Theories on the Origin of Angiosperms

Read this essay to learn about the theories on the origin of angiosperms.

Essay Contents:

Essay on the Isoetes-Monocotyledon Theory
Essay on the Conifer-Amentiferae Theory
Essay on the Gnetales-Angiosperm Theory
Essay on the Bennettitalean Theory
Essay on the Caytonialean Theory
Essay on the Pteridosperm Theory
Essay on the Pentoxylales Theory

Essay # 1. Isoetes-Monocotyledon Theory:

Isoetales, Lycopodales, ferns etc. have shown to possess many common characteristics with monocotyledons. Engler and his associates postulated that the monocotyledons have originated from various groups of Pteridophyte through a hypothetical herbaceous intermediate, the Proangiosperms.

According to Campbell (1928) the herbaceous angiosperms are primitive, those inherited from filicinean ancestor. He also pointed out that the Isoetes, a living heterosporous genus, shows possible relationship with monocotyledons. Like most of the members of Filicineae, the Isoetes grows in aquatic or amphibious environment indicating relationship with monocotyledons.

Campbell suggested that though Isoetes shows similarity with monocotyledons like Najas flexilis with regard to habitat, anatomy of old sporophyte and resemblances in embryo, the differences in respect of simplest angiosperm flower and sporophylls of Isoetes need more evidence to support the hypothesis.

The Lycopodiales and Equisetales are now considered as completely separate and independent evolutionary lines.

The monocots are now considered to be more advanced than dicotyledons and their resemblance with Isoetes is superficial in nature.

Essay # 2. Conifer-Amentiferae Theory:

According to this theory, the higher gymnosperms like Conifers, Cordaites and others are considered as the probable ancestors of angiosperms. Engler (1882, 1892) and Rendle (1904, 1930) found the similarities of angiosperms with the conifers and considered the amentiferous group (Casuarinaceae, Fagaceae, Salicaceae etc.) as the most primitive dicotyledons.

The amentiferous members show the following feature resemblances with conifers:

i. Flowers are simple and naked like conifers;

ii. Stamens of amentiferae are like that of bisporangiate microsporophylls of conifers;

iii. Seeds are covered in angiosperms which have also been found in Araucaria and Agathis of gymnosperm;

iv. During fertilisation in Agathis and Araucaria and in other conifer, the pollens are deposited on the scale and only pollen tubes enter into the micropyle. This, feature is very similar to angiosperm (Doyle, 1945).

It has now been proved that the above similarities are probably due to parallelism between the two groups. The simple flower present in the amentiferous taxa are not considered as primitive, but developed due to reduction at higher level.

The vessel-less angiosperms like Tetracentron, Trochodendron etc. have more primitive secondary xylem than the conifers and amentifers. The amentifers are now considered to be more specialised and not considered as primitive one.

It has now been established that the primitive angiosperm flower is bisexual, whereas the cones of conifers are unisexual.

Lastly, it can be said that this theory does not gain proper rooting, because of the modern concept that amentifers are proved definitely as advanced members of the angiosperms.

Essay # 3. Gnetales-Angiosperm Theory:

Richard von Wettstein (1901) emphasised the close relationship between Gnetales and angiosperms, after modifying the Engler’s system as mentioned in Handbuch der Systematische Botanik.

Later on, Markgraf (1930) and Fagerlind (1947) boldly supported the above view. Fagerlind demonstrated the homology among the three genera of Gnetales (Ephedra, Welwitschia and Gnetum) and proposed that Gnetales and Pro-angiosperms evolved from a common ancestor.

Similarities between Gnetales and Angiosperms:

i. Presence of two cotyledons;

ii. Vessels in the secondary wood;

iii. Venation – reticulate;

iv. Unisexual inflorescence (similar to catkin of many Amentifers);

v. In Welwitschia, the male flowers are actually bisexual formed by reduction of female organs;

vi. Stamens of Ephedra, Welwitschia and Gnetum are apparently similar to angiosperms;

vii. The female flowers of all the above three genera have 2 or more envelops around the nucellus like the perianth of angiosperms;

viii. The well-developed micropylar tube (formed by the elongation of the inner envelop of the ovule) of female flowers looks like the style of angiosperm flower;

ix. Female gametophyte of Gnetum is tetrasporic and without archegonia like angiosperms;

x. First division of zygote in Gnetum is accompanied by cell wall formation like angiosperms.

Differences between Gnetales and Angiosperms:

i. Gnetum have vessels in secondary wood, while it is absent in some primitive dicots like Tetracentron, Trochodendron, etc.;

ii. The vessels of Gnetales originate in an entirely different way from those of angiosperms;

iii. The presence of scalariform pitted tracheids, a characteristic of primitive angiosperms, is absent in both primary and secondary xylem of Gnetum. While circular bordered pits appear in Gnetum during the ontogeny of secondary xylem;

iv. Gnetum also differs from angiosperms in respect of development of vessels.

Based on the above discussions, it can be said that the similarity between Gnetales and certain angiosperms is now considered as superficial and appears due to tilting towards convergence during evolution. Similarities in the number of cotyledons (two) also appear due to convergence.

The angiosperms might have been evolved from polycotyledonous ancestors rather than dicotyledonous one. The polycotyledon (3-4 cotyledones) is reported in an extant genus Degeneria, a primitive group of angiosperm. The absence of archegonia in Gnetum indicates its affinity with angiosperms due to parallelism. But the vascular anatomy strongly opposes any suggestion regarding the relationship between angiosperms and Gnetales.

Essay # 4. Bennettitalean Theory:

According to Saporta and Marion (1885), and Arber and Parkin (1907), the Bennettitales of gymnosperm is the possible ancestor of angiosperms. They considered Benettitales as the possible ancestor of angiosperms due to similarities between the strobili of Cycadeoidea, a Mesozoic genus and the flower of Magnolia.

Similarities:

i. Both the structures (strobili and flower) are bisexual having an elongated central axis;

ii. In Cycadeoidea the different parts like bracts, microsporophylls and megasporophylls are arranged from the bottom towards the apex, while in Magnolia, perianth, stamens and carpels are arranged in a similar way.

In spite of the above superficial similarities, both the groups show much differences by the detail investigation.

The differences are:

Differences:

i. Stem:

Benettitalean stems have a thick cortex, a comparatively thin vascular cylinder and a large pith, but the angiospermic plants have thin cortex, a thick vascular cylinder and a small pith.

ii. Microsporophylls (Stamens):

In Benettitales, the microsporophylls are arranged in whorled and mostly connate, but in Magnolia the stamens are free and spirally arranged on the axis.

iii. Megasporophylls (Carpels):

In Benettitales, the megasporophylls are greatly reduced having a simple stalk on which a single terminal ovule is present. Interseminal scales (sterile scales) are present in between megasporophylls for protection. The flower of Magnolia does not show such structures.

iv. Micropylar Tube:

The micropylar tube is present in Benettitales, but such structure is unknown in angiosperms.

v. Seed:

The seeds of Benettitales are exalbuminous i.e., non-endospermic having a large embryo, but the seeds of primitive angiosperms are albuminous i.e., endospermic and with a small embryo.

The above facts show that Benettitales is not in a position to be considered as the ancestor of angiosperms. Both the groups show much difference than the similarities. The above similarities may result due to common ancestry or parallel evolution.

According to Arber and Parkin (1907), both the groups have a common origin from seed ferns and they might have diversed very early. Later, Takhtajan (1980) mentioned in his classification that Magnoliophyta are regarded to be monophyletic in origin, most probably derived from Bennettitale-like ancestors or stocks ancestral to them.

Essay # 5. Caytonialean Theory:

The Caytonialean theory was proposed by H. Hamshaw Thomas (1925, 1936). His proposition was based on comparative studies of angiosperms with the certain member of Caytoniales (the fossil members of middle Jurassic period).

Some fossil members of Caytoniales had anthers developed singly or in groups on pinnately branched structures that are described as sporophylls (Fig. 4.34). The above structures can be compared with the branched stamens as found in plants like Ricinus (Euphorbiaceae), Calothamnus (Myrtaceae) and Hypericum (Clusiaceae). He also explained how angiosperms originate from Caytoniales.

According to him, the carpel wall of angiospermic plants represents a pair of concrescent cupules and the probable origin of stigma should be judged in the light of these primitive forms.

The Caytoniales show affinity with the Pteridosperms and are now considered as Mesozoic remnants of that group. Based on the morphological studies of the nature of carpel, it can be said that their ancestor should have open megasporophyll with exposed ovule, thus the ovary-like pouches of the form-order Caytoniales cannot be considered as the ancestor of the angiosperm carpel.

Later on, Stebbins (1974) supported the theory based on homology of ovules of the two groups: angiosperms and Caytoniales. According to him, ovule of angiosperm is homologous with the cupule of Caytoniales including both Caytoniaceae and Corystospermaceae.

The anatropous ovule with well-developed funiculus can be compared with the cupule with the stalk. In Caytoniaceae, the cupules are multi- ovulate and dehiscent, but in Corystospermaceae the cupules are uniovulate and indehiscent.

If one considers the cupule of Corystospermaceae to be homologous with the angiosperm ovule, then the outer integument becomes homologous with the cupule wall and the inner one with the single integument of Caytonia ovule as well as with the ovule of other gymnosperms.

The lateral ovules on flattened sporophylls of angiosperms show a different situation in contrast to the terminal position of cupule on branched axes in Caytoniales. According to Stebbins, the above situation can be explained on the basis of structure of Ligettonia (Fig. 4.35), a fossil member of Permian age reported from South Africa in association with Glossopteris leaves.

It was proposed that a branched cupule-bearing rachis became adnate to its subtending bract. Thus the carpel is considered as a compound structure alike to the ovule bearing cone-scale of conifers. In Ligettonia, the flattened and elliptical sporophyll likely bears two rows of dehiscent cupules on its surface.

Thus, according to this theory, the predecessor (forefather) of angiosperms were comparatively recent pteridosperms belonging to Glossopteridalean-Caytonialean alliance available during Jurassic to Permian Age.

Essay # 6. Pteridosperm Theory:

The Pteridosperms are also known as Cycadofilicales or seed ferns. They are so called because the plants show fern-like leaves; some of them bears leaves associated with cycad-like stem showing difference in stem anatomy with ferns. They bear true seeds.

They were reported abundantly from Upper Devonian to Permian of Paleozoic Age. Plants are often monoecious, but micro- and megasporophylls are not arranged in definite strobili. Based on the above characteristics of stem, leaf and seed, they are considered as the probable ancestors of Bennettitales.

The various evidences and interpretations discussed the possibility of Cycadales, Bennettitales or any other gymnosperms to be the ancestor of angiosperms. Therefore, emphasis was given to the Pteridosperms as the possible ancestor. Cronquist (1968) stated that “it is a long way, morphologically, from any known seed fern to an angiosperm, but each of the differences could logically be bridged in the course of evolution”.

The angiosperms show double fertilisation with triploid (3n) endosperm, possibly due to reduction of female gametophyte. Likewise, sepals originate from leaves and petals from both sepals and stamens.

Thus the evolution of an angiospermic flower can be traced from cones or cone-like structures of gymnosperms. Furthermore, the primitive angiosperm like Magnolia lacks vessels in secondary woods is similar to seed ferns. Thus, angiosperms could have evolved from a Pteridospermous ancestry.

Melville (1960) stated that the reproductive branch of Glossopteridales (Pteridosperm) is somewhat comparable with the present-day angiosperm Dichapetalum.

The external similarity of Bennettitales with angiosperms can also be explained in terms of common ancestry of both these groups in the Pteridosperms. Takhtajan (1969) stated that the “angiosperms arose from some very ancient group of gymnosperms, which must have had primitive secondary xylem of scalariform tracheids at least in the early wood, and primitive bisexual strobili. The strobili must have been of a type that could have diversed to give rise to the primitive Bennettitalean strobilus and the primitive angiosperm flower”.

Essay # 7. Pentoxylales Theory:

The Pentoxylales is a group of fossil under Pteridosperms. Meeuse (1961) proposed that angiosperms have originated from Pentoxylales. He found several similarities between Pentoxylales and the living order Pandanales.

The characteristics common with the genus Pandanus are:

a. Plants are erect or suberect and dioecious;

b. Stem slender and cylindrical;

c. Stem bears a terminal tufts of strap- shaped leaves arranged spirally;

d. Inflorescences develop either on short lateral branches in the axil of leafy bract or among the leaves;

e. The peduncle have tracheids with spiral thickening;

f. In Pentoxylales, the seeds are enclosed in sarcotesta, a fleshy layer; seed coat consists of two layers, endosperm copious and embryo minute. All the characteristics are found in the seeds of Pandanus.

The above characteristics indicate the possible origin of angiosperms from Pentoxylales (Fig. 4.36).

Later, Pant and Kidwai (1971) stated that the above similarity appears as a result of parallel evolution.

Theories on the Origin of Angiosperms | Essay | Botany