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Psilotum is a genus of fern-like vascular plants.It is one of two genera in the family Psilotaceae commonly known as whisk ferns, the other being Tmesipteris.Plants in these two genera were once thought to be descended from the earliest surviving vascular plants, but more recent phylogenies place them as basal ferns, as a sister group to Ophioglossales.
Higher up it became elliptical in cross-section, corresponding to the two rows of fertile branching units. Within the branches of these units, the strand was more-or-less rectangular. The conducting elements of the xylem, the tracheids , were of the so-called 'P-type' in which the walls were strengthened by ladder-like (scalariform) bars with ...
Psilotum has been interpreted as producing sporangia (fused in a synangium) on the terminus of a stem. Equisetum always produce strobili, but the structures bearing sporangia (sporangiophores) have been interpreted as modified stems. The sporangia, despite being recurved are interpreted as terminal.
An interesting case is that of Psilotum, which has a (simple) protostele, and enations devoid of vascular tissue. Some species of Psilotum have a single vascular trace that terminates at the base of the enations. [2] Consequently, Psilotum was long thought to be a "living fossil" closely related to early land plants (rhyniophytes).
A strobilus (pl.: strobili) is a structure present on many land plant species consisting of sporangia-bearing structures densely aggregated along a stem.Strobili are often called cones, but some botanists restrict the use of the term cone to the woody seed strobili of conifers.
Vegetative stem: N = node, I = internode, B = branch in whorl, L = fused microphylls Cross-section through a strobilus; sporangiophores, with attached sporangia (spore capsules) full of spores, can be discerned. Strobilus of E. braunii, terminal on an unbranched stem
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Cross-section of a flax plant stem: 1. Pith 2. Protoxylem 3. Xylem I 4. Phloem I 5. Sclerenchyma 6. Cortex 7. Epidermis. In botany, a cortex is an outer layer of a stem or root in a vascular plant, lying below the epidermis but outside of the vascular bundles. [1]