in Anatomy of Flowering Plants by

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The individual member of sieve tube is known as the sieve tube elements. The sieve tube elements are joined at the ends in longitudinal series. When the sieve element matures the disintegration of nucleus occurs colorless plastids are present. The cell wall is very thin and is made up of cellulose. In certain dicot plants proteinaceous slimy material get accumulated as masses called slime plugs. 

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