EVOLUTION OF SEED HABIT IN SELAGINELLA PDF

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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Oxford University Press is esed department of the University of Oxford. However, some heterosporous pteridophytes possess integument-like structures. The evolution of the seed is one of the major events in the history of land plants. Retention and germination of megaspore within the megasporangium.

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The permanent retention of the megaspore within the megasporangium has not become established. Citing articles via Web of Science According to Rashid only 9 genera of Pteridophytes are heterosporous. Under conditions of low light selagine,la, the photosynthetic activity of Selaginella was retarted and it produced microsporangia.

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The phenomenon of heterospory is of great biological significance on account of the following facts: They are differentiated into micro and megasporangia. The condition of Heterospory in Selaginella constitute one of the if important phases in the evolution of the plants. In Isoetes there are only megaspores in megasporangium.

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Heterospory and Seed Habit in Pteridophytes | Botany

Reduction to a single functional megaspore per sporangium: Don’t already have an Oxford Academic account? Sign In Forgot password? Temporary suspension of growth of embryo i. Advantages and Disadvantages of Vegetative Propagation. Development of protective layers and nutritive tissue around megasporangium to some extend: A seed is that ovule which contains an embryo developed as a result of fertilization.

However, in megasporangium, a part of the sporogenous tissue degenerates they provide nutrition to growing sporocytes megaspores. Megaspore is protected by its wall also but not by an integument. Many spore mother cells disintegrate to form nutritive tissue. In microsporangium all the microspores are functional while in magasporangium one megaspore is functional and rest degenerate.

In the microsporangium all the sporocytes undergo meiosis and form a large number of microspores. It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia. By sudden lowering of the temperature, the size of the microspores in the sporocarp of Marsilea increases by six times. What is Vegetative Propagation? Plant Science 4 U.

Some of these forms even arrived at the seed stage. Strong, but incomplete, mate choice discrimination between two closely related species of paper wasp. In Selaginella Isoetes it is distinct before meiosis.

Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory. In Marsilea 64 microspores and 64 megaspores are formed after meiosis in microsporangium and megasporangium respectively.

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Palaeobotanical evidences show that the earlier vascular plants wert all homosporous and the heterosporous condition appeared subsequently in the lowermost upper Devonian. In all Selaginella species. Development of an embryo sac within the sporangium.

In this article we will discuss about heterospory and seed habit in pteridophytes. Anybody can ask a question Anybody can answer The best answers are seoaginella up and rise to the top. Though Selaginella as well as lower Spermatophytes shows homologies in their structure as on The megaspore usually germinates within the megasporangium and is not shed for a time which may vary from species to species.

Heterospory and Seed Habit in Pteridophytes | Botany

This represents an increase in efficiency over Pteridophytic reproduction. Anumber of heterosporous evolutoon belonging to the Lycopsida, Sphenopsida and Pteropsia were known in the late Devonian and early Carboniferous periods. Meaning, General Characters and Affinities.

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