Studies with Azolla (Water Fern)
Meristematic Growth is DETERMINATE.
There are a set number of divisions in each part of the root including the Apical Cell.
Geometric Tissue Wedges (Merophytes - Mero = Divide) are produced. These can be traced back to the Apical Cell.
Formative Divisions send the Daughter Cell along different Developmental Paths
Proliferative Divisions Increase the number of Cells in one path.
Coincides with Initials
Cell Divisions are Anticlinal
(New Walls are Perpendicular to the Long Axis of the Root)
(Periclinal Divisions would be Parallel to the Long Axis)
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This is an example of a Root Apical Meristem in which the cell files that compose the body converge of a few Initials at its apex. | The Initials are more clearly seen in this embossed image. |
They are Quiescent after Embryogenesis
Surrounded by Initials that produce different Tissues in the Root
Initials plus their immediate derivatives = Promeristem
Destroy Initials for one Tissue Group (i.e. Cortical Initial)
Derivatives of another initial -> Replace the "lost tissues"
Summary
Apical Meristems contain Initials which divide infrequently (Quiescent Zone). Their immediate derivatives divide and produce most of the cells for the organ.
The Initials plus immediate derivatives are called the Promeristem.
Seed Plants have more than one Initial.
Seedless Plants usually have one Initial called the Apical Cell.
Cell fates are strongly influenced by the Initial from which they developed BUT this is not absolute.
Cell fate is also influenced by the surrounding cells which determine their position.
Word of Caution: Some root apical meristems have a different overall architecture compared to Arabidopsis. Consequently, the regulation of tissue fates may not be as precise as with Arabidopsis.