Botanique.org

Two sap categories can be found in a vascular plant:

 a sap containing water and minerals taken from the soil by the roots. It is conducted by a tissue called xylem.

The xylem sap mainly goes from the roots to the photosynthesis sites (usually leaves): it is often called ascending sap in French.

The ascent of xylem sap depends on transpiration and water physical properties: the solar-powered bulk flow.

 a sap containing water and solutes, such as amino acids or sugars, mainly sucrose created by photosynthesis. It is conducted by a tissue called phloem.

The phloem sap mainly goes from the leaves to other parts of the plant, especially roots. It is often called descending sap in French but this definition doesn’t take into account translocation of phloem sap from reserve organs (tubers for example) to other parts of the plant.

The translocation of phloem sap is powered by the difference of osmotic pressure between sugar sources (where sugars mainly sucrose are produced) and sugar sinks (where sugar are consumed): the mass flow hypothesis.