Angiosperms

 

* Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialised structures called flowers.

* In angiosperms, the seeds are enclosed by fruits.

* The angiosperms are an exceptionally large group of plants occurring in wide range of habitats. They range in size from tiny, almost microscopic Wolfia to tall trees of Eucalyptus (over 100 metres).

* They provide us with food, fodder, fuel, medicines and several other commercially important products.

* They are divided into two classes: the dicotyledons and the monocotyledons.

* The dicotyledons are characterised by having two cotyledons in their seeds while the monocolyledons have only one.

* The male sex organs in a flower is the stamen. Each stamen consists of a ~lender filament with an anther at the tip. The anthers, following meiosis, produce pollen grains. The female sex organs in a flower is the pistil or the carpel. Pistil consists of an ovary enclosing one to many ovules. Within ovules are present highly reduced female gametophytes termed embryo sacs. The embryo-sac formation is' preceded by meiosis. Hence, each of the cells of an embryo-sac is haploid. Each embryo-sac has a three-celled egg apparatus one egg cell and two synergids, three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.

* Pollen grain, after dispersal from the anthers, are carried by wind or various other agencies to the stigma of a pistil. This is termed as pollination.

* The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule. The pollen tubes enter the embryo-sac where two male gametes are discharged. One of the male gametes fuses with the egg cell to form a zygote (syngamy). The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN). Because of the involvement of two fusions, this event is termed as double fertilisation, an event unique to angiosperms. The zygote develops into an embryo (with one-or two cotyledons) and the PEN develops into endosperm which provides nourishment to the developing embryo. The synergids and antipodals degenerate after fertilisation. During these events the ovules develop into seeds and the ovaries develop into fruit,

Fig: Life cycle of an Angiosperm


 * 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.

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

* The plants of Angiosperms is 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, lilies and orchids

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

(1) Herb: These are small, soft, non-woody plants without persistent parts above ground. 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 mayor 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 a.re grasses but cannot be considered as herb or shrub or tree. e.g., Bambusa (Bans).

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