=195. Scope and procedure.= Control experiments are necessarily carried on in the planthouse, since factors can be controlled in the field only with great difficulty. Their greatest value is in connection with experiments that are being carried on in the habitat, but they also constitute an invaluable means of independent research, since it is not at all difficult to approximate the conditions of a habitat, especially with reference to water-content and light. The essential feature of the method is that the less important factors are equalized as far as possible, while the direct factors, water-content and light, are under the complete control of the investigator. By the equalization of humidity and temperature is meant experimentation in which all the plants of each experiment are subjected to the same amounts of these factors. It is a matter of no importance whatever whether the humidity and temperature are constant or variable. In the case of soil, which is not a variable, it naturally happens that the plants are placed once for all in the same soil mixture. Batteries consisting of thermograph and psychrograph have been kept in the different control houses, but although used at first to give some idea of the hourly and daily fluctuations of temperature and humidity, they have slight bearing upon the evolution of new forms under control. For use in connection with supplementary experiments in adjustment and adaptation, the batteries have proved to be indispensable. Control experiments are regularly made in series which are planned with reference to as many modifications as the efficient difference of the factor and the plasticity of the species concerned permit.