120 THE CELL 



tions a specific stimulative effect is exerted upon them. This, 

 however, can only be perceived indirectly, and can only be ex- 

 plained by the supposition that the attitude, so to speak, of the 

 antherozoids towards malic acid has experienced some modifica- 

 tion. Pfeffer is able in this case to demonstrate a relation simi- 

 lar to that expressed by the Weber-Fechner law for the mental 

 percej)tions of man: " Whilst the stimulus increases in geometrical 

 progression, the perception or reaction increases in arithmetical 

 progression." 



This ratio, which in many respects is very important, can be 

 observed in the behaviour of antherozoids towards malic acid. 



To the fluid, containing the fern antherozoids, some malic acid 

 is added in such a quantity that when the two are well mixed to- 

 gether a solution of '0005 per cent, is produced. If now a capil- 

 lary tube containing a solution of "001 per cent, is inserted, attrac- 

 tive influence, as was the case when the antherozoids were in pure 

 water, can be perceived. The tube must now contain a "015 per 

 cent, solution in order to produce an effect, and if the water, in 

 which the antherozoids are, contains '05 per cent, of malic acid, 

 the solution in the tube must be 1"5 per cent, in strength. 

 Or more generally expressed, the solution in the tube must be thirty 

 times as strong as that from which the antherozoids are to be attracted. 

 Tlie sensitiveness to stimuli, or the stimulation tone of the antherozoids, 

 is affected, if they are present in a liquid which contains a certain 

 'proportional amount of the substance which is to act as the stimulus. 

 Thus it is possible in an artificial way to render them non- 

 sensitive towards weak solutions of malic acid, which under 

 ordinary circumstances constitute excellent stimuli, w r hilst on the 

 other hand they may be made susceptible to attraction from 

 strong concentrations of malic acid, which would repel antherozoids 

 accustomed to living in pure water. 



Individual cell bodies behave very variously towards chemical 

 substances, just as they do towards light. Malic acid, which 

 exerts such a powerful attraction upon fern antherozoids, does 

 not affect those of Feather-moss at all. For these, however, a 1 per 

 cent, solution of cane sugar acts as a stimulus, whilst on the other 

 hand neither of these substances has any effect on Liverwort or 

 Characece. 



A 1 per cent, solution of meat extract or of Asparagin exerts 

 a strong attraction upon Bacterium termo, Spirillum undula, and 

 many other unicellular organisms. Even after a short period, 



