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The first fossil land plants had horizontal and vertical shoots. They were either leafless or had minute leaves. One of these plants is called Rhynia.

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Reconstruction of Rhynia one of the first fossil land plants. Cross section of a Rhynia Stem: Note the concentric circles of primary tissues. The term Stele is is used to indicate the Xylem and Phloem

The Psilophyta contains a genus Psilotum which closely resembles plants like Rhynia. It is essentially a stem that has two interconnected forms. The underground stem (Rhizome) produces Aerial Stems which are Photosynthetic. The Leaves are minute and contribute little to the plant's nutrition. The Rhizome produces Rhizoids which act like root hairs.

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Psilotum Aerial Stems: These have an Apical Cell and they Branch Dichotomously. Psilotum Aerial Stem spread out to show the Dichotomous Branching.
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One terminal branch from Psilotum. The Leaves originate at the Shoot Apical Meristem. They have a Spiral arrangement (Alternate). The stem has ridges and valleys. The stomata (white spots) occur in the depressions. This provides a small amount of protection from excess water loss when the Stomata are open.

Dichotomous Branching occurs at the Apex by the formation of a second Apical Cell. In this case the branches are the same size. This is called Isotomous
(Iso = Same).
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Two Young Rhizome Branches

Psilotum Underground Rhizome: Some Rhizome tips start to grow upwards and are converted into photosynthetic shoots. No Roots are produced. The Rhizomes serve as roots and they produce Rhizoids which act like root hairs.

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Tip of a Psilotum Rhizome
. Note the hair-like Rhizoids.
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The Psilotum Rhizome has the most simple kind of tissue organization with Xylem at the center, surrounded by Phloem, Endodermis, Ground Tissue and Epidermis. Cross Section of the Stele from Psilotum Rhizome: Note the Thick-walled Lignified Xylem which is surrounded by Phloem and the Endodermis.
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Psilotum Stele: The Xylem is star-like and is embedded in Phloem. This is called an Actinostele
(Actino = Star).

The Xylem of Psilotum contains Tracheids which are similar to those seen in other Vascular Plants. These have characteristic Secondary Wall thickenings, unlike the Hydroids we examined earlier. The Phloem contains Sieve Elements which are similar to those seen with other Vascular Plants, unlike Leptoids which do not have all the features of Sieve Elements.

The anatomy of the Aerial Stem is similar to that of the Rhizome. However, the Xylem is star-shaped rather than circular and the Ground Tissue contains Sclerenchyma and Chlorenchyma. PsiloStemX-SMediumMagCropLab200.jpg (72275 bytes)
Outer part of the Stem showing the three types of Ground Tissue and the Epidermis which has a definite Cuticle and Stomata.

There is a debate concerning the ShapeRootShootCropLab200.jpg (26414 bytes)Leaves of Psilotum. They do not have Veins although Leaf Traces may diverge from the Stele and approach the leaf bases. I think that they are leaves. If we assume that they had one Vein, they would be Microphylls.

The next step lead to the evolution of distinct Shoots and Roots. The shoots were specialized for Photosynthesis and the Roots were specialized for absorption and anchorage. Root anatomy would not differ significantly from the cylindrical Rhizome we saw earlier. Furthermore, Root Anatomy is very constant in Land Plants. Branching occurred in the roots and produced a DichotBranchRootShoot200Lab.jpg (20632 bytes)complex root system.

Upright stems were able to overtop flat thalli and became dominant organisms. The production of photosynthetic branches further increased the surface area available for photosynthesis. Branching was initially Dichotomous for stems and roots. Lateral branching developed later in Evolution.

The ascent of aerial stems and their branches required the development of superior support tissues. This was achieved with the Psilophyta. Xylem Tracheary Elements have thick walls and provide some structural support. However, Sclerenchyma (Scler = Hard) tissue evolved and Sclerenchyma Fibers provided extra mechanical strength. Fibers are ShapeLatBranchLab300.jpg (35558 bytes)found in the Vascular Tissues or in close association with them. Sclerenchyma and Parenchyma are the two principal Ground Tissues.

The final step in the evolution of our theoretical plant is the production of Leaves. These are highly specialized for Photosynthesis in many ways. They usually have a wide thin Blade (Lamina) and a thicker Midrib. They may have only one Vascular Bundle (Vein) or a network of interconnected Veins. They are attached to the stem by a nonphotosynthetic petiole. LeafMPhyll3DFlatLab400.jpg (56209 bytes)

The first Leaves were Microphylls and had one Vein/leaf. The vein had a central location and was surrounded by Photosynthetic Parenchyma.

The major functions of ShapeLeavetLab400.jpg (35065 bytes)the stem are translocation of water and photosynthate and structural support for the leaves.

Our theoretical plant has three distinct Organs (Leaf, Stem & Root).

Species in the Lycophyta illustrate simple plants that have Microphylls.

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