Cordaitales: Classification and Features | Gymnosperms | Botany

In this article we will discuss about:- 1. Classification of Cordaitales 2. Characteristic Features of Cordaitales 3. Fossil.

Classification of Cordaitales:

The order is usually divided into three families:

1. Pityeae,

2. Cordaiteae and

3. Poroxyleae.

Of the three families, the best known as well as most extensively studied one is the Cordaiteae.

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

1. Pityeae:

The important genera belonging to this family are Pitys, Archaeopitys and Callixylon. The foliage structure is only known in case of Pitys dayl, which is fleshy and about 4-6 mm. in diameter. There is no definite leaf gap present. The structure of the stem is known.

A cross-section of the stem of Callixylon shows a parenchymatous pith, mesarch primary wood, compact secondary wood with cha­racteristically alternately pit­ted tracheids, feeble growth rings and uni-to multi-seriate rays. A group of pits (occur­ring on the radial walls) alternating with an unpitted area, in a regular pattern is a unique feature of this wood only. No fructification is known as yet.

2. Cordaiteae:

The important genus is Cordaites, which is an assemblage of different form genera.

3. Poroxyleae:

This family is very restricted in its distribu­tion and consists of only one genus, Poroxylon, with two species. The structure of the stem comprises of a large pith, surrounded by a primary exarch wood. The secondary wood is loose, and the leaves are strap-shaped. The seed is known as Rhabdocarpus.

The Cordaitales constitute a group of Paleozoic arborescent gymnosperms which together with the Cycadofilicales contributed the bulk of the seed plants of the Permo-carboniferous coal forests.

Characteristic Features of Cordaitales:

(a) Vegetative Organs:

(1) The members of Cordaitales were tall and slender trees, the trunks often reaching a height of 10-30 metres before branching. The long slender shaft bore a dense crown of branches, on which were produced an abundance of large and simple leaves. No such habit is known among the existing gymnosperms.

(2) The vascular cylinder is collateral and endarch. The tracheids are thick-walled and compactly arranged. The radial walls are heavily pitted and the pits are alternately and multi-serially arranged. The pits are segregated into groups with unfitted spaces which are the characteristic of the group. This is not found in any other group, living or fossil. The innermost wood forms a ring of separate xylem strands. Double leaf trace is present. Pith is large and separate.

(3) The leaf is very variable in shape, rounded to very slender in size. No midrib is present, but a series of parallel lines traverse the length of the blade. They were borne on spiral sequence on the smaller branches that formed the crown of the tree. The bundles were surrounded by bundle sheath and connected by transverse bridge of thick-walled cells.

(b) Spore-Producing Organs:

(1) Fructifications are lax inflorescences on the slender stems among the leaves.

(2) The strobili of the male inflorescence contain 1-6 or more fertile bracts (microsporophylls), each of which supports as many as 6 terminal elongated pollen sacs.

(3) Each female strobilus usually contains 1-4 ovuliferous appendages (megasporophylls). In some cases, megasporophylls are forked and bear two terminal flattened ovules.

(4) Air sacs rendered the pollen buoyant and facilitated easy dispersal by wind.

(5) The winged seeds are flattened and heart-shaped, the nucellus is free from the integument except at the base and the vas­cular supply is double. The nucellar beak of the pollen chamber projects into the micropyle. Fertilization probably took place by swimming sperms. If a pollen tube was present, it probably func­tioned as an absorptive organ as in recent Cycads and Ginkgos.

Fossil Cordaitales:

The Cordaitales, with the Pteridosperms, formed the vegeta­tion of the late Palaeozoic era. The Lower Gondwana rocks of India have yielded potrified wood which are presumably cordaitean.

The fossil Cordaitales which are found in India are as follows:

1. Neoggerathiopsis:

It is a leaf genus which occurs in the Lower Gondwana in India. It has two most common species N. hislopi and N. stoliczkanus; leaves are elongate to spathulate in shape with broadly rounded apex. They vary in size, larger ones reaching more than 20 cm. in length.

2. Samaropsis:

It is the seed genus with few species and occurs in the Karbarbari and Raniganj series of Lower Gondwana in India. The seeds are winged, ovate or flat with emarginate apex and found in close association with Neoggerathiopsis hislopi.

3. Cordaicarpus:

It is also a seed genus with more than one species, and occurs in the Talchir of Lower Gondwana of India. The base is slightly cordate and the apex pointed. They are small, oval or pear shaped and vary in size.

4. Dadoxylon:

It is the stem genus with more than one species. The important Indian species are D. indicum, D. bengalense and D. zalesskyi. These occur in the Barakar series of Lower Gondwana. The species are silicified stems with a large pith containing sclerotic cells. The secondary wood has sharply marked growth rings.