Reproduction and Reproductive Strategies in Plants
Reproductive Strategies – Reproduction is the major drive of all living things. The role of the reproductive drive is to transmit genetic characteristics from one generation to another. The ability of organism to leave behind reproducing offspring is it fitness. Achieving fitness involves among other things, fecundity and survivorship, physiological adaptations, modes of reproduction, age at reproduction, number of eggs, young, or seeds produced, parental care, size and time to maturity. How organisms achieve fitness become the organism’s life history pattern. Reproductive Strategies
Asexual and Sexual Reproduction
The various reproductive systems used by organisms are of two types. sexual reproduction and asexual reproduction.
Sexual reproduction involves the organism producing special cells, the gametes which contain only half the normal number of chromosomes. The two haploid gametes can come from the same organism (self-fertilization) or from different organisms (cross fertilization or outcrossing). Sexual reproduction produces genetic variability among the offspring. Reproductive Strategies
The majority of angiosperms and many ferns show facultative cross-fertilization, i.e., although each organism is able to produce both male and female gametes, they tend to outbreed most of the time. In fact, they often have mechanism that prevent self-fertilization. However, if by the end of the life of a flower or the flowering season, a plant has not set enough seed by outcrossing alone, the plant shift to self-fertilization.
Asexual reproduction does not involve gametes and results in new individuals genetically identical to the parents. In plants a kind of asexual reproduction is present in which a part of the adult (root, stem or leaves) form a new individual. This is called vegetative reproduction. Reproductive Strategies
In some cases, sex cells which would have formed gametes fail to undergo meiosis and develop into an embryo without fertilization. This is known as seed apomixis in plants. The seed produced by apomixis has the same genotype are the parent plant. This leads to lines of individuals which all look very similar. If sexual reproduction also takes place occasionally, genes will be recombined and a new plant form will arise.
Life History Strategies in Plants
The plant ecologists J. Grime has proposed a three-strategy system. In his system, there are R-strategies, competitive or C-strategies, and stress-tolerant or S-strategies.
R-strategists, Weedy species, occupy uncertain or disturbed habitats, have a short growth form, reproduce early in life, possess high fecundity, experience one lethal reproduction, and have well-dispersed seeds.
C-strategists and S-strategists occupy more stable environments and live longer, which often drastically reduced the opportunity for seedling to grow. Reproductive Strategies
Beyond these two characteristics, the two have evolved quite different life history strategies.
C-Strategists, such as grasses in an ungrazed grassland, live in competitive but productive and undisturbed environments, attain maximum vegetative growth, reproduce early, and repeatedly use an annual expenditure of energy stored prior to seed production.
S-Strategists live in stressed environments. such as highly disturbed sites or forest under-story. They have delayed maturity, intermittent reproductive activity, and long-term energy storage’s.
Within these extremes are intermediate types,
Competitive Ruderals (C-R) live in environment of low stress and moderate disturbance, such as fertilized, moderately grazed grasslands. Reproductive Strategies
Stress Tolerant Ruderals (S-R) live in extreme environments such as rock crevices.
C-S-R plants or-copy habitats competition is reduced by the Combined effects of stress and disturbance, such as fields.
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