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Term Paper on Angiosperms
Term Paper # 1. Origin of Angiosperms:
The origin of the present-day angiosperms is a very knotty problem, and is not yet properly understood. The phylogenists have put forward some theories from time to time, a brief resume of some of which is presented below.
Though there is a lack of adequate palaeobotanical records, a large number of workers try to establish the origin or angiosperms from some gymnospermous stock or from some of their ancestral forms, and as such, angiosperms may be monophyletic or poly-phyletic in origin. Arber and Parkin (1907) suggested a hypothetical connecting link in between, the Cycadeoidaceae and the Angiospermae.
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They gave the name Hemiangiospermae to this link, whose imaginary reproductive organs were constructed as in the cycadeoid flower in having an elongated axis bearing spirally arranged perianth leaves, the androecial branch of numerous stamens, and a large number of open carpels with marginal mega-sporangia. From such a type the magnoliaceous flower is supposed to have originated.
According to this view, therefore, the angiosperms are monophyletic in origin, as all of them have come from the same ranalian stock; the dicotyledons have come first, and the monocotyledons constitute an offshoot of them. But there is no fossil record to prove that such a structure was ever present. It is interesting to note, however, that this theory was nevertheless accepted by workers like Bessey.
Hutchinson and others Wettstein (1910-11) postulates that the ancient angiosperms were monosporangiate, and were derived from a gnetalian inflorescence. He regards Casuarina as the most primitive of the existing angiosperms. According to him, the bisexual flower came into existence by the appearance of a terminal pistillate flower on a staminate inflorescence.
Wettstein does not support the view that the origin of the dicots is from the monocots. Markgraf (1930) also lays emphasis on the Gnetales for providing a solution to this problem of angiosperm origin, Fagerlind (1947) considers that there was a common ancestral stock, which had given rise to a line, from which have originated the present-day Gnetum, Ephedra, and Welwitschia among the gymnosperms on one hand, and another in the other direction, from which have evolved the modem angiosperms polyphyletically; he terms the latter as the Proangiosperms.
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Andrews (1947) and Arnold (1947) are both of opinion that the angiosperms have come from the pteridosperms, but Darrah (1939) considers it to be an entirely debatable point. A few workers suggested that the angiosperms might have been evolved from the Caytoniales, a Jurassic group of angiosperm-like plants, recorded by Thomas (1925). But Arnold has shown that they are simply some Mesozoic remnants of the pteridosperms.
Campbell (1925) rejects the idea of the derivation of angiosperms from the cycadeoidean stock, and suggests their origin from some pteridophytic ancestor. According to him, the monocotyledons are more primitive than the dicotyledons. Engler (1936) however, discredits both the cycadeoidean as well as the gnetalian theories of origin, and suggests that the monocotyledons and dicotyledons have arisen independently from a hypothetical group, called the Protangiosperms, existing in the Mesozoic.
The Protangiosperms themselves might have come from some Ophioglossum-like eusporangiate pteridophytic stock, and bore flowers, which were bisporangiate and were either completely achlamydeous or were provided with a very rudimentary perianth. Rendle (1904) supports this idea of Engler. A view quite different from the previous ones was expressed by Sahni (1920), and later on elaborated by Lam (1948).
According to this theory, which is being known as the Stachyosporous theory of the origin of the angiosperms, the seed plants belong to two taxa:
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i. Phyllospermae and
ii. Stachyosporae.
In the former, the mega-sporangia lie enclosed within leafy carpels and contain the majority of the apocarpous dicotyledonous plants and their derivatives. In the Stachyosporae, on the other hand, the mega-sporangia remain covered by some sterile organ; this latter taxon comprises of the Monochlamydeae of Bentham and Hooker and ‘perhaps some monocotyledons and sympetalae’.
Anderson (1934) suggests that the angiosperms might have been evolved as a result of hybridization between very widely different gymnosperms. Goldschimdt (1940) is also of opinion that the evolution of the major angiospermic taxa might have taken place as a result of direct mutations.
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Term Paper # 2. General Consideration of Angiosperms:
The Angiosperms or ‘closed-seeded’ plants are very complex seed-bearing plants (Spermatophytes). They include the great majority of seed plants. The most distinguishing feature of this group is that the carpels unite by their margins to form an ovary in which the ovules are enclosed, and the pollen grains fall upon a receptive surface known as the stigma.
The plant is the sporophyte which is more highly differentiated than those of the gymnosperms. As in gymnosperms, the sporophylls are aggregated to form ‘flowers’ but they are provided with one or two accessory whorls. The sporophyte is always heterosporous producing two kinds of spores. The micro- and megasporophylls are usually found in the same flower.
As in gymnosperms, two kinds of gametophytes, micro-and mega-gametophytes, are produced but they are extremely reduced. No organ corresponding to archegonium is found. The reproduction is effected by motionless gametes. The endosperm is produced after fertilization. Seeds are enclosed within fruit.
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Term Paper # 3. Alternation of Generations:
In the angiosperms there is an ‘alternation of generations’ but it is very much less distinct. The plant itself is the sporophyte because it bears spores (e.g., microspores or pollen grains and megaspores) and it is more conspicuous of the two generations.
The micro-gametophyte is represented by the pollen tube and the mega-gametophyte by the embryo-sac. The female organs or archegonia themselves have perished though their essential cells, the oospheres or eggs, have still persisted. It is to be noted that the gametophyte is completely dependent upon the sporophyte.
It is seen that the sporophyte is followed by the gametophyte and the gametophyte by the sporophyte, and so on. Thus, one stage is alternated by the other. This phenomenon of alternation of two stages or phases is spoken of as ‘alteration of generations’.