Atlas of plant and animal histology

1. Introduction

In the plant embryo, all cells can divide and later differentiate into functional mature cells. Meristems are groups of undifferentiated cells found in some regions of the adult plant body. Meristematic cells proliferate and differentiate into a wide variety of cells and are responsible for the permanent growth of the plant.

Meristems may be permanent and functional during the plant lifespan, as it happens to the shoot and root apical meristems. Others are transient and are active for a short period of time, such as those giving rise to the leaves, flowers, and petioles. Some meristems may appear quite late after germination, such as the pheloderm and vascular cambium.

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 are totipotent, i.e., they are able to differentiate into the entire variety of cell types of an adult plant. The particular cell type and function a differentiated cell performs depends on the spatial position of the meristematic cell this cell is differentiated from.

The classification of meristems is based on their topographic position in the plant body and on the time they become active during plant development (Figure 1).

2. Primary meritems

Primary meristems are first observed in the embryo and are mainly involved in increasing the plant length (Figure 2). Primary meristems are found at the tips of the main and lateral shoots and roots, and hence they are called shoot apical meristems and root apical meristems, respectively. Axillary buds, which give rise to branches, are derived from the shoot apical meristem. Branches bear an apical meristem, similar to the shoot apical meristem. However, the apical meristems of the lateral roots are formed from the root endodermis. Shoot apical meristems are covered and protected by the youngest leaf primordia, whereas the root apical meristem is wrapped by the calyptra. Other meristems are formed from the apical meristems: the protodermis differentiates into the epidermis, the procambium gives rise to the primary vascular tissues, and the ground meristem is responsible for the rest of the cell types of the plant.

However, in monocots, most of the growth in length of shoots depends less on the apical meristems and more on the intercalary meristems. Intercalary meristems are found between nonmeristematic tissues. The best-known examples of intercalary meristems are those located in internodes (mainly at the base of the internodal region) and in leaves.

3. Secondary meristems

In those plants that grow in thickness, i.e., have secondary growth, other types of meristems, referred to as lateral or secondary meristems, are present (Figure 1). These meristems arise later during development and are responsible for the increase in diameter of shoots (produce the wood and bark) and roots. These meristems are typical of gymnosperms and dicotyledonous plants. There are two types of lateral meristems: the vascular cambium, which differentiates from the procambium and produces secondary vascular tissues (secondary xylem and secondary phloem), and the cork cambium (phellogen), which produces bark. Both meristems are arranged as a continuous cylinder, sometimes as an incomplete ring, along shoots and roots.

4. Other meristems

There are two additional meristems involved in the development of the 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 rise to 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.

New meristems may arise from dedifferentiation of parenchyma and colenchyma cells, and from the root endodermis.

In monocots, the secondary growth was lost during evolution, which may have happened because all procambium meristem is differentiated into vascular tissues. The growth in thickness of monocot stems and roots is a consequence of the increase in size of the cells. However, palms and other monocots with tree-like bodies (some asparagales) may develop very thick stems by adding new parenchyma cells and more vascular bundles in regions far away from the apical meristems. This type of growth is called anomalous growth and is produced by the appearance of a new type of meristem known as the peripheral growth meristem.

• Bibliografía ↷

  • Augstein F, Carlsbecker A. 2018. Getting to the Roots: A Developmental Genetic View of Root Anatomy and Function From Arabidopsis to Lycophytes. Frontiers in plant sciences. 9: 1410.

    Miyashima S, Sebastian J, Lee J-Y, Helariutta Y. 2013.The EMBO journal. 32: 178-193.

    Truskina J, Vernoux T. 2018. The growth of a stable stationary structure: coordinating cell behavior and patterning at the shoot apical meristem. Current opinion in plant biology. 41: 83-88.