The reaction of plants to the drought complex (overheating and dehydration) is a complicated phenomenon. It is based on a specific response of a plant to a complicated phenomenon. It is based on a specific response of a plant to a particular situation; at the same time, however, there is a general response to an irritation. Severe overheating causes hydrolysis of proteins to amino acids and the appearance of ammonia in toxic amounts. At the same time, other compounds are also hydrolysed, e.g. polysac-charides. The dominance of hydrolytic reactions over synthetic reactions is observed not only during overheating but during dehydration as well. Drought-resistant plants (xerophytes, drought-resistant varieties, drought-hardened plants) maintain their synthetic reactions at a higher level than nonresistant plants. Dehydration increases protoplasmic viscosity and interferes with the processes of phosphorylation, thus inhibiting the accumulation and transformation of energy. It is possible that injury and death of plants from overheating and dehydration are caused by different factors depending on the suddenness and intensity of the drought as well as on the developmental stage of the plants during the drought. Heat-resistant plants respond to high temperatures with intensive protein synthesis and intensive restorative processes. These are the result of high levels of nucleic acids which stimulate protein synthesis and of high respiratory rates. The protoplasmic colloidal-chemical properties play an essential role in the resistance of plants to high temperatures. High hydrophilic viscosity, the degree of hydrata-tion of colloids, and the increased content of bound water are also important aspects of heat resistance. Adaptation (hardening) of plants during growth does not continuously increase but falls sharply during the development of the reproductive organs (crucial period). During the crucial period all physiological-biochemical properties change in the direction of a lower resistance to soil and atmospheric drought. Reproductive organs are particularly sensitive to drought, especially the androecium, which is much more sensitive than the gynoecium. Pollination and Fertilization are, therefore poor; the result is partially or entirely empty heads. The present theoretical concept of the nature of drought-resistance suggests methods of drought-hardening. Presowing drought-hardening is one that is very efficient. It adapts the plants to soil and atmospheric drought and suggests that high-yielding, drought-resistant varieties of plants can be developed. [There is a bibliography with 115 references.].