Gymnosperms are the taxonomic class which comprises plants whose seeds are not bounded in an ovule (such as a pine cone). Gymnosperm signifies as 'naked seed'. This group is often termed to as softwoods. Gymnosperms generally encompass needles which stay green all through the year. Illustrations are cedars, pines, spruces and firs. Some of the gymnosperms do drop their leaves like ginkgo, dawn redwood and bald cypress and so on.
Microsporangia and Megasporangia:
A microsporangium (plural Microsporangia) is a sporangium which generates spores which give augment to male gametophytes. Microsporangia are distinguished in spike mosses and a minority of ferns. In gymnosperms and angiosperms (that is, flowering plants), the Microsporangia create the microsporocyte, as well termed as the microspore mother cell, which then makes four microspores throughout meiosis. The microspores split or divide via mitosis to make pollen grains. The word is not utilized for bryophytes.
Megasporangia are the comparable female structures on such plants, related with the flower carpel and the megasporangial cone.
Reproduction in Gymnosperms:
As with angiosperms, the life-cycle of gymnosperms is as well characterized through alternation of generations. In conifers like pines, the green leafy portion of the plant is the sporophyte; the cones include the female and male gametophytes. The female cones are bigger than the male cones and are positioned towards the top of the tree; the small, male cones are positioned in the lower area of the tree. As the pollen is shed and blown by the wind, this arrangement makes it hard for a gymnosperm to self-pollinate.
A male cone consists of a central axis on which bracts, a kind of modified leaf, are joined. The bracts, termed as microsporophylls are the sites where microspores will grow up. The microspores build up within the microsporangium. In the microsporangium, cells termed as microsporocyte split or divide through meiosis to produce four haploid microspores. Moreover mitosis of the microspore generates two nuclei: the generative nucleus and the tube nucleus. On maturity, the male gametophyte (that is, pollen) is discharged from the male cones and is taken by the wind to land on the female cones.
The female cone as well consists of a central axis on which bracts termed as megasporophylls are present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore mother cell splits or divides through meiosis to produce four haploid megaspores. One of the megaspores splits or divides to form the multicellular female gametophyte, whereas the others split or divide to form the rest of the structure. The female gametophyte is contained in a structure termed as the archegonium.
On landing on the female cone, the tube cell of the pollen makes the pollen tube, via which the generative cell migrates in the direction of the female gametophyte via the micropyle. It takes around one year for the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte having the generative cell divides or splits into two sperm nuclei, one of which fuses by the egg, whereas the other degenerates. After fertilization of the egg, the diploid zygote is made, which splits or divides through mitosis to make the embryo. The scales of the cones are closed throughout the development of seed. The seed is covered by means of a seed coat that is derived from the female sporophyte. The development of seed takes another one to two years. Once the seed is prepared to be dispersed, the bracts of the female cones open to let the dispersal of seed; no fruit formation occurs as gymnosperm seeds have no covering.
After fertilization takes place, the seed grows up on the scale of the female cone.
a) Pollination: The transfer of pollen from the male reproductive structure to a female reproductive structure is termed as pollination. In gymnosperms, wind frequently carries the pollen from the male to the female cones. The pollen gathers in a sticky substance generated by each ovule.
b) Fertilization: Once pollination has taken the ovule seals and closes in the pollen. The scales as well close and a sperm cell fertilizes an egg cell within each ovule. The fertilized egg then builds up into the embryo portion of the seed.
c) Seed Development: Female cones stay on the tree whereas the seeds mature. As the seeds build up, the female cone rises in size. It can take up to two years for the seeds of a few gymnosperms to mature. Male cones, though, generally fall off the tree after they have shed their pollen.
d) Seed Dispersal: When the seeds are mature, the scales open. The wind shakes the seeds out of the cone and carries them away. Just a few seeds will land in appropriate places and grow up into new plants.
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