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 all Selaginella species. Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory. The highest evolved species, i. They have identical organization but for their size.
Strong, but incomplete, mate choice discrimination between two closely related species of paper wasp. Un, 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. To purchase short term access, please sign selatinella to your Oxford Academic account above. Our major conclusion is that most characters are a direct consequence of habig origin of heterospory and of natural selection for propagules with larger food reserves.
The phenomenon of heterospory becomes distinct either before or after meiosis. Range of thallus structure in Algae. The two types of spores are microspores and megaspores.
Experimental studies on Selaginella Goebel, and Marsilea Shattuck, suggest that nutritional factors mainly govern the heterospory. Evidence for a complex evolutionary history of mound building in the Australian nasute termites Nasutitermitinae. Most users should sign in with their email address. Advantages and Disadvantages of Vegetative Propagation. Development of protective layers and nutritive tissue around megasporangium to some extend: Reduction to a single functional megaspore selaginela sporangium: Retention and germination of megaspore within the megasporangium.
Retention of megaspore inside megasporangium either till the formation of female gametophyte or even after fertilization.
Similar type of abortion of spores was also observed in Stauropteris Chaloner, Lepidocarpon and Calamocarpon. Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia. These are the basic requirements for seed habit: A seed is that ovule which contains an embryo developed as a result of fertilization.
There is reduction to one megaspore in some species, e. Rather, we believe that the decisive character in this success is related to pollination. In Selaginella erythropus megasporangium contains only one megaspore which is functional.
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Importance of Heterospory and Seed Habit of Ferns (581 Words)
General Characters of Pteridophytes Words. In microsporangium all the microspores are functional while in magasporangium one megaspore is functional and rest degenerate. You could not be signed in. Anumber of heterosporous genera belonging to the Lycopsida, Sphenopsida and Pteropsia were known in the late Devonian and early Carboniferous periods. Purchase Subscription prices and ordering Short-term Access To purchase short term access, please sign in to seaginella Oxford Academic account above.
Difference between Racemose and Cymose Inflorescence. In the microsporangium all the sporocytes undergo meiosis and form a large ssed of microspores. Receive exclusive offers and updates from Oxford Academic.
Importance of Heterospory and Seed Habit of Ferns ( Words)
habif Some of these forms even arrived at the seed stage. In the case of homospory the sex may be differentiated only at the gametophytic stage, but here the sex may be differentiated even at the sporophytic stage by the presence of small and large sized microspores which produce antherozoids and eggs respectively. Each microsporangium contains a large number of microspores whereas each megasporangium contains usually only four megaspores.
Such Peridophytes are known as homosporous and this phenomenon is known as homospory. You do not currently have access to this article. The microsporangia contain microspores whereas megasporangia contain megaspores in them. Abstract The evolution of the seed is one of the major events in the history of land plants. Newer Post Older Post Home. The retention and germination of the fertilization of the egg and embryo formation, which is still within germinating megaspore.
However, there are some Pteridophytes which selaaginella two different types of spores differing in size, structure and function. They are differentiated into micro and megasporangia.