Flowering plants are also called angiosperms. About 90% of the plant population belongs to angiosperms. Worldwide, there are about 300,000 species of flowering plants, and the most by a type of plan. It is the most diverse group in the kingdom Plantae. The word ‘angiosperm’ is a Greek term meaning seed container (angio, means container and sperm means seed). Angiosperms are further classified into unisexual and bisexual. In an unisexual flowering plant, there can only be one part i, e, either male or female part whereas the bisexual flower contains both male and female structures within it. Angiosperms differ from the gymnosperms based on how the seed is formed. The seeds of angiosperms are formed in the ovaries of flowers surrounded by a protective fruit. Whereas, the gymnosperm seeds are usually formed strobili- an unisexual part. Angiosperms contain conducting tissues for the circulation of water and nutrients while gymnosperms do not have these.
Lifecycle of angiosperm
The Life cycle of an angiosperm
The fusion between the male and female parts of the flower results in the seed formation. The main phase of an angiosperm’s life cycle is its first phase - the adult, or sporophyte phase. At this phase, angiosperms are heterosporous, and they participate in the formation of microspore. Micropores act as male gametophytes responsible for the production of pollen grains. The megaspores on the other hand help to form an ovule bearing female gametophytes.
Inside the anthers’ microsporangia- male gametophytes undergo meiosis to produce haploid microspores. In the next phase, the haploid microspores further undergo mitosis to produce pollen grains. Each pollen grain has two cells-, a generative cell and a pollen tube cell. Generative cell divide into two sperms and one of which turns into the pollen tube cell.
The ovule found inside the ovary of the carpel has the megasporangium. Megasporangium is protected by 2 distinct layers of integuments and an ovary wall. Within each megasporangium, a megasporocyte undergoes meiosis, resulting in the formation of 4 megaspores (3 small and one large). The 3 smaller megaspores die but the 4th one turns into an embryo sac. The large megaspore undergo replication to produce 8 celled mass, out of which 4 cells migrate to each pole of the embryo sac; two come to the equator and will eventually fuse to form a 2n polar nucleus. The 3 cells away from the egg form antipodals while the two cells closest to the egg become the synergids.
A mature embryo sac contains 1 egg cell with 2 synergids and 3 antipodal cells and 2 polar nuclei in a central cell. When a pollen grain gets close to the stigma, the pollen tube extends from the grain and gets into the style which eventually enters through the micropyle- an opening in the integuments of the ovule. The two sperm cells are deposited into the embryo sac.
Double fertilization can occur when one sperm and the 2 eggs combine, forming a diploid zygote and the other sperm fuses with the 2n polar nuclei to form a triploid cell which can later develop into an endosperm. Endosperm act as the source of nutrition during the cycle.
The zygote grows out to form an embryo with a radicle root (small root), one ( monocot ) or 2 (dicot) leaf-like organs called cotyledons. Depending upon the differentiation in the number of embryonic leaves,2 major groups of angiosperms are formed, the monocots and the dicots. Seed food reserves are stored outside the embryo in the form of complex carbohydrates, lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm help to preserve food, and the well-protected embryo is found at the centre. Some species of angiosperms are hermaphroditic (stamens and pistils are contained on a single flower), some species are monoecious (stamens and pistils occur on separate flowers, but the same plant), and some are dioecious (staminate and pistillate flowers occur on separate plants). Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent (winds).