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प्रश्न
Describe the process of fertilization in angiosperms with the help of a diagram.
उत्तर
- The fusion of one male gamete with an egg and that of another male gamete with the secondary nucleus is called double fertilization. It is the characteristic feature of angiosperms. It was discovered by Nawaschin in the liliaceous plants like Lilium and Fritillaria.
- When pollen grain reaches the surface of the stigma, it germinates and forms a pollen tube.
- Pollen tube penetrates the stigma, style, ovary chamber and then enters the ovule.
- The growth of the pollen tube is guided by the chemicals secreted by the synergids.
- Usually, when pollen tube enters the ovule through the micropyle, it is termed porogamy. But in some cases, it enters through chalaza which is known as chalazogamy. In some plants, it enters by piercing the integuments which are called mesogamy.
- A pollen tube penetrates the embryo sac of the ovule through its micropylar end.
- The pollen tube carrying male gametes penetrates in one of the synergids.
- Watery contents of synergid are absorbed by pollen tube, due to which it ruptures and release the contents, including the two non-motile male gametes.
- As non-motile male gametes are carried through hollow pollen tube, it is known as siphonogamy that ensures fertilization to take place.
- Fertilization mainly involves two processes: Syngamy and Triple fusion.
- Syngamy: It is the fusion of haploid male gamete with haploid female gamete (egg). It results in the formation of a diploid zygote which develops to form embryo. Syngamy is a type of generative fertilization.
- Triple fusion: It is the fusion of the second haploid male gamete with a diploid secondary nucleus. It results in the formation of Primary Endosperm Nucleus (PEN) which develops into triploid endosperm. Triple fusion is a type of vegetative fertilization.
xi. In this process, both the male gametes participate, due to which fertilization occurs twice in the same embryo sac, hence it is described as double fertilization.
संबंधित प्रश्न
On cutting the body of an organism into many pieces it was observed that many of these pieces developed as new individuals. Name the process and list two organisms in which this process may be observed. Draw a schematic diagram to illustrate the changes that are likely to be observed during the development of new individuals in any one of the organisms named.
The stem of a fruit tree X fixed in soil is cut in a slanting way. The upper part of stem of another fruit tree Y of different variety of same species is also cut in a slanting way. The cut stem of tree Y, without roots but having some leaves, is placed over the rooted cut stem of tree X in such a way that their cut surfaces fit together properly. While joining the two cut stems, care is taken to make sure that the layer Z of one cut stem is in contact with layer Z of the other cut stem. The joint of cut stem is bound tightly with a piece of cloth and covered properly with polythene. Soon the cut heals and the two stems grow together and become one fruit tree producing leaves, flowers and fruits.
(a) What is the name of this method of producing plants or trees?
(b) What name is given to the cut stem of tree X having roots?
(c) What name is given to the cut stem of tree Y which has no roots but has some leaves?
(d) Name the layer Z.
(e) Why should the layer Z of one cut stem be in contact with the layer Z of the other cut stem?
(f) Name any four fruit trees which are usually bred by this technique.
(g) State any one advantage of producing fruit trees by this technique.
State whether the following statement is true (T) or false (F):
Cutting and grafting are natural means of reproduction.
What happens when a mature spirogyra filament attains considerable length?
What is regeneration in organism?
In hydra the type of reproduction is ___________.
Write four lines on the following: Budding
Many unicellular organisms reproduce by the process of ______
Spirogyra reproduces by ______.
The part of a seed which is a source of food during germination of seed is ______.
