Psilophyta (Wisk Ferns)

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.

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 called Psilotum which closely resembles plants like Rhynia.

It is essentially a stem that has two interconnected forms.

Its 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.

Psilotum Aerial Stems: Each of these have an Apical Cell and they Branch Dichotomously.	Psilotum Aerial Stem spread out to show the Dichotomous Branching.
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).	A 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.
	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.	Tip of a Psilotum Rhizome. Note the hair-like Rhizoids.

The Psilotum Rhizome has the most simple type 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.
	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.	Outer part of the Stem showing the three types of Ground Tissue and the Epidermis which has a definite Cuticle and Stomata.

Psilotum plants can be Terrestrial but they are often Epiphytes. Their Aerial Stems have a lot of Xeromorphic adaptations which are typical for Epiphytes.

There is a debate concerning the 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. Ultimately, the Bipolarity of Roots and Shoots so that there is a direct, continuous Vascular connection between these two organs evolved. This is illustrated on the right and is the way flowering plants are organized. Lycopods and Ferns have Adventitious Roots that arise from Stems. This is a significant advance compared to Psilotum but is is less effective than a Bipolar arrangement.

The Shoots are Aerial structures specialized for Photosynthesis while the Roots are subterranean structures specialized for Absorption and Anchorage.

Root anatomy would not differ significantly from the cylindrical Rhizome we saw earlier. Root Anatomy is very constant in Land Plants. This is due to the fact that Soil is a much more stable environment than the Atmosphere which is constantly changing.

Branching eventually evolved in roots. This produced a complex root system which can be just as extensive as the aerial system. The root system may be more extensive that the shoot system in dry (Xeric) environments.

Upright stems were able to Overtop Flat Thalli and became the 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 structural support.

However, Sclerenchyma (Scler = Hard) tissue evolved and Sclerenchyma Fibers provided extra mechanical strength. Fibers are found in the Vascular Tissues or in close association with them. However, Fibers are also found in the outer Cortex and this is the best location for structural elements. We see this with Psilotum and this will be a constant with more complex plants.

Sclerenchyma and Parenchyma are the two principal Ground Tissues. Xylem & Phloem can contain both of these.

Sclerenchyma that forms close to the Epidermis provides most of the structural support for soft (Herbaceous) stems.

The final step in the evolution of our theoretical plant is the production of Leaves.

Leaves 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 that differ in diameter. They are attached to the stem by a non-photosynthetic Petiole.

The first Leaves were Microphylls and had One Vein/Leaf. The vein had a central location and was surrounded by Photosynthetic Parenchyma. An Epidermis with Stomata covered the entire Microphyll.

The major functions of the stem are Translocation of Water and Photosynthate, plus Structural Support for itself & the Leaves & Branches.

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

Species in the Lycophyta illustrate simple plants that have Microphylls.