The phloem collects photoassimilates in green leaves, distributes them in the plant and supplies the plant organs. The phloem is specially designed for loading, long‐distance transport and unloading of assimilates. The conducting cells are called sieve elements. These cells are highly modified to create a simple pathway of contiguous living cells, whose long‐term viability is maintained through an intimate association with companion cells. These cells are ideally suited for rapid transport of substances at high rates over long distances. They are elongated and are arranged end to end in files referred to as sieve tubes.
The transport capacity of sieve tubes is dependent on a developmentally programmed degeneration of the cell contents leaving an open, membrane-bound tube. In mature conducting sieve elements, the protoplast is limited to a functional plasma membrane enclosing a sparse cytoplasm containing low densities of plastids, mitochondria and smooth endoplasmic reticulum distributed along the lateral walls. These relatively empty sieve tubes provide a longitudinal network which conducts phloem sap.
Sieve elements are closely associated with one or more companion cells, forming a sieve element–companion cell complex that plays an important role in transport. These distinct cell types result from division of a common procambial mother cell. In mature se–cc complexes, relatively open sieve elements contrast with adjacent companion cells containing dense, ribosome-rich cytoplasm with a prominent nucleus and abundant mitochondria and rough endoplasmic reticulum.
Companion cells are considered to perform the metabolic functions surrendered by, but required for, maintenance of viable sieve elements.
Transport of radioactively labeled substances through phloem has been demonstrated using microautoradiography, providing irrefutable evidence that sieve elements are conduits for transport of phloem sap. Experimentally, a pulse of CO2 is fixed photosynthetically and sugars are given time to reach the stem, which is then excised and processed for microautoradiography.
As C first moves through the stem, most of the isotope is confined to the transport pathway and very little has had time to move laterally into storage pools. High densities of sugars are found in sieve elements, demonstrating that these cells constitute a transport pathway.
There are other factors that are the cause of physical damage could pose a threat to transport through sieve tubes and has undoubtedly imposed strong selection pressure for the evolution of an efficient and rapid sealing mechanism for damaged sieve tubes. Since sieve tube contents are under a high turgor pressure, severing would cause phloem contents to surge from the cut site, incurring excessive assimilate loss in the creation of a seal. Protein is swept into sieve pores where it becomes entrapped, thus sealing off the damaged sieve tubes. Production of callose in response to wounding or high-temperature stress is another strategy to seal off damaged sieve tubes.