Root Directory
Roots are composed of three concentric rings of tissues. These are vascular, ground & epidermis. This organization is illustrated by Selaginella which is a seedless plant. It has a solid core of xylem, surrounded by a ring of Phloem which is surrounded by ground tissue (Cortex). The innermost layer of the Cortex is the Endodermis. An Epidermis is the outermost ring.
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| Cross-Section of a Selaginella root. Locate the concentric circles of tissues. | Center of a Selaginella root with the vascular tissues labeled. |
The classic root which is studied in all Botany courses is that of Ranunculus, a dicot. Its anatomy is identical to Selaginella, except for the fact that the central xylem is star-shaped.
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Cross-Section of Ranunculus Root - Locate the concentric circles of tissues. The vascular tissues are at the bottom center of the image. The densely stained purple cells in the Cortex are Parenchyma that contain many Amyloplasts. Starch storage is one important adaptation of roots. |
Vascular cylinder from a Ranunculus root. The xylem has a star-like appearance. The Phloem occupies the indentations between the Xylem arms. The Endodermis is the layer of small cells which surrounds the vascular tissues. Endodermal cells opposite the Phloem have thick lignified Cell Walls while those opposite the xylem do not. |
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| Diagram showing the organization of Vascular Tissues and the Endodermis. |
