Angiosperm lifecycle

Angiosperms have been evolved 130 million years ago in the earth. They are most dominating plant form in the present.  Angiosperms are categorized into three groups such as monocots, eudicots and basal angiosperms. Earlier both eudicots and basal angiosperms are collectively called as dicots. Basal angiosperms show most evolutionary primitive characters. Compositae (daisies) from eudicots, and Orchids from monocots are evolved with most advanced characters.

                Flowers are the reproductive structure of angiosperms. They are considered as modified strobilus.  Female part of flower is carpel. The carpel is comprised of three parts such as stigma, style and ovary.  Stigma receives pollen grains coming from a stamen. Style connects the stigma to ovary.  Ovary protects the ovules. After fertilization, ovary becomes fruit wall.  Ovules become seeds.  Stamen is the male part of flower. It consists of two parts such as anther and filament. Anther produces pollen grain.  Filament bears the anther. Some species produce flowers bears both female and male parts. This kind of flowers is called as bisexual. Rose produces bisexual flowers.  Some species produce flowers bear only female part of male part. Such flowers are called as unisexual. Coconut, papaya and pumpkin are the example for producing unisexual flowers.   Those carpel and stamen are the fertile parts of the plants. Fertile parts of flowers are covered by infertile parts such as sepals and petals. Sepals protect the inner parts of the flower. Petals which are often in bright colures and large size attract pollinators. Some flowers and their associated pollinators are coevolved.  The evolutionary change in the plants causes the evolutionary change in pollinators too. Pollinators help to disperse the pollen grain into further distance. They help to develop new gene combinations.

                Fertilization of angiosperms refers to as the deposition of pollen grain from an anther on a stigma. Pollen grains may come from a stamen of either same flower or another flower in same tree or flower in another tree.  Self-fertilization refers to as the deposition of pollen grain from an anther of same flower on the stigma. Cross fertilization refers to as the deposition of pollen grain from an anther of another flower on same tree or of another flower on another tree on a stigma. Most of angiosperms are evolved to prevent self-fertilization. Stigma gets mature earlier than anther. Stamen may shorter or longer than stigma. Unisexual flowers are best adaptation to avoid self-fertilization in flowers. Cross fertilization help to develop new gene combinations and enhance the present gene pool of plant.

                Angiosperms produce neither archegonia nor antheridia. They produce ovule and pollen grains. Pollen grains are the partially developed male gametophyte. The diploid microsporocyte(microspore mother cell) undergoes meiosis and produce haploid microspores. Those microspores undergo mitosis and develop into pollen grains.  . It bears two cells such tube cell and generative cell. The pollen grains may deposit on the stigma via animals, water or wind. Pollen grains may disperse via animals when then contains reward for pollinators. Pollen grain can be dispersed by wind or water when they are light weight, have winds like structures or produce in large quantity. 

                Inside ovary, ovules are produced. The diploid megasporocyte (megaospore mother cell)  undergo meiosis and produce four haploid megaspores. Three megaspores are degenerated. Only one megaspore survives. Later it becomes functional megaspore. It undergoes there rounds of mitotic cell division. It results in seven cells and eight nuclei with only seven cytokinesis. The endosperm mother cell contains two nuclei. Those nuclei are called as polar nuclei.

                        After deposition of pollen grain on a stigma, Generative cell produces two sperm. Tube cell develop into large pollen tube. The pollen tube penetrates the stigma and enters through the style into ovule. The sperm are conveyed through the pollen tube. Angiosperms have evolved with another evolutionary advanced feature such as double fertilization. One of sperm fertilizes the egg cell and produce diploid zygote. The zygote divides several time and produce multicellular embryo. Other sperm fertilize the polar nuclei and form triploid endosperms. T The endosperm provides nutrients for developing embryo. Eventually the developing embryo and endosperm are enclosed in a seed. Ovules develop into seed. Ovary wall develops into the fruit.

                        Angiosperms also have seed dispersal animals. The fruits which have not edible part are adopted to disperse their seeds in winds, water or attach to animals. Some fruits are borne with hooks or spine to attach to animal fur. Ex: Achyranthes aspera. Some fruits are associated with wind like structures to disperse their seeds via wind. Ex: Dipterocarpus zelanicaus.  Enclosing seed in a fruit helps to disperse into further area via animals. It helps angiosperms to colonize new area.  Seed have long life span than spores. Seed coat is thicker resist against pest attack and mechanical damage.