Root Directory
Root
Anatomy- Apical Meristems
Root Directory
Root anatomy is very simple.
The root is composed of three concentric circles of primary tissues.
The central core is Vascular Tissue. This central core of cells is derived from the Procambium and is called the Stele.
This is surrounded by a ring of Ground Tissue (Cortex).
The Epidermis is the outer most ring of tissue.
These can be traced back to an Apical Meristem. However, the Root Apical Meristem (RAM) is covered by a Root Cap which will require a little activity by the little gray cells to understand!
Think of the root as being composed of a Cap and a Body.
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The Root Apical Meristem produces the three Primary Meristems (procambium - ground meristem & protoderm) plus the root cap.
In cases where the root cap has a separate primary meristem, it is called the calyptrogen (calypt = cover & gen = produce). This occurs with Closed RAMs.
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Seedless plants like Ferns have prominent Apical Cells in their meristems.
 Dividing Apical Cell in Equisetum (horsetail). Note the Large Apical Cell caught in mitosis. All cells in the organ can be traced back to this cell. |
 Root Apical Meristem with a Central Apical Cell that produces the Root Cap and Body. There is a Calyptrogen. |
The Apical Meristems of Seed Plants are Multicellular but there may appear to be cells analogous to Apical Cells in the roots of some species like corn. These cells are called Initials.
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Banana Root Apex - High Magnification - Locate the Cap and the Body - Follow the cell files in the body towards the RAM. Are Initials Present? Is this an Open or Closed RAM? |
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Banana RAM - Higher Magnification - This has a Closed Organization as the three layers of the body converge on a few "Initial Cells". The Root Cap has a distinctly separate origin from a broad meristem (calyptrogen). However, that is not as clear as it could be in this specimen. |
Closed Apical Meristems
| Corn (Zea mays) is a good example of a Closed Root Apical Meristem as ther is a distinct Root Cap Meristem (Calyptrogen). The Protoderm is readily seen in the thin sections below. |  |
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The Root Cap of these roots appears to have a separate origin from the cells of the body, and a distinct boundary can be seen between the Root Cap and Body of these closed RAMs.
Apical Cells are not found in seed plants but all the cells in the body of some roots can be traced back to a few apical cell-like "Initials". Cell files in the Root Body converge upon these cells in the meristem and they thus bear a resemblance to apices which have an apical cell.
 This image of the Corn Root Apex Sows the Region in which "Initials" can be found. |
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Initials can be designated by careful studies of Cell Lineages. Cell files are traced back to their origins and in some roots there appear to be a small number of cells called Initials that appear to be the source of all the tissues in the root body. In this case cells files in the Procambium are indicated by red lines. Cell files in the Ground Meristem are designated by blue lines and cells of the Protoderm are indicated by yellow lines. This is a simple approximation of the procedure that was actually used by scientists who have studied this in detail. |
The Root Cap of Closed RAMs can be readily plucked from the Body. This trait was used to devise some imaginative experiments on the root cap and geotropism by Swiss and Scottish scientists.
Closed Roots have a Distinct, Macroscopic Root Cap like the ones below.
Colorized SEM Photo of a Root Tip - Note the distinct appearance of the Root Cap. Compare it with the Root Cap on the SEM of an Open RAM below. |
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Three aerial roots of Palm - Based on the appearance of the Root Caps do you think these are open or closed? what is the function(s) of root caps in aerial roots? The root in the center is producing lateral or secondary roots. |
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Open Apical Meristems
The major difference between Open & Closed RAMs is that there are continuous cell files which span the Cortex and the Root Cap in Open RAMs.
The origin of the Epidermis and the ability to distinguish a Protoderm is less obvious in Open RAMs.
The outer covering layer comes off in sheets which include portions of the Dermis/Cortex and the Root Cap. I thought you said that Root Anatomy was Simple!!!!!!!
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 Long Section through the same root - Locate the various parts of the root and the principal tissues. |
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Central Region of Cycad RAM - Note the absence of initials at the focal region of the RAM - The red stain indicates Amyloplasts which are characteristic for the root cap. |
Lateral Region of the Same RAM - Note that the blue lines trace cell files which are continuous from the cortex through the Rooy Cap. |
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Central RAM in a Cycad - Note the General
Cytology of the Meristematic Cells and the numerous Mitotic Figures. |
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| Root Apices of Hyacinth - Open or
Closed???? If ther is no Calyptrogen the root is most probably Open! |
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