Q.10. Give the fertilization process of Cycas? Describe the development of its embryo with the help of diagrams. (1999)
Related Questions -
Q. Explain the development stages of female gametophyte in Cycas. (2008)
Q. Draw a labelled diagram of longitudinal section of ovule of Cycas. (2004)
Q. Describe the development of male gametophyte of Cycas with illustrated labelled diagrams. (2004)
Q. Describe the development of male gametophyte in Cycas. (2003)
Q. Describe the post pollination development of male gemetophyte in Cycas. (2002)
Q. With the help of suitable diagram describe the structure of a sperm of Cycas revoluta. (2001)
Ans. Gametophytic phase: -
The spore are the units of gametophytes. As Cycas there is heterospory so the development of male and female gametophyte takes place from microspores (pollengrains) and megaspores (embryo sacs) respectively.
Male Gametophyte: -
The microspore is the initial cell or unit of the male gametophyte. The microspore is a spherical, unicelled structure which has two walls, an outer thick exine and inner thin intine wall. In the centre there remain present the nucleus which is haploid and cytoplasm. The development of pollengrain is partly completed in the microsporangium and partly in the oval.
Development of Male Gametophyte before Pollination: -
The microspore while still in the microsporangium begins to germinate. First of all microspore divides into two by a transverse division forming a small prothallial cell and large antheridial cell. The antheridial cell soon after undergoes transverse division so as to produce a small generative cell in contact with prothallial cell and a large tube cell. As a result of loss of water, the sporangial wall break open along the lower side, so that the sporangia become boat shaped and the three celled microspores are liberated in air.
Pollination is anemophilous. The microspores or pollen at 3-celled stage are light and dry, so they may be easily blown away or pollinated by wind. At the time of pollination a group of cells at the apex of nucellus breaks down and form mucilage which oozes out of the micropyle and collects in the form of a drop. It traps the pollen grains. As this drop dries up, the pollen grains are sucked inside the pollen chamber. The top becomes very hard forming nucellar break. The grains may lie inert inside the pollen chamber for quite sometime (nearly four months).
Development of Male Gametophyte after Pollination: -
Further development of the male gametophyte from the three celled pollen grain or microspore takes place in the pollen chamber of ovule. Nearly after one week of pollination due to the enlargement of the tube cell, the exine ruptures and the intine protrudes out in the form of a pollen tube. The pollen tube serves as nutritive haustorium. This breaks through nucellar tissue till it reaches neck of the archegonium. The generative cell divides into a stalk cell and body cell in the mean time the prothallial cell penetrate into stalk cell at the time of fertilization the body cell enlarges and two blepharoblasts develop one at each pole nucleus of the body cell. The body cell divides into antherozoids which are multinucleate. The antherozoids and sperms are remarkably large and visible by naked eyes. The sperms start to move the pollen tube becomes more and more turgid and ultimately the pollen tube ruptures and antherozoids are thrown into archegonial chamber. The stalk cell, prothallial cells get disorganised afterwards.
Female Gametophyte: -
During the development of ovule from the megaspore mother cell after reduction division linear tetrad of four megaspores is formed which are haploid (X) in nature. Now out of four megaspore, three megaspores begins to degenerate while the central megaspore, becomes functional. Thus the central functional megaspore is the unit of first cell of the female gametophyte. The functional megaspore increases in size and is nucleus divides by a free nuclear divisions into several nuclei (nearly 100). The functional tissue go on crushing and crushing. As free nuclear division continue, additional cytoplasm is synthesized and, a vacoule is gradually obliterated. At this time wall formation is initiated, at first between the peripheral nuclei and continues in centipetal direction until the female gametophyte is entirely cellular. This is known as the female prothallus of endosperms and consists of nucleated cell. As the gametophyte develops the nucellus is used up and crushed at maturity, is represented only by a papery layer.
The development of archegonium takes place from the cells of the endosperms and female gametophyte and there may 2 to 6 archegonia in a single ovule. Any superficial cell of the gametophyte at the micropylar end functions as archegonial initial cell. This initial cell increases in size and divides by a transverse divisions into a primary neck cell and central cell. The primary neck cell divides by a vertical division into two neck cells. There are no neck canal cells. The canal cell becomes surounded by a enlargement. The canal cell becomes surrounded by a special layer of cell forming, the archegonial jacket layer. Finally the nucleus of the central cell divides into a short lived ventral canal nucleus and egg nucleus but no cell wall is laid down between them. The egg nucleus enlarges and it may be half a millimeter across and visible to naked eye. The ventral canal nucleus soon gets disorganised the archegonium is now ready for fertilization. The gametophytes tissue around archegonium continues to grow upward thus forming a shallow basin, the archegonial chamber. Thus the mature archegonium is an oval shaped structure consisting of two neck cells, a ventral canal nucleus and an egg nucleus.
Pollination: -
The microspores or pollen at, three celled stage are light and dry. So they may be easily blown away or pollination by wind. At the time of pollination a group of cells at the apex of nucleus breaks down and form a drop of mucilage which oozes out through the micropyle of ovule. In the abscence of any carpel distinguishable into stigma, style, ovary. The pollen grain fall directly on the mucilage drops and as the drop dries up the pollen grains are drawn into the pollen chamber below. Further drying of the drop seales the chamber and tap becomes very hard forms nucellar beak.
Fertilization: -
At the time of fertilization the end of the pollen tube containing the tube nucleus and the sperms, grows towards the embryo sac, the targid end of the pollen tube burst and discharges its contents in the liquid of the chamber above archegonia towards the necks in which the sperms swim and make their way down to the egg cell. As the sperms goes into the cytoplasm of the egg, its own cytoplasmic membrane along with the cilia slip off, while the nucleus fuses with the female nucleus forming the oospore or zygote.
Embryogony of Cycas or Post fertilization Changes in Ovule: -
Zygote is the first cell of sporophyte. The zygote undergoes a period of rest. It is spherical in structure and made of diploid nuclei. The zygote nucleus divides repeatedly by nuclear division and formation of numerous nuclei about 1000. It is distributed through out the cytoplasm of the egg. After it wall formation is initiated among free nuclei in the zygote its based and extends gradually towards the neck end of archegonium. The region is called proembryo. One cell of proembryo are differentiated into three regions, the upper free nuclear region - haustorial region, the central elongated cellular region suspensor region and the basal small cellular region.
The suspensor cells continues to elongated till they form exceedingly long coiled suspensor. It helps to force the embryo out of the archegonium into the nutritive tissue of the female gametophyte of endosperm. As more than one archegonia may be fertilized with the result that several suspensors and embryos begins to develop but ultimately one embryo matures and other perish and phenomenon of occurence of more than one embryo in one seed or ovule is known as polyembryony. The embyro cells from the embryo which invades in the endosperms absorbs it completely and for a time increases in size with little differentiation. The mature embryo consists of radicle, plumule and two cotyledons.
Formation of Seed: -
After fertilization several changes takes place in the ovule. The integuments get fused together and dry up the zygote develops into an embryo and the different part of ovule get changed into the seed.
Ovule forms - seed
Integument forms - testa
Nucelles forms - tegmen
Zygote forms - embyro
Endosperm forms - endosperm
Germination of Seed: -
The seeds of cycas germinate immediately and have no resting period. The seed germination is hypogeal i.e., cotyledons remain below the soil after germination. The plumule forms stem and radicle forms the root.