Meristems (SAM & RAM)

Meristems are centers of cell division and growth. In animals, totipotent stem cells, which can differentiate into any tissue type are only found early in development; however, plants contains such embryonic tissues throughout their lives.

Apical meristems are located on the very ends of shoots (shoot apical meristem) SAM and roots (root apical meristem) RAM. They produce three types primary meristems: the protoderm, ground meristem, and procambium. The protoderm gives rise to epidermis, which surrounds the plant. The ground meristem gives rise to ground tissue, a group of tissues with generalized functions such as photosynthesis, storage, and support. Finally, the procambium gives rise to the vascular tissue, which functions in transport. The three primary meristems first appear in the embryo proper, with the protoderm on the outside, the procambium in the center, and the ground meristem in between them.

Figure 1- A longitudinal section of the shoot apex of Coleus includes the shoot apical meristem and the developing leaves (leaf primordia) that surround it. The three primary meristems are visible in the leaf primordia. The protoderm surrounds the leaf primordium, the procambium appears as a line running through the center, and the ground meristem fills the rest of the leaf primordium.
Figure 2- A longitudinal section of the root tip of corn (Zea mays). The large, lighter cells at the bottom form the protective root cap. The remaining visible portion of the root tip (above the root cap) is the root apical meristem (zone of cell division). It gives rise to the central procambium and the ground meristem, which is external to the procambium. The third primary meristem (protoderm) is not visible here. It would be the outer layer of the root tip, just above the current view.
Figure 3- The torpedo stage of the Capsella busra-pastoris embryo proper showing the three primary meristems.

Secondary meristems (lateral meristems) result in secondary growth, a woody increase in girth. These include the vascular cambium and cork cambium. The vascular cambium arises from from the procambium and pericycle in roots. In stems, it arises from procambium cells of the vascular bundles (fascicular cambium) and parenchyma cells between vascular bundles (interfascicular cambium). The vascular cambium gives rise to secondary phloem (part of the bark) and secondary xylem (wood).

The cork cambium arises from the pericycle in roots and the parenchyma cells of the cortex in stems, both of which arise from the ground meristem. The cork cambium produces periderm, secondary dermal tissue that is also a component of bark. (See Roots and Secondary Stem for more details.

Figure 4- Tissue development in stems. The shoot apical meristem produces the three primary meristems: procambium, ground meristem, and protoderm. The secondary meristems (lateral meristems) are the vascular cambium and cork cambium. The vascular cambium arises from the procambial cells of the fascicular cambium and the interfascicular cambium, which ultimately arose from the ground meristem. The ground meristem also produces the cortex, which generates the cork cambium

Figure 5- Tissue development in roots. The root apical meristem produces the three primary meristems: procambium, ground meristem, and protoderm. The secondary meristems (lateral meristems) are the vascular cambium and cork cambium. The vascular cambium arises from the procambium and pericycle. The pericycle also produces the cork cambium.

Other meristems include intercalary meristems which elongate stems from the “middle” (in between nodes) and marginal meristems, which are located along leaf edges and are responsible for leaf development.

Meristems related experiment:-

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