Biology Test 325

The embryo sac of a typical flowering plant is 7-celled and 8-nucleate. It consists of 3 haploid cells- two synergids and one egg cell, grouped together towards the micropylar end which constitute egg apparatus. At the chalazal end also 3 cells are grouped and constitute antipodal cells. The remaining two nuclei are called polar nuclei present inside the large central cell.

Self-incompatibility is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

Integuments are the tough, outer layers that protect the ovule. It will develop into a seed coat after the egg cells are fertilised. In angiosperms, two integuments are found in the ovule, while in gymnosperms, only one integument is found.

Angiosperms are known for having a double fertilization process. This unique feature involves two sperm cells; one fertilizes the egg to form the zygote, while the other fuses with two polar nuclei to form the triploid endosperm, which nourishes the developing embryo. Double fertilization is a key characteristic that distinguishes angiosperms from other plant groups.

At certain places, exine remains unthickened or missing and these places are known as germ pores. These pores are found in the outer wall of a spore or pollen grain through which the germ tube or pollen tube makes its exit.

Water is the most abundant chemical in living organisms. It comprises about 70-90% of the total cellular mass. Proteins, carbohydrates and nucleic acids comprise for about 10-15%, 3% and 5-7% of the total cellular mass respectively.

Viruses are characterized by a protein coat, known as a capsid, which encloses their genetic material. This coat protects the viral genetic material and helps in attaching the virus to host cells. Viruses do not have a cellular structure, contain either DNA or RNA (but not both), and cannot replicate without infecting a host cell, making Option D the correct answer.

The endosperm is triploid in nature in angiosperms, while microspore mother cells are diploid structures.

The number of chromosomes in the endosperm is 24 = 3n. Therefore, n = 8.

Hence, the number of chromosomes in the microspore mother cell will be 2n = 2 x 8 =16.

Primary parietal cells divide to form two layers of secondary parietal cells. The outer layer of secondary parietal cells differentiates to form endothecium and the inner layer of secondary parietal cells divide to form two layers, one of which forms the middle layers and the other forms the tapetum.

The extraembryonic membranes are amnion, chorion, allantois and yolk sac. Amnion is derived from mesoderm on the outer side and ectoderm on the inner side. Chorion is formed from trophoectoderm and mesoderm, whereas allantois and yolk sac membrane have mesoderm on outerside and endoderm in inner side.