Angiosperm plants undergo two phases of growth. These are:
Vegetative growth: During vegetative growth, stems and leaves are produced in large quantities.
Flowering phase: As the name suggests, flowers are produced. In flowers, sexual organs are developed.
Vegetative growth of the shoot takes place at the apical meristem, which is present at the stem. The formation of apical meristem can be regarded as the first step towards the formation of flowers. The apical meristem grows and converts into a floral meristem. This will lead to the formation of a flower.
Formation of Flowers (ABC model of formation of flowers):
The process of conversion of the apical meristem into a floral meristem involves certain signals. These signals include suitable temperature, secretion of gibberellins – a plant hormone, and photoperiod. Along with these signals, leaves also produce a chemical signal. This chemical signal is called florigen. This florigen is transmitted to apical meristems, which convert apical meristem to floral meristem in a stepwise manner. Apart from it, the expression of the gene also shows a significant part in the growth of floral meristem. Studies have proved that one of the proteins of florigen (FT protein) is encoded by Flowering Locus T (FT) gene. The FT protein regulates the gene expression required for flowering. When there are reproductively favorable conditions, florigen is produced in the leaves. It helps to induce bud formation in plants. The genetic level of the formation of flowers can be understood with the help of the ABC model. In this model, three gene functions (A, B, and C) coordinate to decide the growth and development of primordium. The ABC model explains the compound genetic mechanisms that demonstrate the identity of floral organs in Asterids and Rosids plants. When A-genes are expressed in the first whorl, the formation of sepals takes place in plants. On the other hand, equally A and B-genes are expressed in the second whorl. This leads to the formation of petals. B and C-genes interact with each other and accompany the formation of stamens. C-genes act solitarily in the center of the developing flower to induce the formation of carpels.
Parts of a Flower:
When the meristem develops into floral, the development of floral parts initiates. A flower or floral parts generally consists of four basic whorls. These are the calyx, corolla, androecium, and gynoecium.
Calyx: The calyx forms the outmost or the first whorl of a flower. It consists of green leaf-like appendages called sepals. Thus we can say that sepals are collectively called the calyx. The major function of the calyx is to protect the flower or the bud during its early development. The calyx completely covers the flower during the bud stage. The calyx, being green, also performs the process of photosynthesis.
Corolla: The corolla is the second whorl of a flower. It consists of petals. Thus petals are together called the corolla. Petals are usually organized in rings. Petals are mostly brightly colored and larger than the sepals. Different flowers have petals of different shapes. The corolla makes the flower attractive. It helps to attract insects to facilitate the process of pollination in flowers.
Androecium: The androecium is the third whorl of a flower. It contains a group of stamens. Stamens are the male reproductive unit of a flower. Each stamen consists of a filament and an anther. The filament is a long slender structure that connects to and holds the anther. The anther is a sac-like structure situated at the tip of the filament. It produces pollen grains. The outer wall of the pollen grain is called the exine and the inner wall is called the intine. A full-grown pollen grain possesses two cells – a tube cell and a generative cell.
Gynoecium: The gynoecium is the innermost or the fourth whorl of the flower. It contains the female reproductive units of flowers, called carpels or pistils. Each carpel consists of the following three parts:
Stigma: The stigma is the topmost part of a pistil or carpel. It occurs in a variety of shapes and sizes. Its sticky nature helps it to collect pollen grains.
Style: The style is the slender and filament-like structure of a pistil. It connects the stigma to the ovary. The stigma has different shapes in different flowers.
Ovary: The enlarged basal portion of the carpel is called the ovary. The ovary bears many ovules. The ovules are small and contain eggs. After fertilization, ovules form the seed and the ovary forms the fruit.
Formation of gametes in angiosperms:
The gametes of angiosperm plants are produced by gametophyte generation. The gametophytes of angiosperms are directly related to flowers. The gametes link with each other to form a structure called the sporophyte. The sporophyte is commonly related to the plant body. Multiple vegetative structures arise from the shoot meristem of the sporophyte, which at a specific development point shift from vegetative to reproductive structures.
Meiosis and unequal cytokinesis take place in the ovule. These processes result in the production of four megaspores. Out of these megaspores, the largest megaspore experiences three mitotic divisions. This gives rise to a seven-celled embryo sac that contains eight nuclei. One out of these seven cells is the egg. Two cells, called synergid cells, surround the egg. There is a central cell containing two or more polar nuclei. The central cell will fuse with the male gamete develop into the endosperm. At the contradictory end of the developing embryo sac, three antipodal cells are formed. Antipodal cells generally degenerate during the development of the embryo.
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