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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From the above observations it is concluded that the life history of Selaginella approaches towards seed habit because of the following features:. In the last post, we discussed in detail the tendency of Pteridophytes towards seed habit. Receive exclusive offers and updates from Oxford Academic.
Heterospory and Seed Habit in Pteridophytes | Botany
Difference between Racemose and Cymose Inflorescence. Leave a Reply Click here to cancel reply. Therefore, integuments cannot explain the evolutionary success of seed plants. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. Close mobile search navigation Article navigation.
Though Selaginella as well as lower Spermatophytes shows homologies in their structure as follows: Related articles in Web of Science Google Scholar. Abstract The evolution of the seed is one of selagniella major events in the history of land plants. Types of Stelar System and its Evolution in Pteridophytes. While in megasporangia most of the spore mother cells degenerate but in microsporangia only a selagineola mother cells are disorganize.
In this paper, we consider the suite of characters that define the seed habit, and discuss the probable selective pressures that produced each character. Most of the Pteridophytes produce one kind of similar spore.
Importance of Heterospory and Seed Habit of Ferns (581 Words)
Wind-pollination means the propagule is vulnerable to pathogens that mimic pollen, and pathogen pressures may have contributed to some seed characters. These are the basic requirements for seed habit: In Selaginella, nutritive tissue called tapetum is present. When one kind evoluhion spores are present, e. Gymnospermsheterosporyintegumentspollinationseed habit.
These species were C. According to Williamson and Scot two species of Calamostachys form the initial stage that might lead to the heterospory. In the microsporangium all the sporocytes undergo meiosis and form a large number of microspores. Newer Post Older Post Home. If you originally registered with a username please use that to sign in.
The evolution of the seed is one of the major events in the history of land plants. This summarize the successful steps by genus Selaginella towards seed habit. In Selaginella erythropus megasporangium contains only one megaspore which is functional.
In heterosporous Pteridophytes the development of micro and megasporangia follow the same pattern. Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia. The gametophytes of the ferns are, however dependent for their aeed 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.
Importance of Heterospory and Seed Habit of Ferns ( Words)
Learning and Loving Plants. The origin of heterospory can be better discussed on the basis of evidences from paleobotany, developmental and experimental studies.
In microsporangium all the microspores are functional while in magasporangium one megaspore is functional and rest degenerate. Microspores are smaller in size and develop into the male gametophyte while the megaspores are large and develop into female gametophyte. Development of an embryo sac within the sporangium. You do not currently have access to this article.
It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia. Each microsporangium contains a large number of microspores whereas each megasporangium contains usually only four im.
The permanent retention of the megaspore within the megasporangium has not become established. They have identical organization but for their size.
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Sign In or Create an Account. The megaspore usually see within the megasporangium and is not shed for a time which may vary from species to species. Latest Most Read Most Cited Does the shape of forelimb long bones co-vary with grasping behaviour in strepsirrhine primates?
As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size. The two types of spores are microspores and megaspores. 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. Among Pteridophytes, Selaginella rupestris is the one species that is very close to seed habit. The sporangia show greater specialization than those of ferns.
Most users should sign in with ot email address. Some of these forms even arrived at the seed stage. Modification of distal end of megasporangium for pollen capture. Retention of megaspore inside megasporangium either till the formation of female gametophyte or even after fertilization. Many spore mother cells disintegrate to form nutritive tissue. In Marsilea, Salvinia and Azolla the phenomenon of heterospory becomes distinct after meiosis.