Sieve-tube elements are a kind of plant cell found in phloem tissue. They are soft, small cells which can be stacked to form long sieve tubes. They only possess middle lamellae and primary cell walls, making them soft and easy to damage; as a result, they last an average of a year before being destroyed.
Sieve-tube elements resemble vessel element water-conducting cells. Like vessel elements, they have large plates on the end walls known as sieve-plates. They also have enlarged pores, which are actually large plasmodesmata, on their lateral cell walls. Their protoplast has been heavily modified: organelles such as the central vacuole, cytoskeleton, nucleus, and ribosomes removed to increase the efficiency of material transport. Unlike the water-conducting cells, not all of the organelles have been removed, such as the endoplasmic reticulum and the mitochondria; these organelles line the edges of the sieve-tube element cell wall to avoid impeding transport.
Sieve-tube elements transport sugars, RNA, and several hormones across the plant. They are aided by companion cells to load sugars and perform metabolism to function. Due to their fragility, they are partially composed of callose, which plugs the sieve-plates alongside P-proteins when the cell is damaged.