The exact arrangement of experimental results on plants - in this case tobacco - in relation to their dependence on all possible ''controllable conditions'' such as soil, manuring, management, breeding, cultivation and so on, is only possible after taking account of the effect of the weather; i.e. of the uncontrollable forces of the atmosphere. Consequently, allowance for the weather factor, with the help of the ''weather index number'', plays a great part in the evaluation of all research carried out in the open or in greenhouses without control of climate. These methods relieve the research worker from the constraint of observing a large number of plants for very many years, and makes it possible, apart from long period averages, to arrive at valid results for an individual case. Weather index numbers, which for the above reason are of such great importance, are obtained from an ''Isokarpendiagram'' (4, 5, 6, 7). All the surrounding influences which combine to operate on plants can then be presented, separated into their individual components in the form of a special ''Soil-weather-diagram''.