Heterospory and Origin and Evolution of Seed Habitat in Land Plants – Pteridophytes. Evolutionary Significance of Heterospory and Seed Development in. feature of the seed habit, the enclosure of the female spore; ” Der. Embryosack der s]iorous Pteridophytes there may be little or no difference in the size. Heterospory and seed habit. Reference * Botany for degree students – pteridophyta by:Vasishtha B.R Sinha A.K. Singh V.P *
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It thus forms better starting point for the new embryo, than an independent green prothallus which has to manufacture its own food. The exosporic gametophytes have to find the nutrients from the external environment. However, there are some Pteridophytes which produce two different types of spores differing in size, structure and function. While in megasporangia most of the spore mother cells degenerate but in microsporangia only a few mother cells are disorganize.
Moreover, they have a continuous supply of food material from the diploid sporophytic plant. 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.
This is made possible by the development of pollination mechanism. Development of seed dispersal mechanism.
In Selaginella erythropus megasporangium contains only one megaspore which is functional. Successful in some species.
In homosporous plants, the male and female gametes are formed in the same prothallus. From the above observations it is concluded that the life history of Selaginella approaches towards seed habit pterixophytes of the following features:. Structure of Rhynia With Diagram Pteridophytes. Development of organic connection between the megasporangial wall and the megaspores. Temporary suspension of growth of embryo i. The retention of megaspore permanently inside the megasporangium has not been well established.
The chance of the survival of the independent gametophyte is very less. Microspores are small sized spores produced in large numbers inside the microsporangium. Reduction to a single functional megaspore per sporangium. Here, the megaspore remains within the ptreidophytes 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.
In certain spaces of Selaginella only one megaspore develops within the megasporangium.
A seed is that ovule which contains an embryo developed as a result of fertilization. In the figure, Solid red circle indicates: These two types of spore differ in their formation, structure and most importantly its functions and sexuality. The condition of the production of two kinds of dissimilar spores, differing in shape and size in the same species, is termed Heterospory.
During this period important heterosporous genera were Lepidocarpon, Lepidostrobus, Mazocarpon, Plaeuromeia, Sigillariostrobiis members of Lycopsid Calamocarpon, Calamostachys, Palaeostachys members of Sphenosida.
Leave a Reply Click here to cancel reply. The production of two types of spores with different sexuality was first evolved in Pteridophytes. By sudden lowering of the temperature, the size of the microspores in the sporocarp of Hterospory increases by six times.
Production of singe megaspore per megasproangium. In homosporous Pteridophytes, the sex differentiation takes place only during the development of antheridia or archegonia. 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.
One of the most important advantages of heterospory is the formation of seed habitat. Your email address will not be published. Give the names few Pteridophtyes which shows heterosporous condition.
Importance of Heterospory and Seed Habit of Ferns (581 Words)
In angiosperms, it is retained. Retention of young embryo inside the megaspore wall. Gametophytic tissue represented by the female gametophyte. Heterospory and Seed habit in Pteridophytes Ferns.
Retention and germination of megaspore within the megasporangium. The formation of seed is one of the greatest evolutionary vents in the phylogeny of seed plants. The homosporous Pteridophytes show exosporic outside the spore wall gametophytic development. Formation of protective layers integuments around the megasporangium. In microsporangium all the microspores are functional while in magasporangium one megaspore is functional and rest degenerate. The gametophytic generation is a weak link between the sporophyte and seed in the life cycle of terrestrial plants.
Development of protective layers and nutritive tissue around megasporangium to some extend: The sporangia show greater specialization than those of ferns. Production of two types of spores.
What are the possible evolutionary events in the formation of seeds in land plants? This reduces the chance of self-fertilization and increases the chance of variability in the progenies.
Difference between Racemose and Cymose Inflorescence.
Heterospory and Seed Habitat in Pteridophytes | easybiologyclass
Some of these forms even arrived at the seed pterodophytes. It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia. The seed is actually an integumented megasporangium. Advantages and Disadvantages of Vegetative Propagation. This summarize the successful steps of Pteridophytes towards seed habit.