PDF | Summary The origin of the seed-plant life history, and subsequent should be considered two independent, fundamentally distinct evolutionary problems. Historical biogeography of the ancient lycophyte genus Selaginella: Early. Selaginella largely upon the basis of endosperm development, without regard to great diversities in habit and anatomical details. The association may be. The highest evolved species, i.e., S. apoda has advanced almost upto the In Selaginella, there is remarkable approach to the seed habit, on account of the.
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In this article we will discuss about heterospory and seed habit in pteridophytes.
Heterospory and Seed Habit in Pteridophytes | Botany
Most of the Pteridophytes produce one kind of similar spore. Such Peridophytes are known as homosporous and this phenomenon is known as homospory. Sellaginella, there are some Pteridophytes which produce two different types of spores differing in size, structure and function.
Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory. The two types of spores are microspores and megaspores.
Microspores are smaller in size and develop into the male gametophyte while the megaspores are large and develop into female seaginella. According to Rashid only 9 genera of Pteridophytes are heterosporous.
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The origin of heterospory can be better discussed on the basis of evidences from paleobotany, developmental and experimental studies. It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia. As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size. Palaeobotanical evidences show that the earlier vascular plants wert all homosporous and the heterosporous condition appeared evokution in the lowermost upper Devonian.
Heterospory and Seed habit in Selaginella | Plant Science 4 U
Anumber of heterosporous genera belonging to the Lycopsida, Sphenopsida and Pteropsia were known in the late Devonian and early Carboniferous periods. During this period important heterosporous genera were Lepidocarpon, Lepidostrobus, Mazocarpon, Plaeuromeia, Sigillariostrobiis members of Lycopsid Calamocarpon, Calamostachys, Palaeostachys members of Sphenosida.
Some of these forms even arrived at the seed stage. According to Williamson and Scot two species of Calamostachys form the initial stage that might lead to the heterospory. These species were C. Similar type of abortion of spores was also observed in Stauropteris Chaloner, Lepidocarpon and Calamocarpon.
In heterosporous Pteridophytes the development of micro and megasporangia follow the same pattern. They have identical organization but for their size.
While in megasporangia most of the spore mother cells degenerate but in microsporangia only a few mother cells are disorganize. The phenomenon of heterospory becomes distinct either before or after meiosis. In Selaginella Isoetes it is distinct before meiosis. In the microsporangium all the sporocytes undergo meiosis and form a large number of microspores. However, in megasporangium, a part selagiinella the sporogenous tissue degenerates they provide nutrition to growing sporocytes megaspores.
In Isoetes there are only megaspores in megasporangium. In Selaginella erythropus megasporangium contains only one megaspore which is functional. In Marsilea, Salvinia and Azolla the phenomenon of heterospory becomes distinct after meiosis. In Marsilea 64 microspores and 64 megaspores are formed after meiosis in microsporangium and megasporangium respectively.
In microsporangium oof the microspores are functional while in magasporangium one megaspore is functional and rest degenerate. Experimental studies on Selaginella Goebel, and Marsilea Shattuck, suggest that nutritional factors mainly govern the heterospory.
Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia. By sudden lowering of the temperature, the size of the microspores in the sporocarp of Marsilea increases by six times. The phenomenon of heterospory is of great biological significance on account of the following facts: A seed is that ovule which contains an embryo developed as deed result of fertilization.
From the above observations it is concluded that the life history of Selaginella approaches towards seed habit because of the following features:. selaginlla
Retention of megaspore inside megasporangium either till the formation of female gametophyte or even after fertilization. Development of only one megaspore per megasporangium for example, in Selaginella monospora, S. Though Selaginella as well as lower Spermatophytes shows homologies in their structure as follows: The retention of megaspore permanently inside the megasporangium has not been well established. Structure of Rhynia With Diagram Pteridophytes.
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