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In the plant embryo, all cells can divide and later differentiate in functional mature cells. However, as plant grows, groups of undifferentiated cells remains in some parts of the plant body. These groups are known as meristems, and their cells keep the ability of proliferation and differentiation. Meristems are responsible for the permanent grow of the plant since they are present during the whole life of the plant. This is possible because not all proliferating cells of the meristem end up in a differentiated cell but some new cells remain as undifferentiated cells after mitosis. Thus, there is always a pool of undifferentiated cells that maintain the meristematic features as long as the plant is alive.

Meristematic cells show cytological features of undifferentiated cells: they are small, isodiametric and have a very thin primary cell wall. The cytoplasm is rich in ribosomes, proplastids, and many small vacuoles, but is poor in endoplasmic reticulum and inclusions. A well-developed Golgi complex synthesizes the components of the cell wall. The large nucleus contains condensed chromatin and is located centrally. Meristematic cells divide by mitosis and are totipotent, i.e. are able to differentiate into the entire variety of cell types of an adult plant. Plants grow by producing new cells and by the increase in size of these new cells, which is the result of the accumulation of water in their vacuoles. Normally, meristematic cells are densely packed with no empty intercellular space.

The classification of meristems is based on their topographic position in the plant body and on the time that they become active during plant development.

Shoot apical meristem
Shoot apical meristem protected by leaf primordia. Longitudinal view.

Root apical meristem
Root apical meristem protected by the calyptra. Longitudinal view.

Primary meristems are first observed in the embryo and their activity results in primary growth, which is mainly involved in increasing the plant length. During this activity, cells undergo anticlinal cell divisions, i.e. the cell division plane is transverse to the surface of the plant body (see figure). Primary meristems are found in the tips of the main and lateral shoots and roots, and hence they are called apical meristems. The apical meristems of the branches arise from the first shoot apical meristem, whereas the apical meristems of the secondary roots develop from the root endodermis. Shoot apical meristem is covered and protected by the youngest leaf primordia, whereas root apical meristem is wrapped by the calyptra. Three categories of cells can be distinguished in apical meristems. The outermost layer is the protoderm, which differentiates into the epidermis, then comes the procambium, which gives rise to the primary vascular tissues (primary xylem and primary phloem), and finally the ground meristem, precursor of ground tissues (parenchyma, collenchyma and sclerenchyma).

Plant cells do not move and this makes possible to predict what will be the fate of a cell by knowing the spatial location within the meristem. In the apical meristems, as the same time that the central zone produces new cells, the meristem moves away, and these new cells fall under the influence of molecules that direct their differentiation process. This separation between the meristem and new cells is actually a consequence of the continuous mitotic activity of the meristematic cells, which push the meristem away.

Primary meristem
The vascular cambium observed in a transverse section of a stem with secondary growth.

Primary meristem
The cork cambium observed in a transverse section of a stem with secondary growth.

However, in monocots, most of the growth in length of shoots depends less on the apical meristems and it is more the responsibility of intercalary meristems, which are meristematic tissue derived from the apical meristem. Intercalary meristem continues the proliferative activity at some distance from the place where it was originated, i.e. it is inserted (intercalated) among tissues that are no longer meristematic. The best known examples of intercalary meristems are those located in internodes (mainly at the base of the internodal region) and in leaves, particularly in grasses.

In those plants that grow in thickness, i.e., having secondary growth, other type of meristems, referred as lateral or secondary meristems, are present. These meristems arise later during development, are responsible for the increase in diameter of shoots (producing the wood) and roots, and their cells divide by periclinal division. These meristems are typical of gymnosperm and dicotyledonous plants. They are no found in many herbaceous plants, like most pteridophytes and monocots, or in specific organs such as leaves. There are two types of lateral meristems: the vascular cambium, which differentiates from procambium and produces secondary vascular tissues (secondary xylem and secondary phloem), and the cork cambium (phellogen), which arises from various parenchymatous tissues in the cortex and produces phelloderm inward and cork outward. Both meristems are arranged as a continuous cylinder, sometimes as an incomplete ring, along shoots and roots. The vascular cambium is found between the secondary xylem and the secondary phloem, whereas cork cambium locates between the phelloderm and cork.

Plant meristems
Classification of meristems.

There are two additional meristems involved in the development of vascular system: provascular and preprocambium. The provascular tissue is an embryonic tissue located in the prospective vascular cylinder. Although this provascular meristem does not directly give xylem or phloem, it produces precursor cells that will give rise to xylem and phloem. Preprocambium cells are found in leaves and are the precursors of procambium. They cannot be distinguished from the ground tissue cells.

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