Class Angiospermae – Characteristics, Life Cycle, Classification

Characteristics of Angiosperms

Angiosperms have varied attributes. The essential attributes of angiosperms are mentioned below:

• All plants have flowers at some phase in their life. The flowers are the reproductive organs for the plant, supplying them with a method of exchanging genetic information.
• The sporophyte is separated into stems, roots, and leaves.
• They have a true vascular system i.e., proper xylem and phloem.
• The stamens (microsporophyll) and the carpels (megasporophyll) are arranged into a structure called the flower. Each microsporophyll has 4 microsporangia.
• The ovules are confined in the ovary at the base of the megasporophyll.
• Angiosperms are heterosporous, i.e., produce two types of spores, microspore (pollen grains) and megaspores.
• A single functional megaspore is completely maintained within the nucellus.
• The pollen grains transfer from the anther to stigma and recreation occurs by pollination. They are accountable for the transfer of hereditary information from one flower to the other. The pollen grains are much smaller sized than the gametophytes or reproductive cells present in the non-flowering plants.
• The sporophytes are diploid.
• The root system is very intricate and consists of the cortex, xylem, phloem, and epidermis.
• Angiosperms can make it through in a variety of environments, including marine environments.
• The procedure of fertilization is fast in angiosperms. The seeds are also produced rapidly due to the smaller sized female reproductive parts.
• The carpels confine developing seeds that might develop into a fruit.
• The production of the endosperm is among the best benefits of angiosperms. The endosperm is formed after fertilization and provides food for the developing seed and seedling.

Life Cycle of an Angiospermic Plant

The adult plant is a diploid sporophyte mainly differentiated into roots, stems, and leaves. At maturity it produces flowers. A flower is a modified shoot that consists of a pedicel, thalamus or torus, and floral leaves (sepals, petals, stamens, and carpels). Thalamus and floral leaves, especially the stamen and the carpels, are so modified, that they do not even appear like stem and leaves respectively. The sepals and petals are non-necessary or non-reproductive parts, and stamens and carpels are the essential or reproductive parts of the flower.

The sepals and the petals secure the stamens and the carpels. They likewise attract insects for pollination. When the pollination is over, the petals always fall off. The anther contains microspore mother cells which produce haploid microspores through meiosis. Each microspore sprouts to produce male gametophyte. Such microspores containing male gametophytes are called pollen.

The carpel consists of a basal broader part, the ovary, the style, and the terminal part of the style, the stigma. The ovary includes ovules. The ovule consists of an integument (covering) and tissue, the nucellus present inside.

After pollination, the pollen grain is transferred to the stigma. Here it germinates to form a pollen tube. The nucleus of the microspore divides by mitotic divisions to form two male gametes and the tube nucleus. At this stage of development, the pollen grain is called male gametophyte. In the meantime, particular modifications happen in the ovule causing the formation of female spore (megaspore). The megaspore turns into a female gametophyte. This includes 7 cells just. One of these cells is the egg or oospore.

The pollen tube grows through the style, goes into the ovule, and then reaches the female gametophyte. Here it releases the male gametes. The egg and one of the two male gametes fuse to form the oospore. The second male gamete fuses with the secondary nucleus to form endosperm nucleus (double fertilization). The oospore turns into an embryo and the endosperm nucleus develops into a multicellular nutritive tissue, the endosperm.

Seed Formation

Meanwhile, the integuments of the ovule form testa and tegmen and ovary wall become the fruit. Seeds normally undergo a period of rest and after that under ideal conditions, germinate and produce a seedling that gradually becomes a sporophyte. Hence an alternation of dominant sporophyte generation(2n) accompanies inconspicuous gametophyte generation(n).

Double Fertilization

Double fertilization is a special process found in Angiosperms. These two male gametes fuse with two cells concurrently. A male gamete (n) fuses with egg (n) to form a diploid zygote (2n) which develops later into an embryo and 2nd male gamete (n) fuses with another female cell called fusion nucleus (2n) resulting in a triploid (3n) endosperm cell, which develops into food storing endosperm tissue. It is an important evolutionary development in which food storage in fertilized ovule is made only on fertilization i.e., development of the zygote. This really assists the plant to economize its food resources.

Classification of Angiosperms

Based upon the kinds of cotyledon present, angiosperms are divided into two classes. They are monocotyledons and dicotyledons. The dicotyledonous angiosperms have two cotyledons in their seeds and the monocotyledonous angiosperms have one cotyledon.

Monocotyledonous plants

The monocots have some unique functions that consist of the presence of adventitious roots, simple leaves with parallel venation, and trimerous flowers. The number of vascular bundles is more and is closed. Some of the examples consist of bamboos, sugarcane plants, bananas, cereals, lilies, and so on.

Dicotyledonous plants

The dicotyledonous plants have two cotyledons with a tap root system. The venation seen in the leaves is reticulate. Flowers can be tetramerous or pentamerous. Vascular bundles are typically organized in a ring and number between 2 to 6. Examples consist of Grapes, Dandelions, Sunflower, Tomatoes, and Potatoes, etc.

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