Sieve tube cells are placed end to end in phloem The end walls may be transverse or oblique They have many small pores called sieve pits Each sieve pit is lined by a layer of callose Excessive growth of callose closes the sieve pores Due to the presence of sieve pits the end walls are commonly called sieve plates
The xylem vessels and the phloem sieve tubes are connected end to end to transport the water and the food respectively to long distances in a plant. Their end plates are perforated to allow the easy passage of substances from one vessel and the sieve tubes.
When the starfish have been removed experimentally, the mussel populations have expanded rapidly and covered the rocky intertidal shores so exclusively that other species cannot establish themselves.
