Plant leaves normally respond to increasing injurious temperatures with a more or less continuous increase of tissue damage. A discontinuous response comprising a first lethal or sublethal temperature range which is followed by a non-injurious temperature range and a second, the lethal temperature range appeared to be an exception. The data with Populus deltoides x simonii show that a two-phasic response is most pronounced after heat shocks of 15 s and was no longer detected after exposure to heat longer than 9 min. In the course of the growing season the two-phasic stress response appears from the beginning of August until leaf fall in late October. Similar results with Convolvulus arvensis and Ligustrum vulgare show that this stress response is not particular of this hybrid of Populus. The wintergreen leaves of Ligustrum do not continue the two-phasic response until leaf fall in spring. Thus, the two-phasic response cannot be related to leaf senescence, however, it seems to indicate a transitional state of the heat tolerance in the period between late summer and late fall. In this state, the socalled primary thermostability of the organs decreases with decreasing natural heat hardening, however, sublethal heat shocks then have hardening or adaptive effects.