The origin of the exudate from cut maize roots

The exudate that wells out of the cut surface of detached roots is commonly thought to flow from the xylem and to be forced up the vessels by root pressure. Direct monitoring of the cut surface of maize (Zea mays L.) roots by optical microscopy and cryomicroscopy of fast-frozen material showed that the liquid originated not only from cut vessels but was also exuded from living tissues and intercellular spaces over the entire cut surface. Indeed, exuded liquid usually appeared last over the vessels. This pattern of exudation was seen also from partially submerged segments of mature roots in which the lower end had been sealed. Exudation also occurred from similarly prepared root sleeves from which the stele had been removed. Measurements of rates of exudation often showed a periodicity of about 1 h. These observations add to earlier published descriptions of the phenomenon, particularly extensive 19th century studies, which remains unexplained by current understanding of root water relations.