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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The phenomenon of heterospory is of great biological significance on account of the following facts: It thus forms better starting point for the new embryo, than an independent green prothallus which has to manufacture its own food. In Selaginella, there is remarkable sedd to the seed habit, on account of the following important features:.
The microsporangia contain microspores whereas megasporangia contain megaspores in them.
By sudden lowering of the temperature, the size of the microspores in the sporocarp of Marsilea increases by six times. Abstract The evolution of the seed is one of the major events in the history of land plants. General Characters of Pteridophytes Words.
Under conditions of low light intensity, evllution photosynthetic activity of Selaginella was retarted and it produced microsporangia. In heterosporous Pteridophytes the development of micro and megasporangia follow the same pattern. Learning and Loving Plants. Though Selaginella as well as lower Spermatophytes shows homologies in their structure as follows: You must selagihella logged in to post a comment.
Structure of Rhynia With Diagram Pteridophytes. In certain spaces of Selaginella only one megaspore develops within the megasporangium.
Meaning, General Characters and Affinities. Microspores are smaller in size and develop into the male gametophyte while the megaspores are large and develop into female gametophyte.
The effect of a flower-dwelling predator on a specialized pollination system. Seed plants differ from other heterosporous lineages in the capture of microspores before dispersal of the ‘megaspore’.
The highest evolved species, i. The retention and germination of the fertilization of the egg and embryo formation, which is still within germinating megaspore. While in megasporangia most of the spore mother cells degenerate but in microsporangia only a few mother cells are disorganize. When one kind of spores are present, e. The retention of megaspore permanently inside the megasporangium has not been well established.
Importance of Heterospory and Seed Habit of Ferns ( Words)
Advantages and Disadvantages of Vegetative Propagation. In Selaginella, nutritive tissue called tapetum is present. Email alerts New issue alert. As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size.
In Selaginella rupestris and S.
Heterospory and Seed habit in Selaginella | Plant Science 4 U
The permanent retention of the megaspore within the megasporangium has not become established. In the case of homospory the sex may be differentiated only at the gametophytic inn, 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.
This summarize the successful steps by genus Selaginella towards seed habit. However, in megasporangium, a part of the sporogenous tissue degenerates they provide nutrition to growing sporocytes megaspores. According to Williamson and Scot two species of Calamostachys form the initial stage that might lead to the heterospory. In all Selaginella species. These are the basic requirements for seed habit: In Selaginella seec megasporangium contains only one megaspore which is functional.
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Seeds are traditionally defined by the possession of integuments. Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory. Palaeobotanical evidences show that the earlier vascular plants wert all homosporous and the heterosporous condition appeared subsequently in the lowermost upper Devonian.
Sign in via your Institution Sign in. The origin of heterospory can be better discussed on the basis of evidences from paleobotany, developmental and experimental studies.
However, some heterosporous pteridophytes possess integument-like structures.