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The flower, in its basic design, is a shoot in which the internodes (the regions of stem between nodes) do not elongate. As a result, the floral parts that occur successively along this shoot develop as a closely packed cluster. The enlarged portion of the stalk on which the flower develops is known as the receptacle, and the stalk of flower is the pedicel.
Flower develops from a bud known as flower bud. This bud usually develops in the axil of a leaf called the bract. Sometimes the floral bud arises from the apical bud when the bract is absent. The flower buds are homologous with the vegetative buds. It consists of a stem tip, the receptacle, resembling in ontogeny and fundamental structure a vegetative tip.
It consists of nodes and internodes, and bears appendages. The nodes are crowded and brought together by the suppression of internodes. The apical growth is determinate in its growth and typically only four nodes are formed bearing floral leaves at the nodes, the calyx, corolla, androecium and gynoecium, from the base towards the apex respectively.
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The calyx and corolla are more leaf like, and are known as the non-essential organs as they are only indirectly concerned in the process of reproduction and the other two, the androecium and gynoecium are more unlike leaves, and are known as the essential organs, as they are directly required in the process of reproduction.
The various groups of floral parts develop one after another from the receptacle. The first to form are sepals, which are usually green in colour and leaf-like in structure. Collectively the sepals are known as calyx. The calyx encloses the flower bud. Next to form are the petals, which are also usually shaped somewhat like leaves but are often brightly coloured. Collectively the petals make up the corolla.
The special function of the corolla is to advertise the presence of the flower to potential pollinators. In some flowers, such as the lily and tulip, the sepals and petals cannot be distinguished separately, in such cases, they are known as tepals.
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This situation is commonly found in monocotyledons, in which the flower parts are often borne in three’s or multiples of three. Collectively the tepals are known as the perianth. The perianth consists of the sterile, or non- reproductive, parts of the flower.
The next primordia to form on the floral apex are the stamens. Collectively the stamens form the androecium. The number of stamens varies from one to many, depending on the species. The stamens are regarded as the male organs of the flower. Each stamen consists of three parts— filament, anther and connective.
The anther bears four chambers or pollen sacs, pach filled up with pollen grains. The pollen when ripe, are released from within the anther, usually through narrow slits. The haploid male nuclei, which function like sperm but do not swim, differentiate within the pollen tube, which grows from the pollen into the ovary.
The upper and innermost parts of the flower, and usually the last floral parts to mature, are the carpels. The carpels of a flower make up the gynoecium or the pistil. This is the female whorl.
The carpels of most angiosperms consist of a hollow expanded base, or ovary, which later forms the fruit; stigma, which is a flat, often hairy and sticky surface specialized to receive the pollen; and connecting stigma and ovary, a slender stalk, the style, down which the pollen tube travels.
A flower may have one (e.g., Pisuni spp), or several carpels (e.g., Ranunculus spp.) and they may occur singly or fused. The ovary bears some minute egg-like bodies known as the ovules. Each ovule encloses a large oval cell known as the embryo sac. The haploid egg nucleus is formed within the embryo sac.
On the maturation of the flower, the stigma becomes receptive to pollen. A pollen grain, after being deposited on the stigma, produces a long tube which grows down through the stigma and the style and enters one of the ovules in the ovary. Two sperm nuclei are found at the end of this tube. One of these enters to egg of an ovule and fuses with the nucleus of the egg to form the zygote.
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This fusion of male and female nuclei is called fertilization, and the flower is said to be fertilized when this has taken place. After fertilization the ovule develops into a seed, while the whole ovary becomes a fruit. The mature fruit contains one to many seeds, each with an embryo inside it. From these embryos grow the young plants of the next generation, and the life cycle of angiosperm begins again.
