236 PLANT PHYSIOLOGY 



tractive agent, and cane-sugar in the liverworts. This 

 inference is based on experience only to this extent : malic 

 acid is present in fern prothalli bearing sexual organs, and 

 capillary tubes containing a 0.05^ solution of malic acid 

 collected one hundred fern antherozoids in an hour from a 

 drop of water in which there were many. The evidence for 

 cane-sugar is similar. Presumably the free egg-cells of 

 Fucus, etc., are fertilized by chemotactically attracted 

 antherozoids, but proof of this is wholly lacking. Much 

 physiological work of this kind remains to be done on the 

 reproduction of the fresh and salt water alga'. 



Pfeffer's work on chemotaxis was quantitative as well as 

 qualitative. He showed that Weber's law, that "the smallest 

 change in the magnitude of a stimulus which will call forth 

 a response always bears the same proportion to the whole 

 stimulus," applies to chemotactic as to other stimuli. In 

 the case of "Bacterium termo" — now known to be a mixture 

 of several species of short rod-shaped bacteria — Pfeffer 

 found, in the first place, that there must be more of the 

 attractive substance in the capillary than outside, and then 

 that if the hanging-drop culture contain 0.01% meat extract, 

 the bacteria will not swim toward and into a capillary 

 containing less than 0.03% and that they are attracted more 

 and more strongly by solutions containing 0.05%, 0.08%, 

 and 0.1%. The same organisms in a culture containing 

 0.1% meat extract will be attracted only by capillaries con- 

 taining 0.3%, 0.5%, 0.8%, and 1.0% solutions of meat extract. 

 Similarly, the bacteria are drawn from a 1% solution only 

 by a 3% solution or stronger. We see then that the at- 

 tractive substance must be increased by stages of three in 

 order that there may be a change in the direction of loco- 

 motion. This does not imply that there is no stimulation 

 with less, for there may well be, only that there will not be 

 a response in the form of a change in direction of locomo- 

 tion. In order to induce an organism in an environment of 

 a certain degree of favorability to leave this for another, 

 there must be a decidedly larger proportion of the stimulat- 

 ing substance. 



Chemotropism and chemotaxis may be due to funda- 



