23-08-2016, 09:12 AM
I want to know how to prepare a presentation on heterosporyand seed habitat as a seminar topic.And the important subheadings under which the topic should be discussed.
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heterospory and seed habitat
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I want to know how to prepare a presentation on heterosporyand seed habitat as a seminar topic.And the important subheadings under which the topic should be discussed.
23-08-2016, 09:21 AM
I want a quickreply for my question so plz help me to know how to prepare a presentation on heterospory and seed habitat as a seminar topic and important subheadings under which the topic should be discussed
I want a quickreply for my question so plz help me to know how to prepare a presentation on heterospory and seed habitat as a seminar topic and important subheadings under which the topic should be discussed
I want a quickreply for my question so plz help me to know how to prepare a presentation on heterospory and seed habitat as a seminar topic and important subheadings under which the topic should be discussed
23-08-2016, 11:43 AM
Importance of heterospory and seed habit of ferns!
Heterospory:
The sporangia show greater specialization than those of ferns. They are differentiated into micro and megasporangia. The microsporangia contain microspores whereas megasporangia contain megaspores in them.
Each microsporangium contains a large number of microspores whereas each megasporangium contains usually only four megaspores. The condition of the production of two kinds of dissimilar spores, differing in shape and size in the same species, is termed Heterospory. When one kind of spores are present, e.g., Equisetum and ferns, the condition is said to be homospory.
According to modern pteridologists the heterospory is found in seven genera-Selaginella, Isoetes, Marsilea, Salvinia, Azolla, Regnellidium and Pilularia. Here one thing may 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.
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.
The differentiation of microspores and megaspores and their dependence upon the sporophyte has certain advantages. The gametophytes of the ferns are, however dependent for their nutrition 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.
Seed habit:
The condition of Heterospory in Selaginella constitute one of the most important phases in the evolution of the plants. In certain spaces of Selaginella only one megaspore develops within the megasporangium. The highest evolved species, i.e., S. apoda has advanced almost upto the rank of a seed plant. 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.
Biological importance of Heterospory:
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. It thus forms better starting point for the new embryo, than an independent green prothallus which has to manufacture its own food.
In brief, the origin of seed habit is associated with the following important prerequisites:
1. The production of two kinds of spores (i.e., Heterospory).
2. The retention and germination of the fertilization of the egg and embryo formation, which is still within germinating megaspore.
3. Development of only one megaspore per megasporangium.
In Selaginella, there is remarkable approach to the seed habit, on account of the following important features:
1. Selaginella shows Heterospory.
2. The megaspore usually germinates within the megasporangium and is not shed for a time which may vary from species to species.
3. There is reduction to one megaspore in some species, e.g., S. rupestris and S. monospora and a confirmed tendency to reduction in others.
It becomes quite evident that Selaginella has considerably advanced towards the seed habit in a few species, but its approach to the true seed is not complete due to the following features:
1. The megasporangium lacks an integument or covering.
2. The permanent retention of the megaspore within the megasporangium has not become established.
3. After the development of the embryo, the resting period is not there.
Heterospory:
The sporangia show greater specialization than those of ferns. They are differentiated into micro and megasporangia. The microsporangia contain microspores whereas megasporangia contain megaspores in them.
Each microsporangium contains a large number of microspores whereas each megasporangium contains usually only four megaspores. The condition of the production of two kinds of dissimilar spores, differing in shape and size in the same species, is termed Heterospory. When one kind of spores are present, e.g., Equisetum and ferns, the condition is said to be homospory.
According to modern pteridologists the heterospory is found in seven genera-Selaginella, Isoetes, Marsilea, Salvinia, Azolla, Regnellidium and Pilularia. Here one thing may 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.
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.
The differentiation of microspores and megaspores and their dependence upon the sporophyte has certain advantages. The gametophytes of the ferns are, however dependent for their nutrition 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.
Seed habit:
The condition of Heterospory in Selaginella constitute one of the most important phases in the evolution of the plants. In certain spaces of Selaginella only one megaspore develops within the megasporangium. The highest evolved species, i.e., S. apoda has advanced almost upto the rank of a seed plant. 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.
Biological importance of Heterospory:
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. It thus forms better starting point for the new embryo, than an independent green prothallus which has to manufacture its own food.
In brief, the origin of seed habit is associated with the following important prerequisites:
1. The production of two kinds of spores (i.e., Heterospory).
2. The retention and germination of the fertilization of the egg and embryo formation, which is still within germinating megaspore.
3. Development of only one megaspore per megasporangium.
In Selaginella, there is remarkable approach to the seed habit, on account of the following important features:
1. Selaginella shows Heterospory.
2. The megaspore usually germinates within the megasporangium and is not shed for a time which may vary from species to species.
3. There is reduction to one megaspore in some species, e.g., S. rupestris and S. monospora and a confirmed tendency to reduction in others.
It becomes quite evident that Selaginella has considerably advanced towards the seed habit in a few species, but its approach to the true seed is not complete due to the following features:
1. The megasporangium lacks an integument or covering.
2. The permanent retention of the megaspore within the megasporangium has not become established.
3. After the development of the embryo, the resting period is not there.
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