ORIGIN AND EVOLUTION OF ANGIOSPERMS

Angiosperms are the dominant seed bearing plants of the present day earth, consisting of about 300,000 species. They are commonly considered modern seed plants and geologically young. Eames (1961) however, regards them not to be of recent origin.

The unavailable fossil evidences show that the angiosperms originated roughly 165 million years ago in the Mesozoic era, probably in the Jurassic period or perhaps even earlier that, and appear in great outburst in upper cretaceous time, i.e., about 110 million years ago. But the actual origin or true relationship of this large group of plants dominating the earth today is still not known.

            Earliest known fossil angiosperms are the members of Ranales as well as of Amentiferae. Hence, some phylogenists believe that Ranales is the basal group in the evolution of angiosperms while the others believe that Amentiferae should be considered as the most primitive. But, till today, all these are mere assumption about the possible ancestors of angiosperms. Some assumptions consider the seed fern to be the ancient, while other considers some group of Cycads, etc.

            However there has been considerable difference of opinion as to the fact about the origin of angiosperms. Neither the time nor the place of their origin is known for certain. Thus, there are three aspects of problem regarding the origin and evolution of angiosperms.

            1. Time (fossil records) of angiosperms origin.

            2. Nature of probable ancestors of angiosperms and

            3. Place of origin of angiosperms.

      It has been indicated that various plant groups has been considered as ancestral to the angiosperms. Various orders of gymnosperms (Bennettitales, Cordaitales, Cycadofilicales, Cycadales and Gnetales) and pteridophytes (Psilophytales, Lycopodiales and Equisetales) have been considered as probable ancestors of angiosperms by different workers.

            Following are some of the views of workers regarding the origin and evolution of angiosperms.

            1. Isoetales – Monocotyledon Theory:- This theory has been advocated by Campbell (1928). According to this theory Monocotyledons have been derived from Isoetes via Ophioglossales. Linear leaves and cormous habit of Isoetes were co-related with some aquatic plants of monocotyledons. Details of embryo of Isoetes show similarity with some lower aquatic monocots like Nijas flexilis. But there were no fossil evidence to support this theory.

            2. Pteridosperm Theory:- Majority of phylogenists like Andrew (1947), Arnold (1949), Thomas (1955), Cronquist (1968) believe that seed ferns were the ancestors of angiosperms. Vascular histology, stellar structure and unavailable fossil records also favour this theory. The strongest evidence in fovour of this theory is the absence of vessel in secondary wood of some primitive angiosperms and in seed ferns. In both the pteridosperms and angiosperms the sporangial development is eusporangiate and the stele is amphipholic.

            3. Caytonialean Theory:- This theory was first proposed by Thomas (1925, 1936) and was supported by Stebbins (1974) on the basis of homology of ovules of angiosperms with those of Caytoniales. Caytoniales is a fossil gymnosperm of middle Jurassic period, which seems to have angiosperm-like characters. The ovules of these plants were semi-enclosed in small pouches, their leaves had reticulate venation and their sporangia resembled superficially with the anthers of angiosperns.

         4. Glossopteridalean Theory:- Dehgan (1893) has considered the seed fern or Glossopteridales as the probable ancestors of angiosperms. Megasporophylls of glossopterids are identical with a typical leaf of angiosperms. However, there is no similarity between the pollen of two.

            5. Bennettitalean Theory:- This theory was proposed by Saporta and Marion (1885) and followed by Arber and Parkin (1907). According to this theory the nature and organization of the reproductive structures of the members of Bennettitales are similar with the flower of angiosperms.

            The strobili of the Mesozoic genus Cycadeoide resemble with the flower of Magnolia. In both cases the strobili/flower are bisexual and contain an elongated axis having bracts, microsporophylls and megasporophylls. However, the stamens of Magnolia are free while the microsporophylls of Bennettitales are connate. The seeds of Magnolia and allied members have copious endosperm with a large embryo, while those of Bennettitales were non-endospermic with a large embryo.

            6. Coniferales – Amentiferae Theory:- Coniferales are considered as the probable ancestors of angiosperms by several phylogenists including Engler (1892) and Rendle (1904). They pointed out several resemblances between conifers and angiosperms, and treated Amentiferae as the most primitive dicots. The flowers/inflorescences in Amentifers like Casuarinaceae, Salicaceae, Fagaceae, etc., are simple and naked like those of conifers. Fertilization in conifers is similar to that of angiosperms (Doyle, 1945). However, definite differences prevail between the ovuliferous scales of conifers and the angiosperm carpel.

            7. Gnetales – Angiosperms Theory:- Gnetales, the transitional group between gymnosperms and angiosperms, are also considered as the ancestors of angiosperms. Gnetum bears angiosperm-like leaves, female gametophyte and vessel-bearing wood. Gnetales have two cotyledons like dicots. Their ovules bear two integuments as in case with most angiosperms. Stamens of all the three genera of Gnetales (Gnetum, Ephedra and Welwitschia) are similar to those of angiosperms. Gametophytes of Gnetum and Welwitschia are highly reduced like those of angiosperms.

         5. Pentoxylon Theory:- This theory was proposed by Meeuse in 1953. He compared Pandanus, a monocot with Pentoxylales, a group of fossil pteridophytes and listed a number of common features between the two. Pandanus and Pentoxylales resemble each other in their stem and leaf anatomy, male and female inflorescence, pollen and copious endosperm with minute endosperm.

 MONOPHYLETIC AND POLYPHYLETIC ORIGIN OF ANGIOSPERM

The question of monophyly and polyphyly of the angiosperms is extensively debatable and hence not solved, mainly because of their inadequate fossil records. But it is sure that the angiosperms are a natural group of plants and contain such characters which make them unique from all vascular plants.

1. Monophyletic Origin:- Several phylogenists like Faegri (1980), Dehlgren (1983), etc. believe that angiosperms are ‘monophyletic’ in origin, i.e., a group originated from a single ancestor at a single time in the past. They believe so because present day angiosperms show remarkable consistency in their characters like uniform stamina structure, characteristic endothecial layer of the anther wall, double fertilization, formation of triploid endosperm and presence of sieve tube in all. Hickey and Doyle (1977) also support the monophyletic origin of angiosperms on the basis of their studies of monosulcate pollens. Dehlgren (1983) believes that the ancestor of the present day angiosperms was a gymnospermous member. However, no definite fossil evidences are available in favour of the monophyletic origin.

2. Polyphyletic Origin:- The phylogenists like Eames (1961), Cronquist (1965), Krassilov (1977), etc. have been in the view that angiosperms are ‘polyphyletic’, in origin, i.e., dicots and monocots originated at different times from different primitive stocks, and attained their present status through parallel or convergent evolution. Fossil records, variation in perianth and the nature of carpel in both dicots and monocots also support the theory of polyphyletic origin. However, the primitive orders of both dicots and monocots do not show any close relationship in their characters, and thus it also favours the polyphyletic origin of angiosperms.

            However, in the light of adequate fossil records and other characters, it appears that, angiosperms as a group are monophyletic, and their families or group of families are polyphyletic. It needs more fossil records to find out the exact ancestors of angiosperms.

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