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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They are female spores which on germination produce the female gametophyte.
Easy Biology Class
Difference between Bryophytes and Pteridophytes. The phenomenon of Heterospory is of great ehterospory advantage because of the fact that a large megaspore which contains female gametophyte derives its food from the sewd, and is independent of the external conditions as might interfere with the growth of a free living gametophyte. No siphonogamy, therefore water is essential for fertilization. Retention of young embryo inside the megaspore wall.
Modification of distal end of megasporangium for pollen capture. Production of singe megaspore per megasproangium. Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory.
The chance of the survival of the independent gametophyte is very less. In Selaginella Isoetes it is distinct before meiosis. Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia. Types of Stelar System and its Evolution in Pteridophytes. The megaspore usually germinates within the megasporangium and is not shed for a time which pteridphytes vary from species to species.
Difference between Racemose and Cymose Inflorescence. The retention and germination of the fertilization of the egg hetefospory embryo formation, which is still within germinating heterospry.
Development of protective layers and nutritive tissue around megasporangium. Range of thallus structure in Algae. Rounded red circles indicates different steps. Leave a Reply Click here to cancel reply. Your email address will not be published. According to Williamson and Scot two species of Calamostachys form the initial stage that might lead to the heterospory.
Parent sporophytic tissue represented by integuments and nucellus. In pteridophytes, further steps towards seed habit are lacking. This is made possible by the development of pollination mechanism. The next big step in evolution was heterospory or formation of two pteridopnytes types of spores, the smaller ones called microspores and the larger ones known as megaspores.
Formation of protective layers integuments around the megasporangium. The indication of the starting of pollination mechanism is observed anv Selaginella where a temporary retention of the female gametangia within the megasporanigum is evident. They have identical organization but for their size. Thus, the heterosporous condition is considered as precondition for the seed habitat.
In Selaginella erythropus megasporangium contains only one megaspore which is functional.
Here one thing qnd be noticed that the microspores develop male gametophytes whereas the megaspores the megaspores germinate into the female gametophytes which may also be called as microgametophytes and megagametophytes. The permanent haibt of the megaspore within the megasporangium has not become established. However, there are some Pteridophytes which produce two different types of spores differing in size, structure and function. What is the evolutionary significance of Heterospory?
In Isoetes there are only megaspores in megasporangium. Heterospory is a condition of the production of more than one type usually two of spores in a single plant. This reduced the chance of genetic variability among homosporous plants.
Importance of Heterospory and Seed Habit of Ferns ( Words)
In microsporangium all the microspores are functional while in magasporangium one megaspore is functional and rest degenerate. The origin of heterospory can be better discussed on the basis of evidences from paleobotany, developmental and experimental studies. The retention of megaspore permanently inside the megasporangium has not been well established. The offspring sporophyte represented by the embryo.
Moreover, there is no evidence to prove the development of seed habitat from homosporous condition. In this post we will discuss what all stages Pteridophytes successfully completed in forming a seed?
The seed represents three generations: Successful to some extend.