Primary Growth

Primary meristems

Primary meristems are the initial mitotic regions that create plant growth. These are embryonic cells usually found at the tips of plant organs and are therefore called apical meristems (apical = tip).
- 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)
These are actively dividing cells at the tips of all plants.
The primary meristem creates new cells through mitosis to increase the length of the plant.
Apical meristems are found in stem buds and stem tips, as well as the tips of roots

Primary growth in roots & shoots

Mitotic growth from an apical meristem; embryonic growth cells from a primary (apical) meristem
Increases the length 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 is the group of 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., angiosperm leaves)
The leaf primordium is created by the apical meristem
Embryonic cells are created along the edge or margin of 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 the 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 increases 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

Leaves are created by an apical meristem
Leaves then expand through a marginal meristem
Leaves almost always have determinate growth
- There are some exceptions (see reproductive leaves)
Many stems and roots tend to grow indeterminately, although upright determinate stems can exist with rhizomatous plants

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