ADVERTISEMENTS:
In this article we will discuss about Bennettitales. After reading this article you will learn about: 1. Meaning of Bennettitales 2. Origin of Bennettitales 3. Nomenclature, History and Distribution 4. Distinguishing Features 5. Classification 6. Affinities.
Contents:
- Meaning of Bennettitales
- Origin of Bennettitales
- Nomenclature, History and Distribution of Bennettitales
- Distinguishing Features of Bennettitales
- Classification of Bennettitales
- Affinities of Bennettitales
1. Meaning of Bennettitales:
This group of fossil plants flourished well during the Triassic to Lower Cretaceous periods of Mesozoic era. As the Carboniferous period is called the “Ages of Ferns “, the Mesozoic era is called the ‘Ages of Cycads’. It is due to the fact that Cycadeoideales co-existed with Cycadales during Mesozoic era from Jurassic up to Cretaceous period, and hence this period is called Age of Cycads.
ADVERTISEMENTS:
Bennettitales are found either in the form of compressions or petrifactions. Due to Cycad-like form of their fronds and the presence of short stems covered with an armour of presistent leaf bases Bennettitales (Cycadeoideales) have been treated under Cycadophyta by some workers. However, the two groups are quite distinct from each other and maintain their independent identity.
2. Origin of Bennettitales:
Palaeobotanists believe that Bennettitales originated from Pteridospermales. But presence of stalked ovules in Bennettitalean members is a strong evidence against such a theory.
Some workers have, however, shown resemblance between the fronds of Pteridospermales and the stalked ovules and inter-seminal scales of Bennettitales. But it is hard to imagine that stalked ovules and inter-seminal scales are homologous with the fronds.
Discovery of a fossil plant, Westersheimia, from the Triassic of Austria, is again a step towards the possible Pteridospermean ancestry of Bennettitales. This genus occurs along with Bennetticarpus, the seed-bearing organs of Bennettitales.
ADVERTISEMENTS:
In Bennetticarpus wettsteinii and Westersheimia the ovules and inter-seminal scales were present on the ultimate segments of a pinnate structure. But it is still not clear how the entire group of Bennettitales evolved from Pteridospermales.
Some peculiar characteristics present in Bennettitales and not in any other group of gymnosperms, include:
(i) Bisporangiate strobili,
(ii) Synangium-bearing fused microsporophyll’s,
(iii) Close occurrence of ovules and inter-seminal scales, and
(iv) Production of stalked ovules.
3. Nomenclature, History and Distribution of Bennettitales:
The name “Bennettitales” has been given to honour J.J. Bennett, an English botanist. The fossilized trunk of genus Bucklandia was the first specimen of Bennettitales, collected from Great Britain in 1825. A silicified trunk of Cycadeoidea etrusca was discovered in 1867 in an Etruscan tomb.
Williamson (1870) coined the name Williamsonia for a combination of foliage and reproductive organs of Williamsonia gigas. Professor Birbal Sahni (1932) discovered Widiamsonia sewardiana from the Upper Gondwana beds of India.
A rich fossil flora of Bennettitales has been reported from Rajmahal Hills of Bihar (India). Some of the reported members include Bucklandia sahnii, B. indica, Dictyzamites, Otozamites benghalensis, Cycadinocarpus rajmahalensis, Sahnioxylon rajmahalensis, Williamsonia indica, W. sahnii and W. sewardiana.
ADVERTISEMENTS:
American Bennettitales have been described by Ward (1900). Several well-preserved specimens have also been reported from Canada and England. “Germany, Belgium, Poland, Russia, Italy and the Isle of Wight have yielded material, and India has long been a famous fossil Cycad locality”.
T.M. Harris (1964), T. Delevoryas (1968), T.N. Taylor (1970), W.L. Crepet (1974), S.H. Mamay (1970) and B.D. Sharma (1980) are some of the other workers worked on Bennettitales.
4. Distinguishing Features of Bennettitales:
1. These extinct Mesozoic plants were present were present on the earth from Triassic to Cretaceous.
2. Bennettitales were so abundant during Mesozoic era that this period is known as ‘Age of Cycads’.
ADVERTISEMENTS:
3. The members of this group are found either as compressions or petrifactions.
4. The stems were stout or slender and had a wide pith.
5. The stem grew very slowly and had manoxylic wood.
6. Resembling living Cycads, the Bennettitalean leaves were mostly pinnately compound, and only occasionally simple.
ADVERTISEMENTS:
7. Venation was open, and only rarely closed.
8. Syndetocheilic type of stomata were present.
9. The wall of the epidermal cells was sinuous.
10. The reproductive organs were organised in the form of hermaphrodite (e.g. Cycadeoidea) or unisexual (e.g. Wielandiella) “flowers”, which in turn were protected by many bracts.
ADVERTISEMENTS:
11. The ‘flowers’ developed in the axil of leaves.
12. Male reproductive organs were borne in a whorl. They were free or fused, entire or pinnately compound.
13. Microsporangia were present abaxially in the form of synangia.
14. Microsporophyll’s sometimes surrounded megasporophylls forming hermaphrodite “flowers”.
15. Ovules were numerous and stalked and borne on a conical, cylindrical or dome-shaped receptacle.
16. Many inter-seminal bracts were present on the ovule containing receptacle.
ADVERTISEMENTS:
17. The scales or bracts were united at end to form shield through which micropyle protrudes.
18. Seeds were dicotyledonous.
5. Classification of Bennettitales:
Walton (1940) recognized following two families in Bennettitales:
1. Williamsonieae:
Flowers 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 trunk e.g. Cycadeoidea.
Arnold (1948) classified Bennettitales into two families viz. Williamsoniaceae and Cycadeoideaceae while Sporne (1965) divided it into following three families:
ADVERTISEMENTS:
1. Williamsoniaceae, e.g. Williamsonia, Pterophyllum.
2. Wielandiellaceae, e.g. Wielandiella, Williamsoniella.
3. Cycadeoideaceae, e.g. Cycadeoidea (=Bennettites)
6. Affinities of Bennettitales:
Resemblances of Bennettitales with Ferns:
Bennettitalean plants had multicellular ramenta on their entire body, a characteristic also seen in ferns.
Some other features suggesting the filicean affinity of Bennettitales include the presence of:
(i) Direct leaf traces,
(ii) Scalariform tracheids, and
(iii) Large pith.
Resemblances of Bennettitales and Cycads:
Bennettitales resemble cycads in the:
(i) structure of their fronds,
(ii) presence of short stems covered with an armour of persistent leaf bases,
(iii) presence of barrel-shaped trunk,
(iv) presence of very thick cortex, relatively thin wood and large pith in the stem,
(v) manoxylic wood,
(vi) monocolpate pollen grains,
(vii) orthotropous ovules, and
(viii) dicotyledonous embryo.
On the basis of several such affinities, Chamberlain (1935) opined that both Bennettitales and Cycadales arose by parallel evolution from some common ancestor. The ancestral type must have had a foliar megasporophyll having the ovules at the apex as well as on both the sides.
During course of evolution, the lateral ovules must have suppressed and the megasporophyll must have reduced to a stalk-like structure, and thus the Bennettitales must have come into existence. On the other hand there exist several fundamental differences between Bennettitales and Cycadales, already listed in Table 6.1.
Due to so many dissimilarities it will not be possible to visualize any phylogenetic connection between two groups. Andrews (1961) concluded on the basis of such differences that both the groups have evolved along with two different and independent lines.
Resemblances and Difference between Bennettitales with Pteridospermales:
Characters common in both Bennettitales and Pteridospermales include:
(i) Presence of ramenta] hairs,
(ii) Syndetocheilic stomata,
(iii) Direct leaf traces,
(iv) Similar anatomical details,
(v) Leafy microsporophyll’s, and
(vi) Presence of cupule.
The so-called bisporangiate ‘flower’ of Bennettitales could be compared with the bisporangiate fronds of Ptendospermales. Scientists are of the opinion that there exist two lines of evolution from Pteridospermales. Of these, one line gave rise to Bennettitales possessing both uni- and bisporangiate forms, and the other gave rise to mono-sporangiate forms of cycads.
Among the major differences include the:
(i) Presence of smaller and stalked ovules in Bennettitales, which are absent in Bennettitales, and
(ii) Prominent vascular supply to the nucellar tissue in Bennettitales which is not seen in Pteridospermales.
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 specialised inflorescence of the Bennettitales, the presence of inter-seminal scales and some vegetative features fail to show homologies with Gnetum”.
Resemblances of Bennettitales with Angiosperms:
Endarch siphonostelic vasculature of Bennettitales resemble very closely with sympetalous angiosperms. Frequent occurrence of scalariform tracheids in both the groups also brings them close together. Flowers of several primitive angiosperms (e.g. Magnoliaceae) also resemble closely with the strobili of Bennettitales, and on this basis Arber and Parkin (1907) opined that Bennettitales are the ancestors of flowering plants.
Contrary to this, the Bennettitalean stamen is large, front like and compound structure, and cannot be compared with that of the stamen of Magnoliaceae. Moreover, there is no point of comparison between typical carpel of Magnolia and ovule of Cycadeoidea which is strictly gymnospermous.
Ovules are naked in Bennettitales while it is not so in angiosperms. The wood rays of Bennettitales lack marginal cells which are present in angiosperms. The two groups also differ in their general habit and floral morphology.
Regarding the evolutionary tendencies, Pant and Kidwai (1977) opined that Bennettitalean “flower” may represent a parallel line of evolution to angiosperms. Disappearance of Bennettitales during Cretaceous substantiates this view. During this period, however, the angiosperms were already existing by then on the earth.
Differences between Bennettitales and Cycadales:
Bennettitales:
1. Stomata syndetocheilic.
2. Secondary wood monoxylic.
ADVERTISEMENTS:
3. Leaf traces arise always singly at the point of their origin.
4. Fructifications are flower-like structures.
5. Flowers are bisporangiate and plants are monoecious.
6. Microsporophyll’s large, uni-or bi-pinnate, arranged in whorls and fused at the base.
7. Arrangement of stalked ovules along with inter-seminal scales.
8. Seeds exalbuminous.
Cycadales:
1. Stomata haplocheilic.
2. Secondary wood polyxylic.
3. Traces arise as two independent units at the point of their origin.
4. Fructifications are strobilar in nature.
5. Flowers are mono-sporangiate and plants are dioecious.
6. Microsporophyll’s simple with sori on lower side and arranged spirally.
7. Such an arrangement is not found.
8. Seeds always endospermic.
