Angiosperms

Angiosperms are the most advanced group of plants and have been of immense economic importance to humans, more than any other group. They are able to grow in a variety of habitats. They can grow as trees, shrubs, bushes and flowering plants.

In angiosperms, the pollen grains and ovules are developed in specialized structures called flowers. Seeds are enclosed by fruits. The angiosperms plant is the large group and these plants have occupied a variety of habitats. They are varying in size from tiny(Wolffia) to tall trees (Eucalyptus over 100 meters). They have a variety of commercial value such as food, fodder, fuel, and medicine.

Angiosperm Characteristics

1. All plants are having flowers at some stage in their life. The flowers are the reproductive organs for the angiosperms and it gives them a means of exchanging genetic information.
2. Genetic information of the plant spread from flowers to flowers and pollen grains are very small. It is smaller than the gametophytes or reproductive cells used by non-flowering plants.
3. Because of smaller in size, the fertilization process occur quicker in the flowers of angiosperms and makes them efficient at reproducing. Similarly, angiosperms have much smaller female reproductive parts than non-flowering plants, allowing them to produce seeds quickly. Stamens are the male reproductive structures found in flowers and this produces the pollen grains that carry male genetic information. It has carpel that encloses developing seeds that may turn into a fruit. The advantage of the angiosperm is the production of endosperm. After fertilization, Endosperm is formed. This endosperm serves as a highly nutritional food source for the developing seed and seedling.

Classification of Angiosperms

Angiosperms are divided into two classes: the dicotyledons and the monocotyledons. The dicotyledons are having two cotyledons and monocotyledon has only one.

Bentham and Hooker’s system of classification including the seed plants, otherwise it is known as Phanerogams. In This Bentham Hooker system described and classified around 97,205 plant species into 202 families and 7569 genera based on their natural similarities. This system of classification was based on the forming relationship and plant characters which could be correlated with each other. This system of classification was pre-Darwinian, in concept but post-Darwinian in publication and did not mention about the origin of angiosperms.  

It is one of the most popular systems of classification followed even today in many countries. The classification was based on an actual study of the plant specimens and never based on the earlier authors. That is why this system of the classification made it a very authentic, accurate, and popular classification. It gives key characters for the genera and families which was an easy aid for identification.

Phanerogams or the seed plants were divided into three classes They are – Class I: Dicotyledons, Class II: Gymnosperms and Class III: Monocotyledons. 

Angiosperms Reproduction

The highly reduced female gametophytes termed embryo-sacs. The embryo-sac formation is generally preceded by meiosis. Hence each of the cells of an embryo-sac is haploid. Embryo-sac has a three-celled egg apparatus-one egg cell and two synergids. Three antipodal cells and two polar nuclei. The polar nuclei fuse to form a diploid secondary nucleus. Pollen grain transfer to the stigma of a pistil by external factors is termed as pollination. If the male gametes fuse with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN). This event is termed as double fertilization. The embryo is developing from the zygote and endosperm is developing from PEN and it is providing nourishment to the developing embryo. After fertilization, the synergids and antipodals degenerate. The ovules are developing into seeds and the ovaries develop into fruit.