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 Marsilea, Salvinia and Azolla the phenomenon of heterospory becomes distinct after meiosis. Purchase Subscription prices and ordering Short-term Access To purchase short term access, please sign in to your Oxford Academic account above.
Reduction to a single functional megaspore per sellaginella 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:.
In Isoetes there are only megaspores in megasporangium. Anybody can ask a question Anybody can answer The best answers are voted up and rise to the top. Related articles in Web of Science Google Scholar.
Email alerts New issue alert. While in megasporangia most of the spore mother cells degenerate but in microsporangia only a few mother cells are disorganize.
Megaspore is protected by its wall also but not by an integument. Similar type of abortion of spores evolutiion also observed in Stauropteris Chaloner, Lepidocarpon and Calamocarpon.
Importance of Heterospory and Seed Habit of Ferns ( Words)
Receive exclusive offers and updates from Oxford Academic. Close mobile search navigation Article navigation. Strong, but incomplete, mate choice discrimination between two closely related species of paper wasp.
Such Peridophytes are known as homosporous and this phenomenon is known as homospory. In heterosporous Pteridophytes the development of micro and megasporangia follow the same pattern. Sign in via your Institution Sign in.
Many spore mother cells disintegrate to form nutritive tissue. Retention of megaspore inside megasporangium either till the formation of female gametophyte or even after fertilization. Here, the megaspore remains within the sporangium; it itself fertilizes and this way for the first time the gametophyte of Selaginella shows complete dependence of the gametophyte upon sporophyte as in angiosperms.
According to Rashid only 9 genera of Pteridophytes are heterosporous.
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Plant Science 4 U. In the microsporangium all the sporocytes undergo meiosis and form a large number of microspores. It thus forms better starting point for the new embryo, than an independent green prothallus which has selagniella manufacture its own food.
Citing articles via Web of Science Oxford University Press is a department of the University of Oxford. Don’t have an account? General Characters of Pteridophytes Words. The gametophytes of the ferns are, however dependent for their nutrition upon soil and environmental condition, whereas in the case of Selaginella, as far as the nutrition of gametophytes is concerned they derive it from the sporophyte, and there they are more independent to the external condition than those of fems.
Leave a Reply Click here to cancel reply. It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia.
The highest evolved species, i. Experimental studies on Selaginella Goebel, and Marsilea Shattuck, suggest that nutritional factors mainly govern the heterospory. The permanent retention of the megaspore within the megasporangium has not become established. The differentiation of microspores and megaspores and their dependence upon the sporophyte has certain advantages.
Retention and germination of megaspore within the megasporangium to some extend: Difference between Racemose and Cymose Inflorescence. There is reduction to one megaspore in some species, e. However, in megasporangium, a part of the sporogenous tissue degenerates they provide seef to growing sporocytes megaspores.
When one kind of spores are present, e.
Heterospory and Seed Habit in Pteridophytes | Botany
Development of protective layers and nutritive tissue around megasporangium. They have identical organization but for their size.
Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia. Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory.
As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size. Don’t already have an Oxford Academic account? Meaning, General Characters and Affinities.