AP BIOLOGY

Detailed explanation-1: -Sugars are actively transported from source cells into the sieve-tube companion cells, which are associated with the sieve-tube elements in the vascular bundles.

Detailed explanation-2: -Companion cells are found along with sieve tube elements in the phloem. The main function of companion cells is to maintain pressure gradient in the sieve tube elements.

Detailed explanation-3: -Sieve tubes are responsible for the transport of food throughout the plant body while companion cells are responsible for regulating the activity of the sieve element.

Detailed explanation-4: -The sieve tube is the conducting element of the phloem. It is formed from a vertical series of elongated cells, interconnected by perforations in their walls in areas known as sieve plates. The perforations may be restricted to smaller areas, sieve fields, several of which are contained in each sieve plate.