Q.In Gymnosperms, the activity of sieve cells are controlled by:

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.

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