Bennettitales: Introduction, Origin, Features and Affinities

Introduction and Origin of Bennettitales:

Some peculiar characteristics present in Bennettitales and not in any other group of gymnosperms include:

• (i) They bear bisporangiate strobili.
• (ii) Synangium-bearing fused microsporophylls (Triades of Sporangia).
• (iii) Close occurrence of ovules and inter-seminal scales.
• (iv) Production of stalked ovules.

Distribution

The fossilized trunk of the genus Bucklandia was the first specimen of Bennettitales, collected from Great Britain in 1825. Several well-preserved specimens have also been reported from Canada and England. Germany, Belgium, Poland, Russia, Italy, and India has long been famous fossil Cycad locality.

Features

1. These were present on the earth from Triassic to Cretaceous periods.
2. These were abundant during the Mesozoic era (The age of Cycads).
3. Their fossil record is preserved as compressions and petrifactions.
4. The stems were slender and had a wide pith.
5. The Bennettitalean leaves were mostly pinnately compound, and only occasionally simple.
6. They resemble much with cycads.
7. Venation was open, and only rarely closed.
8. Syndetocheilic type of stomata was present (Gymnosperms).

Syndetocheilic type of stomata

1. The reproductive organs were organized in the form of hermaphrodite (e.g. Cycadeoidea) or unisexual (e.g. Wielandiella) “flowers”, protected by many bracts.
2. The ‘flowers’ developed in the axil of leaves.
3. Male reproductive organs were borne in a whorl.
4. Microsporophylls sometimes surrounded megasporophylls forming hermaphrodite “flowers”.
5. Ovules were numerous and stalked and borne on a conical, cylindrical, or dome-shaped receptacle.
6. The scales or bracts were united at the end to form a shield through which the micropyle protrudes.
7. Seeds were dicotyledonous.

Strobilus               Ovules

Classification of Bennettitales:

What do you know about bennettitaceae and williansoniaceae?

They are classified into different groups by different taxonomists as under;

1. Walton (1940) recognized the following two families in Bennettitales:

1. Williamsonieae: Flowers are fully exposed and present on slender stems, e.g. Williamsonia. Williamsoniella and Wielandiella.
2. Bennettiteae: Flowers deeply sunk among the persistent leaf bases and were present on short thick trunks e.g. Cycadeoidea

Williamsonia

1. Arnold (1948) classified Bennettitales into two families

1. Williamsoniaceae
2. Cycadeoideaceae

• Williamsoniaceae
• Wielandiellaceae,
• Cycadeoideaceae,

Affinities of Bennettitales

This group has multiple characters which resemble different other groups of plants.

Resemblances of Bennettitales with Ferns

1. Bennettitalean plants had multicellular ramenta (Thin brownish scales) on their entire body, a characteristic also seen in ferns.
2. Some other features include the presence of;
   1. Direct leaf traces
   2. Scalariform tracheids

• Presence of large pith.

Resemblances of Bennettitales with Cycads

1. The structure of the fronds of both groups is similar in terms of anatomical and morphological features.
2. Both groups have short stems covered with persistent leaf bases.
3. The stem is barrel-shaped or cylindrical.
4. The presence of a very thick cortex, relatively thin wood, and large pith in the stem are the prominent features of both groups.
5. Both groups have monoxylic wood (non-compact wood with a large amount of parenchyma, large pith, and cortex mixed with less amount of xylem tracheids or wood)
6. Monocolpate pollen grains (pollen grains are present in monocots) are present.
7. Ovules are orthotropous (straight ovules with the micropyle at the apex).

Affinities with Cycas

Resemblances of Bennettitales with Pteridospermales

1. Presence of ramenta hairs.
2. Syndetocheilic stomata.
3. Direct leaf traces.
4. Similar anatomical details.
5. Leafy microsporophylls.
6. Presence of cupule (A plant structure made of hardened, cohering bracts, resembling a cup).

Resemblances of Bennettitales with  Gnetales

The two groups resemble each other in their seed structure but differ completely in several aspects.

According to Rodin and Kapil (1969), “the complex and highly specialized inflorescence, the presence of inter-seminal scales and some vegetative features fail to show homologies with Gnetales”.

Resemblances of Bennettitales with angiosperms

1. Frequent occurrence of scalariform tracheids in both groups also brings them close together.
2. Flowers of several primitive angiosperms also resemble closely the strobili of this group. It shows that are the ancestors of flowering plants.

Contrary to this,

1. Ovules are naked in Bennettitales while it is not so in angiosperms.
2. The wood rays of Bennettitales lack marginal cells which are present in angiosperms.
3. The two groups also differ in their general habit and floral morphology.

Following is a list of characteristics of Bennettitales and Cycadales

Cycadales	Bennettitales
1. Stomata haplocheilic. 2. Secondary wood polyxylic. 3. Traces arise as two independent units at the point of their origin. 4. Flowers are mono-sporangiate and plants are dioecious. 5. Microsporophyll’s simple with sori on lower side and arranged spirally. 6. Such an arrangement stalked ovules is not found.	1. Stomata syndetocheilic. 2. Secondary wood monoxylic. 3. Leaf traces arise always singly at the point of their origin. 4. Flowers are bisporangiate and plants are monoecious. 5. Microsporophyll’s large, uni-or bi-pinnate, arranged in whorls and fused at the base. 6. Arrangement of stalked ovules along with inter-seminal scales.

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