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Lycophyta-Lycopodium-3 |
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| As we have
seen, there are several types of stems produced by Lycopodium.
These include an underground Rhizome, a horizontal Stolon and Aerial Stems.
The Apical Meristem is Multicellular. This distinguishes it from the other plants we have seen so far. Multicellularity is a feature of the most advanced plants and probably represents a significant adaptation. However, it is unclear how this might be advantageous. Presumably the multicellular apex may have greater flexibility in terms of leaf initiation & possibly in terms of internal tissue differentiation & regeneration after wounding.
Leaves are initiated from single cells or a small group of cells, close to the Shoot Apical Meristem (SAM). This is typical for most vascular plants. The Epidermis of the stem typically has Stomata. The outer Cortex usually contains Chlorenchyma. This photosynthetic tissue may contain a lot of air spaces & can be continuous with the leaves. The take home message is that the stems can make a major contribution to the total amount of photosynthesis that takes place in Lycopodium. Microphylls represent a significant advance compared to leaves of the Bryophyta. However, plants like Lycopodium may still need a strong contribution from their stems in order to survive & thrive. Some of the internal cortical cells have thick walls which become lignified. These constitute Sclerenchyma tissue which imparts strength to the stem. Sclerenchyma cell walls typically contain Lignin and stain red with Phloroglucinol. Leaf traces are present. |
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 Lycopodium Shoot Tip Showing two Shoot Apical Meristems (SAM) that have resulted from Dichotomous Branching. LP = Leaf Primordia  |
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 Cross Section of a Lycopodium Leaf viewed with crossed Polarizing Filters. The Cuticle of the Epidermis and tissues associated with the central Vein are Birefringent. |
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  Lamina of a Lycopodium Microphyll showing the Chlorenchyma. Note the Air Spaces in the lower Image. Would the term Aerenchyma apply? |
"Midrib" of a Lycopodium Leaf showing the solitary Vein which makes it a Microphyll, and surrounding tissues. |
 Midrib of a Lycopodium Microphyll stained with Toluidine Blue. Thick walled cells in the Xylem have stained positively for Lignin. The Phloem probably surrounds the Xylem but I haven't designated any of the cells as Phloem because I am uncertain about this. |
 Midrib of a Lycopodium Microphyll stained with Toluidine Blue: Cell Walls in the Xylem have stained positively for Lignin. |
| Stomata and adjacent Epidermal Cells from the Abaxial Epidermis of Lycopodium phyllanthum: Stained with Toluidine Blue. Parts of the Guard Cell Wall have stained positively for Lignin. |  |
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 Cross Section of Lycopodium phyllanthum Stem Unstained: Note the Cortex and Stele. |
 Cross Section of Lycopodium phyllanthum Stem Stained with Phloroglucinol: Note the Sclerenchyma in the outer Cortex. |
 Stele of L. phyllanthum Unstained but seen with Crossed Polarizing Filters. Xylem Tracheids are prominent due to their Birefringence. |
 Stele of L. phyllanthum Unstained: Note the Tracheids of the Xylem and the surrounding Phloem. |
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| An Endodermis is visible in young stems but may be hard to find in mature stems. | |
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