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Origin of Angiosperm

This document discusses several theories on the evolutionary origins of angiosperms (flowering plants): 1. The Isoetes-Monocotyledon theory proposes that monocotyledons evolved from various groups of pteridophytes through a hypothetical intermediate called Proangiosperms. However, modern views consider the similarities between Isoetes and monocots to be superficial. 2. The Conifer-Amentiferae theory suggests that angiosperms evolved from conifers like Cordaites. While amentiferous plants like willows share some features with conifers, they are now seen as specialized, not primitive. 3. The Gnetales-Ang

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Dr. Avinash Gholave Department of Botany K.V. N. Naik Arts, Commerce And Science College Nashik. agholave@gmail.com
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. Isoetes Isoetes
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. Najas flexilis
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. Lycopodiales Equisetales
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. Conifers Cordaites Casuarinaceae
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. Trochodendron Mae cone Female cone
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. Gnetum female flower Gnetum Male flower
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). Gnetum wood cross section Gnetum leaf
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. Welwitschia plant Welwitschia male flowers
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.
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: 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. Armen Takhtajan
5. Pteridosperm Theory: The Pteridosperms are also known as Cycadofilicales or seed ferns. They are so called because the plants show fernlike leaves; some of them bears leaves associated with cycadlike 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. Cycadofilicales
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. Further more, 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. Glossopteridales Dichapetalum
Origin of Angiosperm