Current Affairs 11th Class

Category : 11th Class

(Gk. Angeion = covered; sperma = seed)

Introduction

The angiosperms, or flowering plants, constitute the most dominant and ubiquitous vascular plants of present day flora which changed the green and yellow melancholy of the earth's vegetation by the colourful brightness and fragrance of their flower. The term angiosperm means 'enclosed seed' because the ovules or potential seeds are enclosed within a hollow ovary. In this respect they are considered most highly evolved and advanced as compared with the naked seeded gymnosperms.

Characteristic features

(1) Angiospermous plants grow in almost every kind of habitats. In the deserts, these plants grow, flower, shed seeds and complete their life cycle in a few weeks of rainy season. Some flowering plants like Zostera, occur in shallow seas. A small orchid even lives underground. It survives as a saprophyte on decaying organic matter because of the mycorrhizal association which helps to obtain nourishment. In rain forests, some plants grow on the branches of other plants but do not obtain water or food from them. They are called epiphytes (e.g., Vanda).

(2) The angiospermous leaves show reticulate or parallel venation forming areoles. The libriform fibres are present in the xylem and the companion cells are present in the phloem. The true vessels are present in the xylem of angiosperms.

(3) The angiosperms produce flowers which normally consist of 4 whorls of appandages – the two outer accessory and reproductive structure such as sepals and petals and the two inner essential parts – stamens and carpels.

(4) The stamens (microsporophylls) are bilaterally symmetrical. Each stamen consists of a filament and an anther.

(5) The anthers produce tectate pollen grains with exine differentiated into rod-like columellae covered by a tectum.

(6) In angiosperms, the insects and animals also act as pollinating agents. For this purpose the flowers possess bright and showy petals, edible pollen and nectar.

(7) The carpels (= megasporophylls) are rolled and partly sterile so that they enclose the ovules within a hollow ovary that is connected with the stigma and style.

(8) The female gametophyte is highly reduced and consists of single egg cell, two synergids, three antipodals and two polar nuclei. The archegonia are absent.

(9) The most characteristic feature of angiosperms is double fertilization.

(i) The male gamete fuses with the egg producing diploid zygote that develops into embryo or new sporophyte.

(ii) Another male gamete fuses with the polar nuclei (triple fusion) resulting in the formation of triploid endosperm.

(10) After fertilization, the ovules ripens into seeds and ovary ripens into fruits.

Size

(1) The smallest angiosperm is Wolffia. The plant body of Wolffia consists of tiny flat oval green stem (phylloclade) having a few small roots. The plants are about 1 mm in diameter and found free floating in aquatic habitats like ponds, etc.

(2) The tallest angiosperm is Eucalyptus. Their trees may attain a height upto 100 meters or more.

(3) Banyan (Ficus bengalensis) tree covers a large area. It's slanting aerial branches spread in all directions. The tree spreads with the help of prop or pillar roots.

Longevity

Based on the duration of life, the plants are divided into following 4 categories :

(1) Ephemerals : This category includes the plants which live only for a few weeks because of a very short growing season. Such plants are found near deserts or in very cold countries. For example, Arabidopsis species have a life span of 20–28 days.

(2) Annuals : The plants of this category live and complete their life-cycle in a single favourable season. During this period, they grow in size, produce flowers, shed their seeds, undergo senescence and die. They pass the unfavourable period in the form of seeds. Many crop plants (e.g., wheat, rice, maize, etc.) are annuals. The smallest angiosperm – Wolffia is an aquatic annual.

(3) Biennials : The plants of this category complete their life-cycle in two favourable seasons (i.e., in two years). They grow vegetatively in the first season and produce flowers and set seeds in the next. Often they produce some storage organs, as in the sugar beet, where food is stored in their swollen roots.

(4) Perennials : Plants of this category live for more than two years. Generally they live for many years and bear the flowers and fruits during specific seasons. Some perennials continue their vegetative growth for several years and produce fruits and seeds only once in their life time, e.g., Agave, Bamboos, etc. They are called monocarpic. Others produce flowers and fruits every year after attaining a definite stage of maturity, e.g., Mango, Lemon, Apple, etc. Such plants are called polycarpic.

Habit

Depending upon the habit of plants, the angiosperms belong to following categories :

(1) Herb : These are small, soft, non-woody plants without persistent parts aboveground. The height of plants usually reaches upto 1 m. The plants may be annual (Brassica), biennial (Sugar beet) or perennial (Canna). The perennial herbs usually possess underground rhizomes which form the new aerial shoots every year. The plants of banana are perennial herbs.

(2) Shrubs : These are woody plants of relatively low height (1-4 m). They typically branch at or near the base and do not have a main trunk, e.g., Rose. They are mostly perennial.

(3) Trees : These are perennial woody plants with one main trunk. The trunk may or may not be branched. These are of the following types :

(i) Caudex : The stem is unbranched and usually bears a crown of leaves at the apex. e.g., Date-palm.

(ii) Excurrent : The lower part of stem is thicker which gradually tapers above. Branches arise from the main stem in acropetal succession and plant appears conical e.g., Pinus.

(iii) Deliquescent : The apical bud of the main stem dies after some time and branches and sub-branches spread in different directions. e.g., Tamarindus, Ficus.

(4) Culms : In these plants, nodes and internodes are extremely clear. Internodes of such plants are usually hollow. These plants are grasses but cannot be considered as herb or shrub or tree. e.g., Bambusa (Bans).

The plants of Angiosperms divided into two major groups as – Dicotyledons and Monocotyledons.

(1) Dicotyledons : They are show following distinguished characteristics.

(i) Tap roots found in the members of this group.

(ii) The leaves in members of these class exihibit reticulate (net like) venation.

(iii) The flowers are tetramerous or pentamerous having four or five members in the various floral whorls, respectively.

(iv) The vascular bundles arranged in a ring, numbering 2–6, open and with cambium.

(v) The seeds of dicotyledons are with two cotyledons as the name indicate.

(2) Monocotyledons : They are show following distinguished characteristics :

(i) Adventitious roots found in the members of this group.

(ii) The leaves are simple with parallel venation.

(iii) The flowers are trimerous having three members in each floral whorl.

(iv) The vascular bundles scattered in the ground tissue, many in number, closed and without cambium.

(v) The seeds of monocotyledons are with one cotyledons as the name indicate. e.g., Cereals, bamboos, sugarcane, palms, banana, lillies and orchids.