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an elongated conducting element of the phloem of pteridophytes and gymnosperms. The lateral walls of the sieve cells have sieve areas—groups of fine, through channels surrounded by a ridge of cellulose. The sieve areas of ferns, gingkos, and sago palms are angular and many-rowed; those of conifers are rounded and single-rowed. Mature sieve cells usually have no nucleus and function for one year; at the end of the vegetative period the sieve area is covered by a callose. The sieve cells of gymnosperms are physiologically similar to the specialized parenchyma cells of the vertical cords of the phloem parenchyma or of the medullary rays. Such sieve cells are called albuminous cells.