primary education vessels finish crushed by the growth of secondary fabrics.

The secondary second méristème is the Phellogène or sitted subero-phellodermic generator. This one, following the example cambium, carries out radial divisions creating:

towards the interior: Phelloderm. This phelloderm has as a function to regenerate the phellogen.
towards outside: Suber, more commonly called cork. Cork with for role the protection of the plant and is added to that of the skin.

Meristematic organization of fabric

Cells

The meristematic cells have functions similar to the original cells of the animals: they are little or at all differentiated and are able to continue the cellular Division indefinitely. The great difference existing between the specialized original cells and cells, which do not have the potential of differentiation of the original cells, is much marked at the plants: the cells differentiated from the plants are capable of dédifférencier and to rebuild new fabrics or Organe S. the propagation by cutting is an example of these capacities.

These cells are undifferentiated, which results in particular in a cytoplasmic material which fills the cell completely, although the Vacuole S are extremely reduced. The cytoplasm does not contain any Plaste, even if they are present in a rudimentary form. The meristematic cells moreover more narrowly coupled the ones with the others, are separated by very fine pectocellulosic walls and not presenting any intercellular cavity.

Structure of the méristème

The méristème apical primary education can be divided into layers or fields:

Layers

The organization in layer takes into account the cellular lines which constitute each one a different layer (figure 2) .

The most external layer of the cell which forms the skin is called layer L1 , and the subjacent layer is the layer L2 . The internal fabric, which is more than one layer of cells, forms the layer L3 , which will form the medullary Parenchyme. The layers L1 and L2 which have both a division anticline, form the Tunica méristème, while the L3 layer, which has divisions at the same time anticline and pericline, forms the corpus of the méristème.

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Fields

Another presentation of the méristème consists in dividing it into fields (or zones) according to the function of the cells. Some consider thus that the apical méristème is rather a whole of méristèmes specialized in the formation of a specific type of cell (figure 3) .

The first zone, the central zone (ZC) (1) , is also called quiescent center because of the weak mitotic activity of its cells, which form completely undifferentiated cells thus. Those divide and migrate towards the peripheral Zone (ZP) of the méristème (2) . The cells of this peripheral zone are more active and form the primordia foliar which will contitueront bodies like the sheets (organogenic role). Under the central zone the méristème Médullaire (3) is , a cellular zone of strong growth which ensures mainly the creation of internal fabrics: the primary education Parenchyma S and conducting fabrics in particular (4) . Its role is thus histogenic. |} The principal difference which exists between the apical méristème and the méristème racinaire lies in the fact that MAC is at the same time histogenic and organogenic whereas the MAR is only histogenic.

Regulation of the activity of the méristème

The permanent activity of the méristème requires a precise regulation, without what the méristème could for example become exhausted or on the contrary to produce too many cells. Moreover, different the méristèmes must have a given identity (MAC, MAR, méristème floral…). All that is realizable using signals.

Biotechnologies

The méristème is an important fabric in Biotechnologie. Indeed, as one does not find a Virus in the méristèmes of an infected plant, one can regenerate a healthy plant starting from a sick plant by making a culture in vitro of méristème.

This process moreover is very much used in Horticulture to produce plants of same a Génotype given. It is a technique of asexual Multiplication (or vegetative).

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