Life Cycle of Angiosperms

The angiospermic plant is a diploid sporophyte which is composedo f root, stem, leaves and flowers. Flower is the reproductive organ, whiles tamens and the carpels are its reproductive parts. Stamens are maler eproductive parts while the carpels are the female reproductive parts. Eachs tamen consists of an anther with four pollen sacs. A large number of microspores are produced by meiosis in each pollen sac. The wall of microspore becomes thick and is known as pollen grain.

During pollination the pollen grains are transferred to the stigma of the carpels. The pollen grain germinates and develops into male gametophyte or microgametophyte. The nucleus of the pollen grain divides into a generative nucleus and vegetative or tube nucleus. The generative nucleus divides into two male gametes. The pollen grain sends down a tube called the pollen tube which contains two male gametes and tube nucleus. The pollen tube together with the two male gametes and a tube nucleus constitutes the male gametophyte.

The carpel consists of a basal swollen part, the ovary, which contains one or many ovules. The ovule consists of a tissue called nucellus, which is covered by the integument. Certain changes occur in the ovule, leading to the formation of megaspore. The megaspore generally develops into seven celled female gametophyte or embryo sac. One of these seven cells is the egg or cosphere, che as endosperm mother cell which is diploid in nature, two synergid cell and 3 antipodal cells.

The pollen tube enters the female gametophyte. The tip of the pollen tube ruptures and the two sperms are released into the female gametophyte. One sperm fuses with the egg to form zygote or cospore and the other sperm fuses with the endosperm mother cell to form fusion nucleus. The fusion of one sperm with the egg to form zygote and that of the other with the endosperm mother cell to form fusion nucleus is called double fertilization, which occurs only in the angiosperms. The cospore develops into an embryo which consists of a radical, hypocotyle, plumule and one or two cotyledons. The fusion nucleus develops into a nutritive tissue called endosperm. After fertilization the ovule matures into seed. The integuments of the ovule form the seed coats called testa and tegmen. The walls of the ovary develops into fruit on ripening. Under favourable conditions, the seed germinates to produce seedling which on development becomes sporophyte. The two kinds of generation i.e.gametophyte and sporophyte, one after the other show alternation of generation.

Life cycle demonstrates an adaptation of angiosperm on land

Angiosperms are successful because they can adapt themselves to almost all kinds of environments. Moreover they produce flowers, fruits and seeds which show various adaptations for dispersal over large areas. Double fertilization is of common occurrence. They range in size from 1mm up to 100 m(Wolfia = 1mm, Eucalyptus = 100 m). They posses broad leaves and may be annual, biennial or perennial.

Inflorescence and its major types

In some angiosperms individual flowers are quite large and are borne singly on the pedicel. Such flowers are termed as solitary flowers. In most angiosperms the flowers are small and occur in groups. Such cluster of flowers arranged on the floral axis is called an inflorescence. Cluster of flowers in some way ensures pollination, fertilization and thus the reproductive success of the species. The inflorescence may be racemose or cymose. In a racemose inflorescence the main axis continue to grow indefinitely until the last flower is formed at its apex. The oldest flowers are toward the base of the inflorescence and the youngest ones toward the apex. In a cymose inflorescence the main axis soon ends in a flower. Onc, two or more lateral branches develop below the terminal flower, each ending in a flower like the main axis. In this case the terminal flower is the oldest and the lateral flowers are younger.

Chief types of racemose inflorescence are:

1. Typical Raceme

In a typical raceme the main axis is elongated and bears laterally a numbero f flowers. Each flower has a pedicel or stalk e.g. Cassia fistula (amaltas)

2.The Spike

The spike is a racemose inflorescence in which the main axis is elongated like ike raceme but the flowers are sessile i.e. without stalk e.g. Achyranthus( puth kanda) and bottle brush.

3. Catkin

It is a spike that usually bears only pistillate or staminate flowers. Examples

are mulberry and willow.

4. Corymb

In this case the main axis is comparatively short and the stalks of the lower flowers are longer than those of the upper younger ones. As a result, all the flowers lie at about the same level e.g. Iberis (candytuft).

5. Umbel

In umbel the main axis is shortened. Flowers are stalked. Due to shortening of main axis, the flowers appear to arise at one level e.g.

Hydrocotyl (brahmi booti). In some cases a number of umbels are present on the tip of the main axis. Such a compound inflorescence is called umbel of umbels or compound umbel e.g.carrot.

6.Panicle

Abranched raceme is called panicle e.g.grapes, mango and oat.

7. Capitulum

In capitulum or head, the flowers are sessile and are crowded together on a very short axis. It looks like a single flower e.g. sunflower. Chieftypes of cymose inflorescence are:

1. Uniparous Cyme (Monochasial Cyme)

In this case the main axis ends in a flower below which it produces one daughter axis only. The daughter axis as well as each succeeding one again end in flower and gives rise to one daughter axis only e.g. Begonia,Tradescantia.

2. Biparous Cyme (Dichasial Cyme)

In biparous cyme the main axis ends in a flower and produces two daughter axis. Each of the two daughter axis again ends in a flower and produces two daughter axis which may continue the branching in the

same manner e.g. Silene, Ipomoea.

3. Multiparous Cyme

In multiparous cyme the main axis ends in a flower and produces three or more daughter axis each of which continues the branching in the similar manner e.g. Euphorbia.

Benefits of Angiosperms for Humans

Angiosperms contribute huge quantities of food and of great variety, e.g. cereals such as corn, wheat, barley, rye, rice; legumes like

beans, peanuts, soybeans, fruits, vegetables and nuts, etc., Besides food,

they also supply fibres, such as cotton, limen (flax), jute. Indian hemp manila hemp. Sisal hemp e.t.c which are used in the manufacture of rope string nets and bags. The most important of these fibres are the cotton fibres , used in the manufacture of clothing. Uses of plants in various

industries and as medicines contribute to the well-being of mankind. Various articles of daily use and of economic importance are obtained from the flowering plants. Some of these, such as wood for building and

furniture, fats and oils, fibres, cereals, fruits, sugar, drugs, (e.g, quinine, digitails, ephedrin, morphine, cocaine, etc.), dyes (indigo, haematoxylon, saffranin,

etc.), tea, coffee, spices, tobacco, paper, tannins, resins, gums, essential oils, rubber, fuel, alcohol, etc. Green flowering

plants are also responsible for purifying the atmosphere as they absorb carbon dioxide and give out oxygen during photosynthesis.

One of the important aspect of angiosperms is their medicinal nature. Some of the more prominent plants in this regard are discussed below:

1. Capnabis (Cannabis sativa)

Used medicinally for thousands of years, today it is used in the treatment of sleeping disorders , autoimmune diseases and glaucoma, eye diseases.

2. Coca (Erythroxylon coca)

Cocaine was widely used as a local anaesthetic in the 19th century and coca leaf tea is taken for altitude sickness in South America.

3. Daffodil (Narcissus spp.)

Galantamine hydrobromide, a compound derived from daffodil bulbs, is being used to treat Alzheimer's disease.

4. Deadly nightshade (Atropa belladonna)

All parts of this plant, especially the berries, contain the extremely toxic chemical atropine. Atropine is used to relax the muscles of the eye and to stop muscular-spasms. (Spena tihedal)

5. English yew (Taxus baccata)

The leaves of this yew are used in the synthesis of compounds called taxols, which are used in the treatment of breast cancer.

6. Fever tree (Cinchona succiruba)

A native of Latin America, the bark of the fever tree produces quinine, which is used to treat malaria.

Vascular plants as successful land plants --

Angiosperms form one of the most highly evolved sub-classes of Spermatophyta (Seed plants), the other being gymnosperms. The

spermatophyta have been recently estimated to include about 300 families, 12500 genera and 300000 already known species. New species are being added each year by the exended survey of the vegetation of the earth and by a more critical examination of the older materials. Evidently, the angiosperms constitute a rapidly expending group which dominates the world of plants today. To this vast assemblage of flowering plants must also be added thousands of new varieties, races or strains, etc. which owe their origin as also their perpetuation to man and to the care be bestowed upon them on account of their economic or commercial importance. The most successful and important of these plants belong to Gramineae or the grass family with 7500 species, which have colonized great areas of the earth surface in practically all sorts of soil and climatic conditions and constitute the main source of man's food supply.

The factors responsible for the success of angiosperms are;

1. Their adaptability to all kinds of environments.

2. The production of flowers, fruits and seeds which show various adaptations for dispersal over large areas.

The conifers (gymnosperms) and other vascular plants (Pteridophytes) though widely distributed are, however, not found to flourish in such diverse habitats as do the angiosperms.

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