There is now a substantial body of evidence that shoot growth and physiology of plants rooted in drying soil may be regulated by chemical signals moving from the root to the shoot in the xylem stream. Although some evidence suggests that soil drying can reduce the supply of promoters of leaf growth and stomatal opening, there is now compelling evidence for an enhanced flux of inhibitors in the xylem stream of droughted plants. Some of this inhibitory activity is still to be identified but at least in some plants the bulk of activity can be explained by the enhanced concentration of the plant hormone abscisic acid (ABA). A series of field experiments has now shown that ABA, moving as a signal from the roots to the leaves in the transpiration stream, can provide a measure of the access that the plant has to water in the soil in the rooting zone. We show here how this signal may be a variation in the concentration of ABA arriving at the sites of action in the leaf. The response to such a signal apparently varies as a function of the physiological state of the leaf. The basis of such variation in the sensitivity of response is also discussed. One other interpretation of the field data is that leaves respond to the amount of ABA arriving in the leaf, rather than the concentration. We show some evidence for this contention.