Cycadofilicales: Features and Classification | Gymnosperms |

Origin of Cycadofilicales:

The Cycadofilicales or Pteridosperms are popularly known as ‘seed ferns’. They were trees or small plants bearing fern-like foliage. They flourished greatly in the Carboniferous and the period was so very rich in fossil fern leaves that it was thought to be the ‘Age of Ferns’.

Owing to the presence of fern-like leaves they were formerly believed to be a kind of fern, but subsequently when fossil leaves with attached seeds were discovered (1903) the Cycadofilicales were designated as Pteridosperms by Oliver and Scott. They are regarded as the primitive group of seed-plants.

In the year 1877, Grand Eury first discovered a few species of fern leaves which later on went by the name of the form genus myeloxylon. In 1887 Williams pointed out that Heterangium (stem) and Kaloxylon (root) combined characters of both ferns and cycads, which led potonie (1899) to create a new systematic group the Cycadofilicales.

With the discovery of the fact that Lagenostoma lomaxi belongs to the stem of Lynginopteris oldhamia by Oliver and Scott in 1903, the name Pteridospermae was pro­posed for the seed-bearing Cycadofilicales. The seed-bearing habit of the Pteridospermae was again verified next year by Kidston in Neuropteris heterophylla.

The Cycadofilicales or Pteridospermae are popularly known as ‘seed ferns’. Though a few petrified stems have been discovered in the Devonian, yet no positive seed has been obtained before the Lower Carboniferous is reached. The plants first appeared in the Devonian, became very prominent in the Carboniferous, but remained wide-spread during the Permian, but a few survived even up to Mid-Jurassic. The best known Cycadofilicales are reported from Great Britain.

Characteristic Features of Cycadofilicales:

(a) Vegetative Organs:

1. Plants were trees or small plants with monopodial bran­ching and presence of ramentum.

2. Leaves were large and fern-like bearing micro-and megasporangia.

3. Leaf traces relatively large, single or double.

4. Leaves possessed resistant cuticle.

5. Primary xylem well-developed, usually mesarch or endarch.

6. Secondary xylem and phloem present, tracheids bearing numerous multiseriate bordered pits on the radial walls, and a large pith.

(b) Spore-Producing Organs:

1. Pollen-producing microsporangia are of marattiaceous type. They are exannulate, sometimes grouped into synangia and difficult to distinguish from the fructifications of ferns, but never grouped into cone.

2. The ovule had an integument differentiated into three layers, the vascular supply was divided into two sets of branches, a well-developed nucellar beak and a pollen chamber.

3. Seeds were not in cones or inflorescences, but on slightly modified and undistinguished foliage and they resembled those of Cycads.

Classification of Cycadofilicales:

The Cycadofilicales have been divided into two major groups:

i. The Paleozoic Cycadofilicales and

ii. The Mesozoic ones.

The former is further divided into three families, viz., Lyginopteridaceae, Medullosaceae, and Calamopityaceae, while the latter also consists of three families, viz., Peltaspermaceae, Corystospermaceae, and Caytoniaceae. Of the six families mentioned above, the most exten­sively studied one is Lyginopteridaceae.

A brief account of each of the families is given below:

1. Lyginopteridaceae:

In the family the leaves are of the filicoid type, and the stem is of monostelic organization. The leaf, traces and foliar bundles are single or double; the xylem is mesarch. Seeds are of cycadean type, and the cupule is present. The best known lyginopterid is Calymmatotheca, hoeninghausi. This is the plant to which both Lyginopteris and Lagenostoma belong.

2. Medullosaceae:

In this family the stem is polystelic, and many bundles are scattered in the petiole. Seeds are large.

3. Calamopityaceae:

This is the oldest and the largest of all the families of Pteridosperms. The stem is monostelic, and scattered bundles are present in the petiole.

4. Peltaspermaceae:

The leaves in this family are small, bipinnate, and the rachis is characterized by swellings. The microsporophyll is branched, and microsporangia open longitudinally, and smooth pollens are discharged.

5. Corystospermaceae:

The leaves in this family are linear-lanceolate and the seeds are characterized by ovoid, elliptic shape, and curved bifid micropyle.

6. Caytoniaceae:

This family is characterized by leaves formed usually in two (occasionally three) pairs. Anatomically it has transfusion tissue and mesophyll with palisade layer. The pollen grains show a symmetrical bilobed structure, and these are winged.

The ovules are orthotropous, and the nucellus is free from the integument except at the base. This is an advanced character. Formerly the Caytoniales were compared with the Angiosperms, but later on it was found out that members of this family are essentially gymnospermous.