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the sieve plate is a filter for the water that enters the "Water Vascular System" in Echinoderms.
they allow the sucrose to enter in phloem.
by flowing along with water through perforations in the sieve plate
so that water and solutes can pass.
Sieve plates are pores in the plant cell walls that facilitate transport of materials between two sieve tubes.
from: http://en.wikipedia.org/wiki/Sieve_tube_element