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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This summarize the successful steps by genus Selaginella towards seed habit. Development of an embryo sac within the sporangium. Palaeobotanical evidences show that the earlier selagunella plants wert all homosporous and the heterosporous condition appeared subsequently in the lowermost upper Devonian. The effect of a flower-dwelling predator on a specialized pollination system.
In microsporangium all the microspores are functional while in magasporangium one megaspore is functional and rest degenerate. Many spore mother cells disintegrate to form nutritive tissue. Answer Now and help others. General Characters of Pteridophytes Words. Strong, but incomplete, mate sflaginella discrimination between two closely related species of paper wasp.
Heterospory and Seed habit in Selaginella | Plant Science 4 U
In Isoetes there are only megaspores in megasporangium. The phenomenon of heterospory is of great biological significance on account of the following facts: Difference between Racemose and Cymose Inflorescence.
In Selaginella, there is remarkable approach to the seed habit, on account of the following important features:. There is reduction to one megaspore in some species, e.
Heterospory and Seed Habit in Pteridophytes | Botany
The phenomenon of Heterospory is of great biological advantage because of the fact that a large megaspore which contains female gametophyte derives its food from the sporophyte, and is independent of the external conditions as might interfere with the growth of a free living gametophyte.
Here’s how it works: Reduction to a single functional megaspore per sporangium: Reduction to a single functional megaspore per sporangium.
In the last post, we discussed in detail the tendency of Pteridophytes towards seed habit. The retention of megaspore permanently inside the megasporangium has not been well established.
Each microsporangium contains a large number of microspores whereas each megasporangium contains usually only four megaspores. Megaspore is protected by its wall also but not by an integument.
Microspores are smaller in size and develop into the male gametophyte while the megaspores are large and develop into female gametophyte.
However, there are some Pteridophytes which produce two different types of spores differing in size, structure and function.
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These species were C. It becomes quite evident that Selaginella has considerably advanced towards the seed habit in a few species, but its approach to the true seed is not complete due to the following features:.
The selagunella of microspores and megaspores and their dependence upon the sporophyte has certain advantages. The permanent retention of the megaspore within the megasporangium has not become established.
Here one thing may be noticed that the microspores develop male gametophytes whereas the megaspores the megaspores germinate into the female gametophytes which may also be called as microgametophytes and megagametophytes. Don’t have an account? The evolution of the sekaginella is one of the major events in the history of land plants.
This article is also available for rental through DeepDyve. Anumber of heterosporous genera belonging to the Lycopsida, Sphenopsida and Pteropsia were known in the late Devonian and early Carboniferous periods. Retention and germination of megaspore within the megasporangium to some extend: Purchase Subscription prices and ordering Short-term Access To purchase short term access, please sign in to your Oxford Academic account above.
Learning and Loving Plants. In Marsilea 64 microspores and 64 megaspores are formed after meiosis in microsporangium and megasporangium respectively.
The condition of Heterospory in Selaginella constitute one of the most important phases in the evolution of the plants. You could not be signed in. In the microsporangium all the sporocytes undergo meiosis and form a large number of microspores. To purchase short term access, please sign in to your Oxford Academic account above.
Meaning, General Characters and Affinities. Citing articles via Web of Science However, some heterosporous pteridophytes possess integument-like structures. The condition of the production of two kinds of dissimilar spores, differing in shape evoltion size in the same species, is seeed Heterospory. Abstract The evolution of the seed is one of the major events in the history of land plants.
As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size.