Anatomy of Flowering Plants

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These

are

long,

tube-like

structures,

arranged

longitudinally and are associated with companion cells.

Their end walls are perforated to form the sieve plates. A

mature sieve element has a peripheral cytoplasm and

a large vacuole but lacks a nucleus. The functions of

sieve tubes are controlled by the nucleus of companion

cells. The first formed primary phloem (protophloem)

consists of narrow sieve tubes. The later formed phloem

(metaphloem) has bigger sieve tubes.

Function: Conduction of food materials from leaves.

SIEVE TUBE ELEMENTS

Specialized parenchymatous cells closely associated

with sieve tube elements (AIPMT 2012). Sieve tube

elements & companion cells are connected by pit fields

present between their common longitudinal walls.

Function: Maintain the pressure gradient in sieve tubes.

COMPANION CELLS

It is made up of elongated, tapering cylindrical cells

which have dense cytoplasm and nucleus. The cell wall

is composed of cellulose and has pits through which

plasmodesmatal connections exist between the cells.

Phloem parenchyma is absent in most of the monocots.

Function: It stores food material and other substances

like resins, latex and mucilage.

PHLOEM PARENCHYMA