Meristems-3   
311 Syllabus

Meristem Architecture

Fern Apical Meristems have Apical Cells rather than hundreds of Meristem Cells

This is true for most NON-Seed Plants

The Apical Cell gives rise to all the other cells in that Organ (Root or Shoot)

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 vs Proliferative Divisions

Formative Divisions send the Daughter Cell along different Developmental Paths

Proliferative Divisions Increase the number of Cells in one path.

Quiescent Center = Cells that  form a "Nucleus" which rarely divide.

Coincides with Initials

Nondividing Cells are less likely damaged by UV radiation

Can regenerate entire root with appropriate tissue pattern.

Cell Divisions are Anticlinal
(New Walls are Perpendicular to the Long Axis of the Root)                                       
(Periclinal Divisions would be Parallel to the Long Axis)

Produces long Cell Files that can be traced to their Origins which surround the Quiescent Center

Most Enlargement is in the Longitudinal Plane -> Relatively small increase in Diameter

Longitudinal Section of a Root Showing the Location of the Quiescent Center	Longitudinal Section of a Root Showing the Long Cell Files that are produced by Periclinal Divisions in the Root Apical Meristem & adjacent cells.
Banana Root Apical Meristem: Note how the longitudinal Cell Files in the Root Body converge on a small group of cells located at the apex. This is the area in which the Initials and Quiescent Center are found.

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.

Arabidopsis. Root Apical Meristem has Multiple Initials

Center of Meristem has Four Central Cells

They are Quiescent after Embryogenesis

Surrounded by Initials that produce different Tissues in the Root

Initials plus their immediate derivatives = Promeristem

Cell Fates are determined by their Position not by their Initial

Destroy Initials for one Tissue Group (i.e. Cortical Initial) 

Derivatives of another initial -> Replace the "lost tissues"

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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.