Amphimixis is the formation of new individuals through the normal process of sexual reproduction by meiotic formation of gametes and fusion of gametes during fertilization.
Apomixis is the formation of new individuals directly through asexual reproduction without involving the formation and fusion of gametes. It occurs by three methods – Agamospermy, spore formation, and vegetative reproduction.
Agamospermy is of three types:
- Adventitive embryony
- Recurrent Agamospermy
- Non-recurrent Agamospermy
Adventive embryony is the formation of embryo adventitiously from a cell other than egg. polyembryony was discovered by Leeuwenhoek (1719) in case of Citrus.
Apospory is the formation of gametophyte directly from the sporophyte without forming spores.
Apogamy is the formation of sporophyte directly from gametophyte without gamete formation and gamete fusion.
Parthenogenesis is the development of the new individual from a single gamete (usually female).
In diploid parthenogenesis, the egg is diploid as it has been formed from either a nucellar cell or diploid megaspore.
Haploid parthenogenesis occurs when the embryo is formed from the haploid egg(‘normal embryo).
Natural vegetative propagation in higher plants:
Fleshy adventitious roots (Sweet Potato, Dahlia and Asparagus), Rhizome (Ginger), Suckers (Mint and Chrysanthemum), Leaves (Bryophyllum, Kalanchoe, and Adiantum caudatum), and Bulbils (Oxalis, Agave, Dioscorea, Lily).
Artifical methods of vegetative propagation:
Cutting (Stem cutting – Sansevieria and Root cutting – Tamarind and Lemon), Layering (Jasmine, Grape Vine), Grafting (Mango), and Micropropagation.
Micropropagation is the raising of new plants from a small plant tissue with the help of tissue culture technique
Tissue culture technique was first thought of by Haberlandt (1902) and Hanning (1904) and White (1932) made the first successful attempt in case of Tomato root.
Cellular totipotency is the ability of a somatic cell to produce the complete organism. This vegetatively produces individual is called Clone. Two hormone help in this process: Auxin (Favors root formation), and Cytokinin (Favors shoot formation).
Shoot tip culture is used to produce virus-free plants.
In Haploid or Pollen grain culture, the young anthers are used to produce haploid plants.
The tissue culture variations produced in plants is called Somaclonal Variations.
In Microsporogenesis the archesporial cells divide into outer primary parietal cells and inner primary sporogenous cells. Parietal cells form the wall of microsporangium. It consists of Endothecium (fibrous layer), 1-3 middle layers and a layer of tapetum.
Tapetum is the major nourishing layer and its cell becomes large, multinucleate and polypoid through endomitosis and endopolyploidy. It provides Ubisch granules for forming exine of pollen grains.
The sporogenous cells divide to form pollen grain or microspore mother cell (P.M.C). The mother cell then undergoes meiosis and form tetrads of microspores or pollen grains.
Two cells are present in male gametophyte:
- Large tube or vegetative cell
- Small generative cell
The generative cell may divide into two male gametes.
The embryo sac is oval, 7-celled and 8-nucleated structure. The three micropylar cells (egg apparatus), the Central cell, and three chalazal cells (antipodal cells).
The central cell is the largest and it has two polar nuclei which often fuse in the center to form single diploid secondary or fusion nucleus. The secondary nucleus is the only diploid structure in the embryo sac.