ANGIOSPERM

Angiosperm:

Introduction:

Angiosperm is the largest group of plant on earth. There are approximately 2,70,000 known species alive today. It includes all plants that have flowers and account for approximately 80% of all known living plants.

Occurrence:

They are able to grow in variety of habitat. They can grow as trees, shrubs, bushes, herbs and small flower of plants.

Salient Features:

Ø  Sporophyte (dominant plant in life cycle) is differentiated into roots, stem and leaves.

Ø  Highest degree of perfection of the vascular system with true vessels in the xylem and companion cells in the phloem.

Ø  The organization of microsporophyll’s (stamen) and megasporophyll (carpels) into a structure called flower which is typical of angiosperm.

Ø  Presence of microsporangia (pollen sacs) per microsporophyll (stamen).

Ø  Ovules are always enclosed in an ovary which is the basal region of megasporophyll.

Ø  Production of two kinds of spores, microspores (pollen grains) and megaspores. Thus, angiosperms are heterosporous.

Ø  Male gametophyte consists of pollen grains and pollen tube contains the tube nucleus and two male gametes or nuclei. The male cells or gametes are non-ciliated.

Ø  Female gametophyte lacks any extensive development of vegetative tissue. It consists of 3 egg apparatus cell, 3 antipodal cells and 2 pollen nuclei in the centre of embryo sac.

Ø  Non- motile male cells or nuclei are carried bodily to the neighborhood of egg apparatus by the pollen tube.

Ø  Seeds remain enclosed in the ripened ovary called the fruit.

Ø  Phenomenon of double fertilization or triple fusion is the characterstic of angiosperm.

Ø  Endosperm develops after fertilization. It is triploid.

Ø  Angiosperms are completely adapted to life on land.

Distinguishing feature of angiosperm:

Ø  Angiosperms have ovules that are enclosed in an ovary. There are subtypes of angiosperm; monocot and dicot.

Ø  They grow as trees, shrubs, bushes as well as herbs. These plants have diploid (2n) sporophyte.

Ø  Root system is also very complex. The root also contains cortex, phloem, xylem and epidermis. They have root hair that helps in absorption of water, minerals from the soil.

Ø  Flowers are most differentiating feature of angiosperm. They are the reproductive structure of angiosperms.  The flower has a thalamus i.e. short axis and the 4 whorls of sporophylls are arranged on the thalamus.

Ø  Prime features that distinguish angiosperms also include double as well as triple fusion. This leads to the formation of zygote (2n) and triploid endosperm cell (3n). Endosperm cell produces endosperm and zygote develops into a sporophyte.

Ø  Fertilization takes place by pollination includes insect, wind, pollination etc. Endosperm produced after fertilization and before the zygote undergoes first division. Endosperm helps in providing nutrition to the developing embryo. Fertilized ovule develops into seeds and then ripens into fruit.

Main feature of angiosperm:

1)      Benefit of flowers: Due to flowers, angiosperms are able to adapt a wide range of ecological niches, making them largely dominated terrestrial system.

2)      Reduced male and female parts: Instead of cones in gymnosperm, the angiosperms have stamens are much lighter than the corresponding organ of gymnosperm and have contributed to the diversification of angiosperm through time with adaptation to the specialized pollination method.

3)      Dominant sporophyte: Main plant of angiosperm is diploid sporophyte which is divided into root, stems and leaves. Male gametophyte in angiosperm is significantly reduced in size compared to those of gymnosperm seed plant.

Double fertilization:

It is a rule on angiosperm. This means that the fertilization in angiosperm involves the joining of female gametophyte (megagametophyte or embryo sac) with two male gametes (sperm).

Classification of Angiosperm:

Based on the nature of embryo in the seed, angiosperms are classified into two groups. There are two main groups of flowering plants:

1.       Monocot:

Ø  Monocots have one seed leaf or cotyledon in their seeds. Cotyledons are significant part of the embryo found in seed. They are the first part of the seedlings emerge out from the soil when it germinates.

Ø   The monocots are characterized by single cotyledonous seeds, parallel venation in leaves and trimerous flowers having three members in each floral whorl.

Ø  Plants also vary in number of their floral parts. Monocots usually have flower parts of three or multiples of three. This means that they could have 3, 6 or 9 petals or they could have sepals that make up their flower.

Ø  Vascular bundles in stem are numerous and scattered.

Ø  No cambium is present in monocots or there is no secondary growth found in stem.

Ø  Stems are usually hollow.

Ø  Seed germination is usually hypogeal.

Ø  Monocots are generally herbaceous plants.

Ø  Fibrous root system is found in monocots. They also have adventitious roots.

Ø  Monocots include grasses, cereals, lilies, sugarcane, banana, onion, palms, bamboo, orchids etc.

2.       Dicots:

Ø  Dicots are characterized by seeds having two cotyledons, reticulate venation in leaves and tetramerous or pentamerous flowers i.e. having 4 or 5 members in each whorls.

Ø  Dicots have two seed leaves in their seeds. The seed leaves stores food for the young sporophyte and becomes the first seed leaves when the seed germinates.

Ø  Dicots have 4 flower parts of as in mustard and have five flower parts as in hibiscus or they have multiples of 4 and 5. Flowers are usually complete.

Ø  Vascular bundles in stem are fewer and arranged in circular rings.

Ø  Cambium is present and secondary growth occurs.

Ø  Stems are usually solid.

Ø  Seed germination is either hypogeal or epigeal.

Ø  Tap root system is present in dicots.

Ø  Dicot includes pulses, pear, apple, cucurbis, coriander, marigold and many more ornamental flowering plants.

Ø  Dicots have woody and non – woody plants. Dicots are generally herbs, shrubs and trees.

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