What are Angiosperms?

An angiosperm is a word obtained from the two Greek words angio, which refers to "covered," and sperms, which means "bean". Flowering plants are what the angiosperms are called. The flowers of a plant develop into fruits, which contain seeds.

The Diversity of Angiosperms

The Anthophyte is the phylum in which angiosperms are numbered. Modern angiosperms tend to be a monophyletic race, implying that they all descended from a single common ancestor. The arrangement of the cotyledons and pollen grains, among other things, divides flowering plants into two main classes. Grass and lilies are monocots, while eudicots and dicots form a polyphyletic group.

Classification of Angiosperms

Cotyledons, or seed leaves, are structures found within the embryos of angiosperm seeds. During seed germination, these structures are usually green in color. Angiosperms are classified into two groups based on the number of cotyledons present in the seeds: dicotyledons and monocotyledons.

Monocots

The presence of a single cotyledon in the seedling is the most common way to identify monocot plants. Veins that run parallel to the length of the leaves and flower sections that are arranged in a three- or six-fold symmetry are other anatomical characteristics shared by monocots. Monocots have very little true woody tissue. The trunk of palm trees is made up of vascular and parenchyma tissues formed by the primary and secondary thickening of meristems. The first angiosperm pollen was monosulcate, with a single furrow or pore running through the outer layer.

This characteristic can still be seen in modern monocots. The stem's vascular tissue isn't structured in any specific pattern. With no big tap root, the root system is often accidental and oddly located. Monocots include plants like true lilies and orchids, as well as grasses and palms. Monocots include rice and other cereals, barley, sugarcane, and tropical fruits like bananas and pineapples, to name a few.

Dicots

The existence of two cotyledons in the developing shoot distinguishes eudicots from true dicots. In leaves, veins form a network, while flower sections are divided into four, five, or more whorls. In dicots, the vascular tissue is arranged in a ring in the stem, while in monocots, the vascular tissue is dispersed across the stem. Eudicots develop either herbaceous (like grasses) or woody tissues. Pollen from most eudicots is trisulcate or triporate, meaning it has three furrows or pores. One main root formed from the embryonic radicle normally anchors the root system. Two-thirds of all flowering plants are eudicots.

The Life Cycle of an Angiosperm

Angiosperms are seed-producing plants that give rise to both male and female gametophytes. These gametophytes fertilize during sexual reproduction.

The adult or sporophyte phase of an angiosperm's life cycle is the most significant. Angiosperms, including gymnosperms, are heterosporous. As a result, microspores are produced, which produce pollen grains as male gametophytes, and megaspores are produced, which form an ovule containing female gametophytes. Male gametophytes divide by meiosis within the microsporangia of anthers to produce haploid microspores, which then undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells: a generative cell that will split into two sperms and a pollen tube cell.

The megasporangium is covered by two layers of integuments and the ovary wall within the ovule, which is sheltered within the ovary of the carpel. A megasporocyte undergoes meiosis within each megasporangium, producing four megaspores: three small and one large. Only the massive megaspore persists, and it releases the embryo sac, a female gametophyte. To form an eight-cell level, the megaspore divides three times. Four of these cells migrate to each of the embryo sac's poles; two reach the equator and fuse to form a 2n polar nucleus. Antipodals are formed by the three cells furthest away from the egg, while synergids are formed by the two cells nearest to the egg.

A single egg cell, three antipodal cells, two synergies, and two polar nuclei in a central cell make up the mature embryo sac. A pollen tube stretches from a pollen grain when it hits the stigma, grows down the style, and enters the ovule through the micropyle, a hole in the integuments of the ovule. In the embryo sac, the two sperm cells are placed.

Double Fertilization

A diploid zygote, or potential embryo, is formed when one sperm and one egg combine. The second sperm fuses with the 2n polar nuclei to give rise to a triploid cell that will mature into an endosperm (a food reserve tissue). The zygote matures into an embryo with a radicle, or small root, and one (monocot) or two (dicot) cotyledons, which are leaf-like organs. The two main classes of angiosperms, monocots, and eudicots, are distinguished by the number of embryonic leaves they have.

Outside the embryo, seed food reserves are preserved as complex carbohydrates, lipids, or proteins. The cotyledon functions as a conduit, carrying broken-down food supplies from the seed's storage site to the developing embryo. The coat of the seed is made up of toughened integuments, the endosperm with food reserves, and the well-protected embryo in the middle.

Some angiosperm species are hermaphroditic (stamens and pistils are found on the same flower), while others are monoecious (stamens and pistils are found on separate flowers but the same plant), and still, others are dioecious (stamens and pistils are found on separate flowers but the same plant) (staminate and pistillate flowers occur on separate plants). Cross-pollination is promoted by anatomical and environmental barriers, which are mediated by a physical agent (wind or water) or an entity, such as an insect or bird. Cross-pollination boosts a species' genetic diversity.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for;

• Bachelors in Biology
• Bachelors in Botany
• Masters in Botany