Primary Growth

Primary meristems

  • Primary meristems are called apical meristems

    • Growth may come from a single apical cell in ancestral plants (e.g. mosses, ferns), or a cluster of cells called the apical meristem in seed plants (e.g. seed plants)

  • Actively dividing cells in the tips of all plant

  • The primary meristem produces cells through mitosis, to increase the length of the plant

  • Apical meristems found in stem buds and tips, as well as the tips of roots (apical = tip)

Primary growth in roots & shoots

  • Mitotic growth from an apical meristem

  • Increases length and/or depth of shoots and roots of the plant

  • Each apical meristem creates three (3) intermediate primary meristems which are the sites of cell differentiation and maturation

Protoderm

  • Protoderm is the thin layer of cells on the periphery of the developing shoots and roots that will eventually become the mature epidermis

Ground meristem

  • The ground meristem is a mass of cells inside the developing shoots or roots that eventually become mature ground tissues, such as parenchyma and sclerenchyma

Procambium

  • The procambium are the cells within the developing shoots or roots that will mature into the vascular tissues, such as xylem and phloem

Special Primary Growth

Marginal meristem

  • This meristem increases the size of a leaf blade (e.g. angiosperms)

  • The leaf primordium is created by apical meristem

  • Embryonic cells are created along the edge or margin this leaf

  • These cells actively divide to increase the size of the blade

  • Growth is almost always determinate1

Basal meristem

  • This meristem adds new cells at base of leaves to increase the size (e.g. monocots, Welwitschia)

  • Embryonic cells are created at the base of a leaf

  • These cells actively divide to increase the length of the blade

  • Youngest tissues are at the base; oldest at the tip

  • Growth is usually indeterminate2

Intercalary meristem

  • This meristem increase the length of stems through dividing cells in the internodes (e.g. monocots, horsetails)

  • Initial shoot formation is created by an apical meristem

  • Embryonic cells are created between nodes

  • Shoot exhibits "telescopic growth", in which internodal cells actively divide to increase the length of the shoot system

  • Growth is almost always determinate1

1Determinate Growth

  • Growth that terminates in an organ; meristematic cells in these organs are genetically programmed to cease dividing (e.g. most leaves, flowers)

2Indeterminate Growth

  • Growth that does not terminate; meristematic cells in these organs continue to divide for the life of the plant (e.g. rhizomes, stems of perennial woody plants, woody roots)

Above: Diagrams showing both the activity of a marginal meristem in a eudicot leaf, and a basal meristem in a grass leaf

Below: Diagram showing the activity of a intercalary meristem in the stems of grasses

Apical vs. Intercalary growth

  • Below are two examples of how the same final form could be created using either apical growth or intercalary growth using a simple, early plant model to demonstrate

Above: Model of an early land plant, Aglaophyton, employing apical growth to grow aerial stems. Stems are created as apical meristems grows upwards leaving cells to mature.

Above: Model of an early land plant, Aglaophyton, employing intercalary growth to grow aerial stems. Stems are pre-formed and expand through expansion between the branching points.

Primary growth in leaves