This thesis presents the results of an investigation into the enigmatic phenomenon of xylem sap pressurisation (or exudation) from dormant trees of Acer pseudoplatanus. L. The characteristics of the system were established, through initial experimentation, and found to relate to previous studies of exudation from trees of Acer saccharum (Marsh), More detailed investigations were then undertaken. Studies were performed both in the field, using mature trees, and in the laboratory, using stem sections. Previous hypotheses appeared inadequate to account for the new observations and an alternative mechanism was conceived. This new hypothesis, based on sap priming through internal frosting, is proposed. Experimentation revealed that sap uptake, caused by sub-atmospheric stem pressures during cooling, was biphasic. The first phase of uptake was shown to be the result of thermal effects prior to freezing. The second phase was associated with freezing of sap within the tissue. Experiments already described suggest that uptake of sap (or water) before ice formation is a consequence of contraction of gas and liquid and dissolution of gases.