AP BIOLOGY

Detailed explanation-1: -Sieve elements are “semi-alive” at maturity. They lose their nucleus and other organelles but retain the endoplasmic reticulum, mitochondria, and plastids. Like tracheary elements, sieve elements are oriented end-to-end, forming a tubelike continuum.

Detailed explanation-2: -Sieve tube cells do contain vacuoles and other organelles, such as ribosomes, before they mature, but these generally migrate to the cell wall and dissolve at maturity; this ensures there is little to impede the movement of fluids.

Detailed explanation-3: -as sieve cells are a very important component in transportation of water they dispose of all their organelles to make extra space to accommodate more water and to ensure faster transport and all their requirements are fulfilled by the companion cells.

Detailed explanation-4: -Sieve tubes are tubular cells with perforated walls. They are living but do not have a nucleus and are the important component of phloem. Phloem parenchyma and companion cells are elements of phloem and both are living cells. Phloem fibres also called as bast fibres are the only dead cells present in the phloem.

Detailed explanation-5: -The unique features of sieve tube members are that they have no cell nucleus, ribosomes or vacuoles.