Sexual Reproduction in Flowering Plants Test - 61

Sporopollenin is a major component of the tough outer (exine) walls of plant spores and pollen grains. It is chemically very stable and is usually well preserved in soils and sediments. The exine layer is often intricately sculptured in species-specific patterns, allowing material recovered from (for example) lake sediments to provide useful information to palynologists about plant and fungal populations in the past. The chemical composition of sporopollenin is not exactly known, due to its unusual chemical stability and resistance to degradation by enzymes and strong chemical reagents. Analyses have revealed a mixture of biopolymers, containing mainly long chain fatty acids, phenylpropanoids, phenolics and traces of carotenoids. Tracer experiments have shown that phenylalanine is a major precursor, but other carbon sources also contribute. It is likely that sporopollenin derives from several precursors that are chemically cross-linked to form a rigid structure.

Parents: 2n (Diploid female)   ×​       4n (Tetraploid male)

Gametes:   n (Female gamete)   ×       2n (Male gamete)

The endosperm is formed by the process of triple fusion, wherein one male gamete fuses with the two polar nuclei. 

In this case, the two polar nuclei are haploid but the male gamete is diploid (2n). Hence, the endosperm would be tetraploid (n + n + 2n = 4n).

so, option A is correct.

Exine is the outer covering of pollen grain which is highly sculptured with sporopollenin; a chemically inert polymer of carotenoid ester which is highly resistant to degradation by any enzyme, as known so far. Leaf cuticle is protective covering on leaf epidermis and is composed of cutin, an insoluble heterogenous lipid polymer of long chain substituted aliphatic acids. It is degraded by cutinase enzymes (an esterase that breaks the ester linkages between cutin molecules). Cork is derived from phellogen cells and is the secondary extrastellar tissue. Cork cells are dead and are heavily deposited with lignin and suberin which are degraded by peroxidase enzymes. The fibres present in secondary xylem are known as wood fibres and are characterized by presence of strongly lignified vessels. Lignin is oxidized by lignin peroxidase. 

Gymnosperms are characterized by development of endosperm from haploid megaspores before fertilization. Here, the endosperm is gametophytic tissue and is haploid. A gymnopsermic leaf is a somatic organ which has diploid chromosome number in its cell. According to question, the number of chromosomes in leaf is 16 (2n) which means that its haploid chromosome number (the chromosome number present in gametes) is 8 (n). The megaspores, being the product of meiosis in megaspore mother cells (2n), are haploid and therefore chromosome number of its endosperm is n=8. Option A is correct. Since angiospermic endosperm is product of triple fusion, it is triploid; thus angiospermic plant with 2n=16 have endosperm 3n=24. Option D is incorrect. A plant with 2n=16 cannot have endosperm with 12 or 16 chromosomes which are neither haploid nor triploid for it. Options B and C are incorrect. Correct answer is A. 

Phytochrome is a photoreceptor, a pigment that plants (and some bacteria and fungi), use to detect light.The most important plant responses regulated by the phytochrome system include photoperiodic induction of flowering. As angiosperms are the flowering plants, the phytochromes are present in angiosperms.
Thus, the correct answer is option (D), 'Angiosperms'.

Endosperm is developed from triploid primary endosperm nucleus in central cell of embryo sac. The triploid nucleus is formed after vegetative fertilization i.e. fusion of male gamete with secondary nucleus of central cell, also known as triple fusion. Endosperm serves to provide essential nutrients to the growing embryos. Seeds having a fleshy storage endosperm that lie just beneath tegmen and persists upto maturity are termed as albuminous seeds. Castor, coffee, lily and cotton seeds are albuminous. Pericarp is residual nucellus that persists in seeds. Castor, cotton, lily and coffee seeds have pericarp present just below the seed coat and around the kernel. Pericarp surrounds endosperm in these seeds. Castor seeds exhibit enlargement of cells present at the tip of outer integument which forms a fleshy outgrowth at micropylar end of the seed is called as caruncle. It serves in seed germination and seed dispersal by animals. Cotton, lily and coffee seeds lack caruncle.

Radicle is the first part of a seedling to emerge from the seed during the process of germination. The radicle is the embryonic root of the plant and grows downward in the soil.
Plumule is the young shoot of  a plant embryo above the cotyledons, consisiting of the epicotyl and often of immature leaves.
Cotyledon is the significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling.
Ovule is the structure that gives rise to and contains female reproductive cells.
Thus, the correct answer is option (B), 'Plumule'.

Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes. Pollen grains are present in the male reproductive part o the flower, the anther.
Thus, the correct answer is option (C), 'Anther'.

Reproduction by special sporophytic or gametophytic generative tissues without fertilisation (fusion of male and female gamete) is known as apomixis. Citrus is characterised by the development of an apomictic embryo from integument/nucellus (the maternal sporophytic tissues in an unfertilized ovule) late in ovule development. Simultaneous fertilisation of meiotically derived adjacent embryo sac results in the formation of endosperm. The apomictic embryo (i.e., adventitious embryo as developed from maternal sporophytic tissues) obtains nutrition from the endosperm and mostly causes the death of the zygotic embryo.  

Rice, maize and onion are monocots and have endospermic seeds. Aleurone grains store the granular protein in a thin aleurone layer which forms the outer most covering of endosperm. The single cotyledon of monocots is called as scutellum and is present between developing embryo and endosperm. Scutellum is separated from endosperm from by an epithelial layer. The aleurone secretes enzymes during germination that degrade the protein, carbohydrates and lipids present in endosperm. The degradation products are absorbed by scutellum and are transported to developing embryo .