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DESCRIPTIONAngiosperm Reproduction. The Flower, Pollination, the Seed, and Photoperiodism. Sexual Reproduction in Angiosperms. Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. - PowerPoint PPT Presentation
The Flower, Pollination, the Seed, and Photoperiodism
Sexual Reproduction in Angiosperms
Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely.
There must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals.
The functions of the flower solve both of these problems.
Life of the Flower
The act of pollen (male gamete) being transferred from anther to stigmaUsually accomplished by wind or animals
The FlowerThe flower consists of 4 concentric whorls of tissue
a whorl of sepals (collectively called the calyx) a whorl of petals (collectively called the corolla) stamens in which the microsporangia (sperm) form carpels in which the megasporangia (eggs) form.
Anatomy of the FlowerNon-reproductive StructuresSepals: protectionPetals: attracts pollinatorsPedicel: connects flower to stem
Anatomy of the FlowerMale structuresAnther: produces pollen Filament: supports and lifts antherStamen = anther + filament
Female structures (collectively known as carpels)Stigma: attachment site for pollenStyle: connects stigma to ovaries; has pollen tubeOvary: location of ovules
SEM of various pollen grains
Sperm vs. EggMale gameteHaploidProduced in anthersCalled microsporangium or pollenEach pollen grain has two cellsGerm cellTube cellFemale gameteHaploidProduced in ovaryCalled megasporangium or ovuleEach ovule consists of 8 nuclei
Fertilization:The fusion of a male gamete (pollen) with female gamete (ovule) to produce a zygote
Life Cycle of Angiosperms
Dicot Seed Anatomy5-15% waterCan remain dormant and viable up to 300 yrs (species dependent)
Seed Coat: testaEmbryonic Leaf:plumuleEmbryonic Root:radicle
Conditions of GerminationFor successful germination seeds require a combination of oxygen for aerobic respiration water to metabolically activate the cells temperature for optimal function of enzymes
Each seed has its own particular combination of the above three factors.
In addition particular species need other more specialized conditions such as:fire freezing passing through digestive system of a seed dispersing animal washing to remove inhibitors (beans) erosion of the seed coat (Poppy)
The particular conditions required by a seed allows it to match germination to favourable conditions
Seed GerminationRehydration of cells stimulates metabolism (enzymatic pathways startup)Gibberellin (hormone) produced in cotyledonGibberellin stimulates amylase productionStarch is digested in maltoseMaltose is transported to radicle and plumuleMaltose converted to glucose, used for cellular respiration or cellulose production
PhotoperiodismMany plants have the ability to measure the length of the sun cycle.
This allows them to schedule their flowering period for a specific time of year
Experiments have proven that plants measure night length, not day
PhotoperiodismLong Day Plants (LDP)Plants will only flower after a minimum amount of daylight is achieved.Typical of plants living in higher altitudesUsually during spring or summerShort Day Plants (SDP)Plants that only flower when the day is shorter than a maximum time periodUsually during fall or winter
Photoperiodism also explains why some plant species can be grown only in a certain latitude. Spinach, a long-day plant, cannot flower in the tropics because the days never get long enough (14 hours)
Ragweed, a short-day plant, fails to thrive in northern Maine because by the time the days become short enough to initiate flowering, a killing frost in apt to occur before reproduction and the formation of seeds is completed.
Phytochrome System: The receptor of photoperiod is located within the leaf.The cellular location of the receptor is unclear.The chemical nature of the receptor is a the molecule phytochrome. Phytochrome can be converted from one form to another by different types of light.
Flowering in SDPShort day plants flower when the night period is long. In day light or red light, Phytochrome Red (Pr) is converted to Phytochrome Far Red (Pfr). The conversion actually only requires a brief exposure to white or red light.In the dark, Pfr is slowly converted back to Pr. A long night means that there is a long time for the conversion. Under short day conditions (long night) at the end of the night period the concentration of Pfr is low.In SDP, low Pfr concentration is the trigger for flowering.
Flowering in LDP:Long day plants flower when the night period is short.In day light (white or red) the Pr is converted to Pfr.During periods when the day light period is long but critically the dark period is short, Pfr does not have long to breakdown in the dark. Consequently there remains a higher concentration of Pfr.In LDP, high Pfr concentration is the trigger to flowering.