This method of cultivation represents the simplest type of experiment. Its great advantage is that the investigator has absolute control over all the experimental conditions. Nutritive salts and toxic substances can be supplied in exact quantities and do not suffer loss or change by interaction with other substances which are beyond control. Any precipitates which may form in the food solution are contained within the culture vessel and are available for use if needed. The results are thus most useful as aids in interpreting the meaning of those from the field experiments, the results of the one method frequently dovetailing in with those of the other in a remarkable way. The disadvantage of the water culture method is that it is more or less unnatural, as the roots of the plants are grown in a medium quite unlike that which they meet in nature, a liquid medium replacing the solid one, so that the roots have free access to every part of the substratum without meeting any opposition to their spread until the walls of the culture vessel are reached. The conditions of aeration are also different, for while the plant roots meet with gaseous air in the interstices of the soil, in water cultures they are dependent upon the air dissolved in the solution, so that respiration takes place under unusual conditions. It is possible that the poverty of the air supply can be overcome by regular aeration of the solution, resulting in decided improvement in growth, as L. M. Underwood (1913) has shown in recent work on barley in which continued aeration was carried out. 2. _Sand cultures._