Meristem: Definition, Types, and Examples

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Meristem Definition

The meristem is the region of tissue in plants where new growths emerge. Plant cells in the meristem are constantly multiplying and have not yet differentiated. Meristem tissue can give rise to new leaves, flowers, or roots depending on where it is on the plant and which signals it receives. The meristem is essential for plant growth; without it, plants would be unable to produce new cells.

Function of Apical Meristem

The apical meristem of a plant can be located at the ends of roots (root apical meristem) or at the tops of shoots (shoot apical meristem) and is responsible for the plant’s length or height development. Primary growth is the term for this sort of development.

When an apical bud (or terminal bud) is present, it displays apical dominance over the axillary buds, encouraging vertical development while inhibiting lateral growth. When the apical bud is removed, the axillary buds’ growth-blocking signals are eliminated, allowing lateral growth to occur.

Shoot Apical Meristem

Above ground, the shoot apical meristem is made up of undifferentiated cells with one of three fates. The protoderm, ground meristem, or procambium are the three major meristems that they can develop into. The protoderm will become the plant’s epidermal tissues, the ground meristem will become the plant’s cortex and pith, and the procambium will become the plant’s vascular tissues, xylem and phloem.

The leaves of a plant sprout from the sides of the apical meristem and are included in the shoot. The node develops a bulge, called an axillary bud, when the leaf begins to grow at the node. The axillary buds will remain dormant if the terminal bud is near to the axillary bud.

When the terminal bud is eliminated or the distance between the terminal bud and the axillary bud grows—as it does as the plant grows—the inhibitory forces that cause apical dominance lessen or vanish, enabling leaves to develop at the apical meristem’s lateral buds.

When flowering plants are ready to bloom, the plant’s shoot apical meristem transforms into an inflorescence meristem, which produces petals, sepals, stamens, and other flower components.

Root Apical Meristem

The root apical meristem, which is situated below ground, is in charge of a plant’s root growth and development. The root meristem generates cells in both directions at the same time, resulting in two types of tissues.

The plant’s primary roots supply proliferative, undifferentiated cells for continuing growth, while the other tissue forms a root cap that covers the apical meristem and serves as a source of new cells. Because the roots develop and the root cap is continually crushed down into the soil, the root cap’s cells are regularly lost and replaced by new cells supplied by the main root. 

The lateral development of roots from the main root into massively branching root systems is accounted for by lateral root meristems. Lateral root development aids the plant’s efficiency in water and nutrient absorption, as well as nutrient storage and aerial growth stability.

Function of Basal Meristem

The basal meristem, also known as an intercalary meristem, is located between mature, differentiated tissues. The intercalary meristem is separate from the apical meristem, despite its proximity and composition of mainly undifferentiated cells. It promotes the plant’s vertical development independently of the apical meristem.

However, here, growth occurs at the base of the plant, rather than at the tip. This permits leaves to keep growing even after being clipped, similar to how grass blades keep growing after being mowed.

Function of Lateral Meristem

While the apical meristem is in charge of vertical growth, the lateral meristem is in charge of lateral, or diameter, growth. Because it occurs around an already existing stem, this form of development is referred to as secondary growth.

The vascular cambium and the cork cambium are two forms of lateral meristems found in all woody plants and some herbaceous plants. The vascular cambium, like the procambium of the apical meristem, generates secondary xylem and phloem, but the procambium is also responsible for the formation of wood, which increases a plant’s girth.

The periderm, which is identical to the protoderm, is formed by the cork cambium. The protoderm generates a plant’s original epidermis, but the periderm replaces that epidermis to produce bark. The bark protects the plant from physical injury and prevents water loss through the use of a waxy compound called suberin.

Meristem Citations
  • Grass inflorescence architecture and meristem determinacy. Semin Cell Dev Biol . 2018 Jul;79:37-47.
  • Axillary meristem initiation-a way to branch out. Curr Opin Plant Biol . 2018 Feb;41:61-66.
  • CLAVATA-WUSCHEL signaling in the shoot meristem. Development . 2016 Sep 15;143(18):3238-48.
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