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) which is covered by an Epidermis. These can be readily 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 wherein the root cap has a separate primary meristem, it is called the calyptrogen. This occurs with closed RAMs.
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There are two basic types of root apical meristems. They are called Open or Closed. Seedless plants like ferns have prominent Apical Cells in their meristems. All cells in the Root Cap & Root Body can be traced back to the Apical Cell. This is the ultimate Closed RAM.
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| 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. |
The Apical meristems of seed plants are multicellular but there appear to be cells analogous to Apical Cells in the roots of some species like corn. These 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. However, that is not as clear as it could be in this specimen. |
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 converge upon these and they bear a resemblance to apices which have an apical cell. Consequently, these are closed as well. However, 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. 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.
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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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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. Do you think that this colorized root above has an Open or Closed Organization? |
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 distinct in open RAMs because of this. 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 of 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 cap. |
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Central RAM in a Cycad - Note the General Cytology of the Meristematic Cells and the numerous Mitotic Figures. Can you find a Calyptrogen? |
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| Root Apices of Hyacinth - Open or Closed???? | |
