REPRODUCTION

     The ability to produce new living individuals in a characteristic feature of all living organisms and is known as reproduction. Reproduction is the only means by which life is maintained. Every living thing, whether it be a plant or an animal, strives to exist forever but this is not possible as each living form has only a certain life span.

Therefore, living organisms have resorted to the production of new individuals resembling themselves so that their species can go on existing. 

     Each plant and animal usually gives rise to several new individuals each time it reproduces. If this went on indefinitely, the number of individuals of each kind would increase greatly and there would hardly be any space for Man himself to live. However, such a problem has never occurred. This is because the older organisms die and in doing so , make room for the younger ones. At the same time, not all the newly produced young ones survive. When there are large numbers of a particular plant or animal in any one place, competition occurs among the individuals and only the stronger ones manage to survive. The weak ones die off sooner or later. Thus, of the total number of organism produced, there will always be only a few that survive. These in turn will reproduce new ones to keep the species going. 

     This theory was put forward by a biologist named Darwin. He also thought that there would always be only a certain number of a particular plant or animal on earth and as years go by, accompanied by climatic changes, new types of plants and animals would be produced. The formation of new types of plants and animals from old types is known as evolution. The fact that plants and animals which existed a million years ago are not the same as those which exist today proves that evolution has occurred over the ages. 

   REPRODUCTION IN PLANTS

Plants can reproduce in two ways— by asexual and sexual In asexual reproduction, only one individual is involved. It divides into two or more portions, or cuts off bits of its body to produce new individuals. For example, unicellular such as chlamydomonas and euglena reproduce by dividing into two, four or eight parts, each of which becomes a new cell resembling the adult in every way. 

     In sexual reproduction, two individuals, a male and a female, are involved. Both these individuals produce reproductive cells or gametes (namely male and female gametes). These male and female gametes then fuse in pairs to form zygotes. Each zygotes then undergoes division and multiplication of cells to produce a new multicellular individual.

     In flowering plants, the main reproductive organs are the flowers. Flowers may be bisexual or unisexual. The male gametes (contained in the pollen grains) produce by one flower fuse with the female gametes (contained in the ovules) of the same flower or another flower of the same kind. This is known as fertilization. A flowering plant, therefore, has two parts— a reproductive part (the flowers) and a vegetative part (roots, stem, leaves and buds).  

     Besides sexual repriduction, some flowering plants resort to an asexual method of reproduction known as vegetative propagation. In this case, a particular part of the vegetative body of a plant, when isolated, is able to grow into a new plant. For example each cutting of the stem of a Hibiscus plant is able to grow into a new Hibiscus plant by producing new roots, branches and leaves. Although the stem is a vegetative part of the plant, a small section of it can produce a new plant. 

SEXUAL REPRODUCTION IN PLANTS— FLOWERS

     As flowers are the main reproductive organs of flowering plants, we shall study about the structure of flowers, how they are pollinated and fertilized, and how they develop into fruits containing seeds. 

         FLOWER STRUCTURE

     Fliwers are the most attractive as well as the most important parts of a plThey usually grow from the axils of leaves (axillary flowers) but some are found at the apex of a stem or branch (terminal flowers). 

Some occur singly and are known as solitary flowers (e.g. Hibiscus) while others grow in clusters or inflorescences (e.g. sunflower). An inflorescences may be terminal or axillary. 

     If a flower or a group of flowers is borne in the axil of a foliage leaf, this leaf is known as a bract. The flower stalk is called the pedicel. However, the main axis of an inflorescence is known as the peduncle while the stalk of each individual flower of an inflorescence is called the pedicle. In the inflorescence the flowers which have stalks are known as pedicillate flowers while those which have extremely short stalks or no stalks at all are known as sessile flowers. 

     The fleshy base of a flower to which the different floral parts are attached is known as the receptacle, or thalamus. A flower has four different types of floral parts arranged in four different whorls on the receptacle. The floral parts are the calyx, corolla, androecium and gynoecium. The flower is considered to be a modified shoot, where the receptacle is a modified branch and the floral parts are modified leaves. 

The Calyx

     The calyx is the outermost whorl of floral parts on the receptacle. It is uaually composed of two to five small leaf like parts known as sepals. However, in some flowers like those of certain cacti, numerous sepals are found. Sepals are usually green in colour and serve to protect the inner parts of the flower during the bud stage. They may remain completely free from one another (e.g morning glory) or may be fused together to form a cup like structure ( e.g crotolaria). The free state is known as a polysepalous condition while the fused State is known as a gamosepalous condition. In some flowers, there may be another whorl of floral parts outside the calyx (e.g Hibiscus). The whorl is known as the epical Each unit, an episepal is usually green in colour. 

     The sepals of some flowers instead of being small and green, are large, brightly coloured and attractive. Such a situation is seen in the flowers of caesalpinia and clerodendron, in which the colours are orange and white respectively. In these flowers the sepals are said to be petaloid.yx.ant.means.