X58 THE PLANT 



experiments that are being carried on in the habitat, but they also 

 constitute an invakiable 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 diflferences 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 

 Irarrel 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 



