In addition to sugars, Phloem sap may also contain inorganic ions,
amino acids and hormones in transit from one part of the plant to
another, but its main solute is usually the disaccharide sugar sucrose.
Xylem sap flows upward from the roots, Phloem sap moves
throughout the plant in various directions.
Sieve tubes always carry sugars from a sugar source to a sugar sink.
Sugar source is a plant organ that is a net producer of sugar by
photosynthesis or breakdown starch. Leaves are the primary sugar
sources in mature plants.
Sugar sink is an organ that is a net consumer or storer of sugar.
Growing roots, buds, stems, and fruits are sugar sinks.
The pressure flow mechanism explains why phloem sap always
flows from a sugar source to a sugar sink, regardless of their
locations in the plant.
o Contrast the forces that move Phloem sap with the forces
that move Xylem sap
Pressure is generated at the source end of a sieve tube
by the loading of sugar and the resulting osmotic flow of
water into the phloem. This pressure pushes phloem
sap from the end to the sink end of the tube. In contrast,
transpiration generates a pulling force that drives the
ascent of xylem sap.