The flower is commonly borne on short or long stalk called the pedicel. It has an upper swollen region known as receptacle thalamus or A typical angiospermic flower consists of four whorls of floral appendages attached on the receptacle: It is the outermost whorl of the flower.
It is composed of leaf like green sepals. The sepals are essentially green in colour but in some cases they are coloured like petals. Such a condition of calyx is called petaloid. This is the second whorl of the flower and consists of a number of petals. Petals are generally brightly coloured and sometimes fragrant which make the flower to become attractive.
It is the third whorl of flower and is the male reproductive organ consisting of stamens. Each stamen is made of filament and anther. The filament supports anther at its tip.
This is the last and the fourth whorl of flower and is the female reproductive organ of the flower. It occupies the central position on the receptacle and composed of ovary, style and stigma and the component parts are called carpels. The process of the formation and differentiation of microspores pollen grains from microspore mother cells MMC by reductional division is called microsporogenesis.
The fertile portion of stamens is called anther. Each anther is usually made up of two lobes connected by a connective. In turn each anther lobe contains two pollen chambers placed longitudinally.
Each pollen chamber represents a microsporangium and is filled with a large number of pollen grains or microspores. This is the outermost single layered and protective.
In Arceuthobium, cells of epidermis develop a fibrous thickening and the epidermis is designated as exothecium. Inner to epidermis, there is a single layer of radially elongated cells. Cells of endothecium develop fibrous thickening made up of cellulose with a little pectin and lignin which help in Sexual reproduction in flowering plants definition dehiscence of anther. In between these cells, a few cells without thickening are also present.
These thick walled cells collectively form the stomium. Three to four layers of thin walled cells situated just below the endothecium are known as middle layers. Cells of this layer are ephemeral and degenerate to provide nourishment to growing microspore mother cells. This is the innermost layer of the wall. The cells are multinucleate undergo endopolyploidy and polyploid.
Tapetal cells are nutritive. It soon becomes four lobed. The outer layer is called exine. It is made up of sporopollenin derived from carotenoid. The inner intine is thin, delicate and is made of cellulose and pectose.
The generative cell loses its contact with the wall of microspore and becomes free in the cytoplasm. The pollen grains are shed from the anther at this bicelled stage rarely three celled.
The tube secretes exogenous pectinases and other hydrolytic enzymes to create a passage for its entry. The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule. Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule.
The point Sexual reproduction in flowering plants definition attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe.
The body of the ovule shows two ends: The main body of the ovule is covered with one or two envelopes "Sexual reproduction in flowering plants definition" integuments. These leave an opening at the top of the ovule called micropyle. The integuments enclose a large parenchymatous tissue known as nucellus. The process of development of female gametophyte or embryo sac from megaspore is called megagametogenesis.
In this type, only one megaspore situated towards chalazal end takes part in the development of embryo sac. This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant. Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. It is also called xenogamy.
The fusion of two dissimilar sexual reproductive units gametes is called fertilization. This process was discovered by Strasburger Pollen grains reach the receptive stigma of the carpel by the act of pollination.
Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates in vivo to produce a pollen tube which grows into stigma towards the ovarian cavity.
After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:. When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type.
The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common. Casuarina, Juglans, Betula, etc. It was first observed by Treub in Casuarina. The pollen tube enters the ovule through its middle part i. Cucurbita, Populus or through funicle e. The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus.
Sexual reproduction in flowering plants definition synergids direct the growth of pollen tube by secreting some chemical substances chemotropic secretion. The tip of pollen tube enters into one synergid.
The penetrated synergid starts degenerating. After penetration, the tip of pollen tube enlarges and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid. The nuclei of both the male gametes are released in the embryo sac. One male gamete fuses with the egg to form the diploid Sexual reproduction in flowering plants definition. The process is called syngamy or generative fertilization. This syngamy was discovered by Strasburger The diploid zygote finally develops into embryo.
The other male gamete fuses with the two polar nuclei or secondary nucleus to form the triploid primary endosperm nucleus. The process is called triple fusion or vegetative fertilization. These two acts of fertilizations constitute the process of double fertilization. The process was discovered by S. Nawaschin and Guignard in Lilium and Frittillaria.
Double fertilization occurs in angiosperms only. DearPreparing for entrance exams?