14 Methods of Working 



always immediate. Too great concentrations kill the plant at once, 

 too weak solutions fail to have any appreciable immediate action and so 

 appear indifferent. Between the two extremes there exists a range 

 of concentrations of which the effect varies with the plant's growth. 

 A solution may be of such a nature and strength that at first growth is 

 seriously checked, though later on some recovery may be made, while it 

 is also possible that a concentration which is apparently indifferent at 

 first may prove more or less toxic or stimulant at a later date, according 

 to circumstances. Consequently too much stress must not be laid upon 

 the results of the short time experiments with regard to the ultimate 

 effect of a poison upon a particular plant. 



An examination of the various experimental methods shows that 

 while no one of them is ideal, yet each of them has a definite contribu- 

 tion to make to the investigation of toxic and stimulant substances. 

 Each method aids in the elucidation of the problem from a different 

 standpoint, and the combination of the results obtained gives one a 

 clearer picture of the truth than could be obtained by one method 

 alone. Water cultures, with their exactitude of quantitative control 

 lead on by way of sand cultures to pot cultures, and these to field 

 experiments in which the control is largely lost, but in which the 

 practical application is brought to the front. 



