CHEMOTAXIS 699 



an alkaline culture. Thus Greely has found that while 

 alkali-reared Parametria invariably give the initial response 

 by swimming towards the kathode, the reverse is generally 

 the case with those which have been subjected to acid — that 

 is to say, the latter as a rule begin by swimming towards the 

 anode. 



Chemotactic movements. — Similar effects are, again, 

 observed under chemical stimulation. The opposite reactions 

 of acids and alkalis which have already been described are the 

 occasion of opposite chemotactic responses. Thus Jennings 

 found Parametria moving towards, or showing positive reaction 

 to, acids, whereas to alkalis they exhibit negative response, 

 or movement away. Chemotactic reaction, again, is modified 

 by the strength of the solution, that is to say by the intensity 

 of stimulation. The opposite effects of strong and feeble 

 doses (p. 488) are here illustrated by the fact that in many 

 organisms positive response is observed under the action of a 

 weak solution, and negative under a strong. Connected with 

 the subject of chemotaxis is the interesting phenomenon of 

 the response of the antherozoids of ferns and of Selaginella 

 to malic acid, as discovered by Pfeffer. In the response of 

 growth we found that dilute solutions of sugar gave rise to 

 one kind of response, that is to say to acceleration of growth, 

 and that with very strong solutions, say about 10 per cent., 

 the opposite occurred — that is to say, retardation (p. 482). 

 Now, it is found by Pfeffer that whereas the dilute solution of 

 a malate exerted an attractive influence on the antherozoids, 

 a 10 per cent, solution had a repellent effect. 



Summary 



A rhythmic vegetable organ, in a state of standstill, has 

 its autonomous movement renewed by a supply of energy 

 from incident light. 



Too strong an intensity of light may, by causing fatigue, 

 arrest such movement. Or the greater fatigue of the more 

 excitable half of the organ may cause a reversal of the relative 

 rapidities of the up and down beats. 



