Q.The tallest living gymnosperm among the following is ..........

Gymnosperm endosperm is the haploid female gametophyte tissue, and it is already present in the ovule before fertilization even takes place.

In angiosperms, the endosperm is a post-fertilization product: it forms only after a male gamete fuses with the two polar nuclei of the central cell (triple fusion), making it triploid (3n) and produced after fertilization.

Gymnosperms are different. In a gymnosperm ovule, the megaspore mother cell undergoes meiosis and the surviving haploid megaspore develops, well before fertilization, into a multicellular female gametophyte that fills most of the ovule. This female gametophyte tissue itself acts as the nutritive tissue for the developing embryo and is conventionally called the (gymnosperm) endosperm. Since this tissue is already differentiated and present in the ovule prior to pollination and fertilization, the endosperm in gymnosperms is described as being formed BEFORE fertilization — and, being derived from the haploid megaspore, it is haploid (n), not triploid.

Gymnosperm literally means "naked seed" (gymno = naked, sperma = seed) — their ovules and seeds are not enclosed in an ovary wall, unlike angiosperms whose seeds develop inside a fruit.

Among the four options:

• Angiosperms are flowering plants whose seeds develop enclosed within a fruit (the mature, ripened ovary) — the opposite of "naked seed."

• Pteridophytes (ferns and their relatives) do not produce seeds at all; they reproduce via spores.

• Bryophytes also do not produce seeds; they too reproduce via spores.

• Gymnosperms (e.g. Pinus, Cycas, Cedrus, Gnetum) bear their ovules directly on the surface of megasporophylls (often arranged in cones), with no ovary wall around them, so the mature seeds remain exposed/"naked."

In gymnosperms, phloem lacks companion cells (found only in angiosperms); the equivalent supporting role is played by albuminous cells, and it is the nucleus within these albuminous cells that controls the metabolic activity of the associated (enucleate, mature) sieve cells.

Gymnosperm phloem consists of sieve cells (not sieve tube members) as the conducting elements. Sieve cells are elongated, tapering cells with sieve areas on their lateral walls, and like angiosperm sieve tube elements, they lose their nucleus at maturity. Since a mature sieve cell has no nucleus of its own to direct its ongoing metabolic activity, it depends on an associated parenchymatous cell — the albuminous cell (also called a Strasburger cell) — which lies adjacent to it and remains nucleate. The nucleus of the albuminous cell is understood to regulate/control the physiological activity of the associated sieve cell, in a role analogous to (though not identical to) the companion cell-sieve tube relationship in angiosperms.

The other options are incorrect: gymnosperms do not have sieve tube members or companion cells (those are angiosperm phloem features); phloem parenchyma cells are present but are not specifically the regulators of sieve-cell activity in the way albuminous cells are.