Rapid and systemic defence responses occur in various higher plants, i
ncluding the tomato (Lycopersicon esculentum L.). The long-distance si
gnalling mechanisms which permit these responses are not clear, but th
ree models are currently considered in the literature: phloem transpor
t, hydraulic dispersal in the xylem, and electrical transmission. Expe
riments presented here are designed to discriminate between these thre
e models on the basis of some key predictions. It is demonstrated that
wound signalling call be prevented by enclosure of the shoots in poly
thene bags, to generate high humidity. This effect can be reversed by
addition of mannitol solution to the roots, showing that it depends on
saturation of the plant's water status rather than on changes in the
gaseous environment of the shoot. In addition, wound signals are shown
to pass freely across heat-killed tissue. These results are predicted
by the hydraulic-dispersal model of signalling, but they are not comp
atible with the other two models. We therefore conclude in favour of h
ydraulic dispersal.