158 THE PLANT 
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 what- 
ever 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 supple- 
mentary 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 con- 
cerned permit. 
196. Water-content series. An account of the experiments which 
have been carried on for four generations with Ranunculus sceleratus will 
serve to show the application of culture methods to the origin of new forms 
in response to varying water-content. This species was chosen because it 
grows readily in the planthouse, is plastic, and, since it is naturally am- 
phibious, permits of much modification in both directions. The smallest 
amount of water per day under which the seedlings would grow was found 
to be 25 cc. This was taken as one extreme for the series, and deep water 
in which the plant could be submerged as the other. An arbitrary series 
was tentatively made as follows: 25 cc., 50 cc., 100 cc., 150 cc., 200 cc, 
mud, shallow water, and deep water. Further study justified these 
divisions, since the first six gave efficient differences in water-content, and 
the resulting forms all showed differences of structure as well as of growth 
and form. Seedlings of the same age, and as nearly alike as possible, were 
transplanted to large pots of which there were four for each of the first 
six; they were placed in half-barrels for mud and floating forms, and in a 
barrel for submerged forms. After a few days, when they had become well 
established, the plants in the pots were watered in the amounts indicated, as 
often as was necessary to keep the most xerophytic form alive; the soil for 
