Talk about sieve cells

Superposed elongated living cells with cellulosic cell wall. They are found in gymnosperms and pteridophytes, having their longitudinal and transverse walls perforated with pores interrupting the cell walls and the middle lamella allowing the flow between adjacent cells. Unlike the vessels and tracheid, their nuclei disintegrate at maturity but their cytoplasm remains and is very active in conducting food material in solution throughout the plant.