4.6.2: Meristems

Learning Objective

Define and locate the two apical meristems and the three primary meristems.

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; Figure \(\PageIndex{1}\)) and roots (root apical meristem; RAM; Figure \(\PageIndex{2}\)). 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 \(\PageIndex{1}\): A longitudinal section of the shoot apex of Coleus includes the shoot apical meristem and the developing leaves (leaf primoridia) that surround it. The three primary meristems are visible in the leaf primordia. The protoderm surrounds the leaf primoridum, the procambium appears as a line running through the center, and the ground meristem fills the rest of the leaf primodium. Smaller, younger leaf primordia are closer to the shoot apical meristem than older leaf primordia are. Image labeled from Maria Morrow (CC-BY-NC).

Figure \(\PageIndex{2}\): 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. Image labeled from Maria Morrow (CC-BY-NC).

Figure \(\PageIndex{3}\): The torpedo stage of the Capsella busra-pastoris embryo proper showing the three primary meristems. Image labeled from Wisewire (CC-BY).

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; Figures \(\PageIndex{4-5}\)).

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; Figures \(\PageIndex{4-5}\)).)

Figure \(\PageIndex{4}\): Tissue development in stems. The shoot apical meristem produces the three primary meristems: procambium, ground meristem, and protoderm. The procambium produces vascular tissues. The primary xylem, fascicular cambium, and primary phloem arise from the procambium. The ground meristem produces the pith and cortex, which are ground tissues. The pith generates pith rays, which give rise to the interfascicular cambium.The protoderm produces the epidermis, a dermal tissue. 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. The secondary xylem and secondary phloem arise from the vascular cambium. The cortex gives rise to the cork cambium. The cork cambium and tissues it produces (phelloderm and cork) are dermal tissues. Image by Melissa Ha based on diagram from Maria Morrow (CC-BY).

Figure \(\PageIndex{5}\): Tissue development in roots. The root apical meristem produces the three primary meristems: procambium, ground meristem, and protoderm. The procambium gives rise to vascular tissues, including the primary xylem and primary phloem. The ground meristem produces the cortex, a ground tissue. The protoderm produces the epidermis, a dermal tissue. The secondary meristems (lateral meristems) are the vascular cambium and cork cambium. The vascular cambium arises from the procambium and pericycle (which arose from the procambium). The vascular cambium produces secondary xylem and secondary phloem (both vascular tissues). The pericycle also produces the cork cambium, which gives rise to the phelloderm and cork (all dermal tissues). Image by Melissa Ha (CC-BY-NC).

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.

Attribution

Curated and authored by Melissa Ha using 5.1 Tissues from Introduction to Botany by Alexey Shipunov (public domain)