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
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
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