The thresholds of the critical doses of the high positive temperatures were established, which affect the growth of the cells of the apical root meristems and the secondary meristems of the hypocotyl hook, causing irreversible lesions, which lead to the complete inhibition of the growth of the main roots and the secondary roots, respectively. Applying of different doses of heat shock (HS) on cucumber seedlings, it has been shown that, at the same duration of action, the initials of the lateral roots, due to the apical dominance, resist to the HS temperature with 7°C higher, compared to that, characteristic for the main root meristems.