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.

Reproduction

The male sex organ in a flower is the stamen.

Each stamen consists of a filament with an anther at the tip. The anthers following meiosis produce pollen grains.

The female sex organ in a flower is the pistil or the carpel.

Pistil consists of an ovary enclosing one too 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 grains, 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.

Size

(1) 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.

Classification

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

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 exhibit 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 indicates.

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

Longevity

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

(i) 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.

(ii) 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.

(iii) 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.

(iv) 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.

Bamboo or Agave are monocarpic perennial plants.

A small archid grows underground as a saprophytes.

Vessels are major water conducting cell in angiosperm.

Double fertilization is a unique character of angiosperm.

While most algal genera are haplontic, some of them such as Ectacarpus, Polysiphonia, kelps ate haplodiplontic. Fucus, an alga is diplontic.

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 up to 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).

Habitat

Warming (1895) divided the plants, on the basis of their adaptation to water, into four major groups – hydrophytes, mesophytes, xerophytes and halophytes. A fifth group epiphytes can also be included.

(i) Hydrophytes : The plants which grow in aquatic habitats are called hydrophytes. They are further grouped as –

(a) Submerged (e.g., Hydrilla)

(b) Attached floating (e.g., Nymphaea)

(d) Amphibious or partly emerged hydrophytes (e.g., Sagittaria).

(ii) Mesophytes : These are the plants which grow under moderate moisture and temperature conditions. They have no special adaptations to grow either in very dry or in very wet conditions (e.g., Sun flower, Brassica). These plants do not possess special adaptations to reduce transpiration.

(iii) Xerophytes : The plants which grow in dry or xeric habitats (i.e., under deficient supply to available water) are called xerophytes. These plants face acute shortage of water and therefore, develop morphological, structural and physiological adaptations in order to survive under such habitats. The adaptations in plants are mainly to check the transpiration and survive under acute shortage of water. e.g., Cynodon (Doob grass), Casuarina, Euphorbia tirucalli, Asparagus, etc.

(iv) Halophytes : Halophytes are those plants which grow in saline habitats, i.e., in salt marshes, alkaline soils, river estuaries, saline ponds near seashore or sandy and heavy soils having excess of salts. In such habitats, the water is present in sufficient amount but due to high osmotic concentrations it is physiologically not available to normal plants. Such conditions are said to be physiologically dry. e.g., Spartina, Atriplex, Portulaca etc.

(v) Epiphytes : These are the plants which grow on other plants for space only. The plants are autotrophic and occur both in aquatic and terrestrial habitats. e.g., Vanda (an orchid).

Plant Life Cycles & Alternation of Generation

In plants, both haploid and diploid cells can divide by mitosis. This ability leads to the formation of different plant bodies haploid and diploid.

The haploid plant body produces gametes by mitosis. This plant body represents a gametophyte.

Following fetilisation the zygote also divides by mitosis to produce a diploid sporophytic plant body.

Haploid spores are produced by this plant body by meiosis. These in turn, divide by mitosis to form a haploid plant body once again.

Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between, gamete producing haploid gametophyte and spore producing diploid sporophyte.

Different plant groups, as well as individuals, representing them, differ in the following patterns:

1. Sporophytic generation is represented only by the one-celled zygote, There are no free-living sporophytes, Meiosis in the. zygote results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase in such plants is the free-living gametophyte. This kind of life cycle is termed as haplontic. Many algae such as Volvox, Spirogyra and some species of Chlamydomomas represent this pattern.

2. The type wherein the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. The gametophytic phase is represented by the single to few-celled haploid gametophyte. This kind of lifecycle is termed as diplontic. All seed-bearing plants i.e. gymnosperms and angiosperms, follow this pattern.

3. Bryophytes and pteridophytes, interestingly, exhibit an intermediate condition (Haplo-diplontic); both phases are multicellular and often free-living. However, they differ in their dominant phases. A dominant, independent, photosynthetic, thalloid or erect phase is, represented by a haploid, gametophyte and it alternates with the short lived multicelluler sporophyte totally or partially dependent on the gametophyte for its anchorage and nutrition. All bryophytes represent this pattern. The diploid sporophyte is represented by a dominant, independent, photosynthetic, vascular plant body. It alternates with multicellular, saprophytic autotrophic, independent but shortlived haploid gametophyte. Such a pattern is known as haplo-diplontic life cycle. All pteridophytes exhibit this pattern.