Reproduction In Flowering Plant

A. ARTIFICIAL VEGETATIVE REPRODUCTION

❒ ” In artificial vegetative propagation, a portion is separated from the body of the plant and then it is grown independently.

❒ ” This is useful commercially because the new individuals produced maintain the desirable characters of the parents and the offsprings are called as true type.

❒ ” A population of these genetically identical plants obtained from an individual is called a clone.

❒ ” Following techniques have been developed for the artificial vegetative propagation of economic plants:

Cuttings: This is the most common method of vegetative propagation practiced by gardeners and nurserymen all over the world. A portion of root, stem or leaf is taken and rooted in the soil to form a new plant. Stem cuttings are most suitable for vegetative propagation as they readily establish themselves into new plants by forming adventitious roots. Factors such as the optimal length and diameter of the cuttings, age of the parent plant and the season have to be taken into consideration for each species. Plants like sugarcane, roses, grapes, cocoa, tapioca, carnation, Bougainvillea, Coleus etc. are propagated by stem cuttings.

➢ Stem cuttings of some plants do not produce roots readily and have to be treated with hormones like IAA, IBA and NAA.

➢ Root cuttings of lemon, tamarind, ipecac, etc. when put into moist soil, sprout forming new roots and shoots.

Layering: In this method, roots are induced on the stem while it is still attached to the parent plant. There are two common types of layering:

➢ Mound layering: In this technique, the lower branch of the plant is bent down close to the ground and covered with moist soil in such a way that its growing tip remains above the soil surface. This bent branch is now called a layer. After a few days the covered portion of stem usually produces adventitious roots. The rooted branch is then cut and is grown as an independent plant. This method is commonly employed for propagating strawberry, jasmine, grape vine, raspberry, gooseberry, apple, etc.

➢ Air layering: In this method, the stem is girdled or slit at an upward angle, and covered with moist moss or cotton and wrapped with a polythene sheet to prevent drying. In drier climate, an earthern pot with a hole at the bottom is hanged over the sheet in a convenient position and the two are connected by a soft cotton cord. The pot is filled with water, and water trickles down the cord and keep the sheet moist. After the injured part produces roots, the branch is cut and planted separately to propagate a new individual. This technique is usually employed in plants with thick branches which can not be bent to ground
e.g., pomegranate, orange, guava, litchi, etc.

B. MICROPROPAGATION

 Micropropagation is the raising of new plants from a small plant tissue with the help of tissue culture technique.

Tissue culture is the technique of maintaining and growing cells, tissues, organs, etc. and their differentiation on artificial medium under aseptic conditions inside suitable containers.

For micropropagation, axillary buds, shoot tips or any other plant part can be used as an explant. Virus free plants can be produced if shoot apical meristems of about 0.25 mm long with one or two leaf primordia are inoculated.

Explants are properly sterilized when collected from the field. Multiple shoots can be obtained by inducing callus. Callus is irregular, unorganized and undifferentiated mass of actively dividing cells.

Plants produced in culture are carefully transferred to the soil. They are planted first in vermaculite and kept under high humidity (90-100%) for 10-15 days. They are gradually taken to the green house and can be transferred to the soil after 4-6 weeks.

Tissue culture technique is being commercially used for micropropagation of orchids, carnation, Gladiolus, Chrysanthemum and many other ornamental plants. The method is also useful for producing healthy plants in several important crops like potato, tapioca and sugarcane. Even haploid plants can be produced by tissue culture technique.

c. megasporogenesis

❒ ”The process of the formation of megaspores in megasporangium (ovule) from megaspore mother cell by reductional division is called megasporogenesis.

❒ ” Megasporogenesis is well studied into following two heads :

❒ Structure of ovule

❒ Formation of megaspores

structure of ovule

❒ ”The integumented megasporangium is called ovule. On the inner wall of ovary, there is a parenchymatous tissue, known as placenta. The placenta bears one or more ovules.

❒ ”The ovule has a stalk and a body. It is attached to placenta by means of stalk, called funicle or  funiculus. The point of attachment of funiculus to the body of ovule is called hilum.

❒  ”In anatropous ovule (the most common type) the funicle continues beyond the hilum alongside the body of ovule, forming a sort of ridge, called the raphe. The main body of ovule is made of parenchymatous tissue called nucellus.

On the basis of nucellus, ovules are of two types ; tenuinucellate and crassinucellate. In the former type, nucellus is till developed, e.g., Asteraceae, Rubiaceae, etc. In the latter type, the nucellus is massive and well developed, e.g., most of the angiosperm.

❒  ” Nucellus is enclosed and protected by sheath, known as integument. On the basis of number of integuments, ovule may be unitegmic (e.g., Asteraceae and gymnosperms) or bitegmic (e.g., Polypetalae of dicots, and monocots) or ategmic (e.g., Santalum, Loranthus, Liriosoma, Olax, etc.).

❒ ”Bitegmic ovules are more common in angiosperms and have been found in 208 families. The integuments cover entire nucellus except at its apex where a small opening is left out, called micropyle.

D. TYPES OF OVULES

On the basis of mutual relationship between funicle, chalaza and micropyle, there are six types of ovules :

❒  ”Orthotropous or atropous : Such ovule is erect or straight so that the funicle, chalaza and micropyle lie on one and same vertical line. This is the most primitive type of ovule, e.g., Polygonaceae, Piperaceae, Casuarinaceae, Urticaceae, etc.

❒ ”Anatropous : The body of the ovule is completely inverted so that micropyle and hilum come to lie very close to each other. The micropyle and the chalaza, but not the funicle, lie on the same straight line. It is the most common type of ovule found in angiosperms (82% families).

Hemianatropous or hemitropous : It is intermediate between ortho and anatropous types of ovule with the body of ovule lying at right angles to funiculus. Micropyle and chalaza lie on the same straight line, e.g., members of Ranunculaceae, Primulaceae, and some members of Brassicaceae.

❒  ” Campylotropous : The body of ovule is bent in such a way that micropyle gets directed downwards and is not in straight line with chalaza, e.g., Capparidaceae, Chenopodiaceae, Caryophyllaceae, some legumes and some members of Brassicaceae like mustard, Capsella, etc.

Amphitropous : The body of ovule is curved in such a way that embryo sac also gets curved and becomes horse-shoe shaped, e.g., Papaveraceae, Alismaceae, Butomaceae, etc.

❒  ”Circinotropous : The ovule is initially orthotropous but becomes anatropous due to unilateral growth of funicle. The growth continues till the ovule once again becomes orthotropous. In this manner, the funicle takes a complete round around the body of ovule, e.g., prickly pear (Opuntia tuna), members of Plumbaginaceae.

E. CONTRIVANCES FOR CROSS POLLINATION

Nature favours cross pollination. All unisexual flowers and a large number of bisexual flowers are naturally cross-pollinated.

The main contrivances ensuring cross-pollination are as follows :

❒  ” Unisexuality or dicliny : The presence of only one reproductive organ in a flower is called unisexuality or dicliny and flower is said to be unisexual or diclinous. Unisexual flowers are borne by both monoecious and dioecious plants. Unisexual flowers are of two types, male and female flowers. Monoecious plants bear both male and female flowers, e.g., maize, castor, coconut, banana, cucurbita, grapes, etc. Dioecious plants bear either male or female flower. Such plants are of two types ; male plant bearing male flowers and female plant bearing female flowers, e.g., papaya, hemp, Asparagus, date palm, spinach etc.

❒  ”Cross pollination is the rule in dioecious and monoecious plants.

 ❒ Dichogamy : The maturation of anther and stigma of a bisexual flower at different times is called dichogamy and flower is said to be dichogamous. Dichogamy is of two types Protandry : Anthers mature prior to stigma, e.g., Compositae (Helianthus, Tagetes, etc.), Malvaceae (Gossypium, Abelmoschus, etc.), Apiaceae (Foeniculum, etc.).

❒  Protogyny : Stigma matures earlier than anthers e.g., Anona, Polyalthia, Magnolia, Michelia, Aristolochia, Pennisetum, Ficus, Plantago, Scrophularia, etc.

❒ ” Self sterility or Incompatibility : When pollen grains of an anther do not germinate on the stigma of the same flower, then such flower is called self sterile or incompatible and this condition of flower is called self sterility, e.g., potato, Passiflora, Malva, tobacco, radish, Petunia axillaries, etc. Quicker growth of pollen of another plant pollen of the same plant is called prepotency, e.g., apple.

F. AGENTS FOR CROSS POLLINATION

❒  Cross pollination is brought about by external agents, such as wind, animals and water. Accordingly the types of cross pollination are anemophily or anemogamy, zoophily or zoogamy and hydrophily or hydrogamy. Anemophily : Pollination by wind is called anemophily and such flower is called anemophilous. The adaptation of anemophily are as follows :

➢They are devoid of scent, nectar, etc.

➢They are small, inconspicuous and not very long.

➢They are devoid of showy bracts, bracteoles, etc.

➢The calyx and corolla may be highly reduced or absent.

➢When monoecious, the male flowers are more abundant than female.

➢The inflorescence or the flowers may be pendent so that they may swing.

➢The anthers may be versatile.

➢They produce enormous amount of pollen grains. A single plant of Mercurialis annua has been estimated to produce 1352 × 105 pollen grains.

➢The pollen grains are very minute, light, dry, non sticky and smooth so that there is no clumping. Pollen grains of some plants are said to be blown to 1,3000 km.

➢Pollen grains may be winged as in Pinus.

➢Presence of large, hairy and branched stigmas.

➢The number of ovules is generally reduced (biological significance).

➢Anemophily is found in most cereals, palms and in a number of archichlamydeous families like Salicaceae (poplar, willow), Betulaceae (alder, birch), Fagaceae (oak, beech), Ulmaceae (elm), Urticaceae (Ultica), etc.

➢ Anemophily is a primitive character in the evolution of plants.

❒ ” Zoophily : Pollination by animals is called zoophily and such flower is called zoophilous. According to the type of animals involved, zoophily is further sub divided into entomophily, ornithophily, chiropteriphily and malacophily.

❒ Entomophily : Pollination by insects is called entomophily and such flower is called entomophilous flower. Main insects which help in pollination are bees, flies, wasp, beetles and moth. Bees alone carry out pollination in 80% cases. These are colour blind for red and are attached towards yellow, violet and purple colour. Moths carry out pollination in nocturnal flowers and rest of the insect in diurnal flowers.

The adaptations of entomophily are follows :

➢Conspicuousness : It may be due to the individual flower or to the aggregation of flowers forming inflorescence. A flower becomes very conspicuous due to its bright colour of its petals or perianth leaves. Certain insects show preference to certain colours as bees prefer blue colour, butterflies red and so on.

➢ Scent : Some flowers emit nauseous odour to attract insect. Most nocturnal flowers emit a sweet scent at night to attract insects from a distance. Common examples are Nyctanthes, Cestrum, Jasminum, Quisqualis, etc.

➢ Nectar : Usually all flowers with gamopetalous corolla secrete nectar. Nectar is sugary substance edible for insects. Nectar is contained in a special gland called nectary. It occurs at the base of floral whorls.

G. endosperm

 ❒  Endosperm is a nutritive tissue formed from vegetative fertilization in angiosperms. It is meant for nourishing the embryo. It is generally triploid. On the basis of mode of development, there are 3 types of endosperm in angiosperms : nuclear type, cellular type and helobial type.

 ❒ These three types of endosperm can be distinguished only after the first division of primary endosperm nucleus (PEN). Nuclear endosperm : When the first and all subsequent division of primary endosperm nuclei are free nuclear division, i.e., nuclear divisions are not followed by wall formation, nuclear endosperm is formed. At maturity, centripetal wall formation occurs to make the tissue partly cellular. Cellular endosperm : When the first and all subsequent division of primary endosperm nucleus are followed by wall formation, cellular endosperm is formed. This type of endosperm formation is common in the members of Gamopetalae and is reported in about 72 families.

➢ Helobial endosperm : In this type, first division of primary endosperm nucleus is followed by wall formation of form two chambers, but further nuclear divisions in both chambers are free nuclear divisions. This type of endosperm is intermediate between the nuclear and cellular types. It is mostly found in monocots (about 17 families) of order Helobiales.

    It is generally believed that nuclear endosperm is primitive, helobial intermediate and cellular advanced.