In this essay we will discuss about:- 1. Stems of Medullosa 2. Leaves of Medullosa 3. Petiole 4. Roots 5. Pollen Organs 6. Ovules (Seeds).
Essay on Medullosa
Essay Contents:
- Essay on the Stems of Medullosa
- Essay on the Leaves of Medullosa
- Essay on the Petiole of Medullosa
- Essay on the Roots of Medullosa
- Essay on the Pollen Organs of Medullosa
- Essay on the Ovules of Medullosa
Essay # 1. Stems of Medullosa:
The well-known reconstructed Medullosa noei shows a general appearance of a tree fern of 3.5 m to 4.5 m height with a crown of leaves. The lower portion of the stem is covered by periderm and adventitious roots.
The scars of spirally arranged leaf bases are arranged on the upper portion of the stem. The T.S. of young stem shows two to four segments of vascular tissue. In older stem, a ring of secondary parenchyma is present that superficially resembles another vascular segment (Fig. 1.5A).
The number of vascular segments varies not only from level to level, but also within the species. Vascular segment is elliptical or band shaped in cross-section and each segment is protostelic in nature, composed of clusters of large metaxylem cells separated by abundant parenchyma. There are one to four mesarch sympodia or axial bundles near the edge of the primary body. Trachieds of secondary xylem are very long (up to 24 mm) with 12 rows of bordered pits on radial wall.
The periderm cells are isodiametric, thick-walled and produced towards inside the stem. The cortical parenchyma and abundant elongated secretory canals are present towards outside. Patches of sclerenchyma fibres form interrupted bands around the periphery of the stem.
Most of the geologically younger (Permian) medullosan stems differ markedly from the older (Carboniferous) forms, principally on the basis of the arrangement of vascular segments. Medullosa anglica and M. thompsonii of Lower- Middle Pennsylvanian are the two other important medullosan stems.
Colpoxylon, recorded from Permian of France, is characterised by the complete absence of central vascular strands in the ground tissue. The leaf traces are small and circular with secondary xylem. Sutcliffia of Lower Coal Measures of England represents bigger stem (15 cm in diameter) with two large central vascular segments surrounded by many subsidiary segments.
Essay # 2. Leaves of Medullosa:
Medullosan fronds are very large, dichotomously branched or slightly unequal in lower portion, pinnately compound. The most common leaf-genera are Neuropteris and Alethopteris. Pinnae of Alethopteris are attached with Medullosa noei stem.
Pinnules show a prominent mid-vein from which secondary veins develop at sharp angles. T. S. of pinnule shows it’s revolute (turned under along the margin) nature and the midrib contains one to four vascular traces. One to two layers of palisade cells are present beneath the upper epidermis, while in the lower part loosely arranged mesophyll cells are found.
In Neuropteris (Fig. 1.5B), the pinnules possess a cordate base. The prominent midvein and secondary veins dichotomise several times before entering the pinnule margin. T.S. of Neuropteris rarinervis pinnules shows upper epidermis with undulating margins and parenchymatous bundle sheaths.
Other important leaf genera of medullosan members are Reticulopteris, Mixoneura, Cyclopteris, Odontopteris, Lonchopteris, etc.
Essay # 3. Petiole of Medullosa:
The detached petioles of Medullosaceae are called Myeloxylon which have numerous secretory canals. T.S. of Myeloxylon shows a striking similarity with maize stem in having many scattered vascular bundles and sclerenchymatous fibre at the periphery.
Essay # 4. Roots of Medullosa:
Roots of Medullosa are adventitious. T.S. of roots attached to stems of Medullosa anglica are triarch with broad periderm in the older roots. In some Middle Pennsylvanian Medullosa roots, xylem exhibit exarch actinostele (pentarch) with secondary xylem around the primary body.
Essay # 5. Pollen Organs of Medullosa:
Pollen organs of Medullosaceae are synangiate structure with tubular sporangia showing longitudinal dehiscence. Bernaultia is a well-described pollen organ, consists of a hemispherical campanulum (bell- shaped structure) with an eccentrically placed pedicel.
There are radiating rows of paired sporangia with multicellular hairs. Whittleseya (Fig. 1.5B) is a hollow synangiate compression- impression campanulum formed by the fusion of a large number sporangia side by side. Whittleseya pollen organs are associated with Neuropteris frond. Codonotheca is another compression-impression pollen organ, made up of 6 sporangia in a ring that are fused at bases and the apex remains free.
A complete fusion of a ring of 6 sporangia is observed in compressed Aulacotheca. Dolerotheca is a large and complex pollen organ, consisting of four independent radial sporangia that are fused together to form a compound structure. It was observed that Bernaultia is associated with Myeloxylon frond of Alethopteris pinnules.
Essay # 6. Ovules (Seeds) of Medullosa:
The seeds of Medullosaceae are frequently found as casts, although many permineralised seeds have been reported. Brongniart in 1828 proposed Trigonocarpus nomenclature for cast specimen of Medullosa seeds. It has been observed that permineralised Medullosan ovules, Pachytesta (Fig. 1.6A-C) are associated with Alethopteris fronds. So there has been controversy regarding the validity of the two generic names for the medullosan ovules (seeds).
It has now been universally accepted that the name Trigonocarpus is to be restricted to those seeds which are preserved as cast or compression-impression fossils and the name Pachytesta to be retained for permineralised (petrified) ovules (seeds). Both the genera show trimerous organisation.
Panchytesta ovules are the largest among seed-ferns. Pachytesta (Fig. 1.6C) shows its massive nucellus which is free from the integument except at the base. Nucellus forms a simple bell-shaped pollen chamber.
The ovule reveals double vascularisation where both the nucellus and integument are supplied with vascular strands. The integument is three- layered (Fig. 1.6B): an outer fleshy layer (sarcotesta), a stony, fibrous, middle layer (sclerotesta) and an inner thin layer (endotesta). Megagametophytes have been reported to be preserved in Pachytesta seeds.
