long. slender phloem cells in gymnosperms.

sieve cells (Wikipedia)

Sieve elements are specialized cells that are important for the function of phloem, which is highly organized tissue that transports organic compounds made during photosynthesis. Sieve elements are the major conducting cells in phloem. Conducting cells aid in transport of molecules especially for long-distance signaling. In plant anatomy, there are two main types of sieve elements which are sieve cells and sieve tube members. Sieve cells are specialized cells in the phloem tissue of flowering plants. Companion cells and Sieve cells originate from meristems, which are tissues that actively divide throughout a plant's lifetime. They are similar to the development of xylem, a water conducting cells in plants whose main function is also transportation in the plant vascular system. Sieve elements' major function includes transporting sugars over long distance through plants by acting as a channel. Sieve elements elongate cells containing sieve areas on their walls. Pores on sieve areas allow for cytoplasmic connections to neighboring cells, which allows for the movement of photosynthetic material and other organic molecules necessary for tissue function. Structurally, they are elongated and parallel to the organ or tissue that they are located in. Sieve elements typically lack a nucleus and contain none to a very small number of ribosomes. The two types of sieve elements, sieve tube members and sieve cells, have different structure. Sieve tube members are shorter and wider with greater area for nutrient transport while sieve cells tend to be longer and narrower with smaller area for nutrient transport. Although the function of both of these kinds of sieve elements is the same, sieve cells are found in gymnosperms, non-flowering vascular plants, while sieve tube members are found in angiosperms, flowering vascular plants.

Companion cell (left, light pink), nucleus (dark pink), sieve tube (right, solid green), sieve tube plates (dashed green), dissolved nutrients (yellow)
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