530 LECTURE XX. 



and observing the movement of the organisms towards the tubes or 

 even their entrance into them. 



The strength and nature of the stimulating effect varies 

 with the concentration of the solution. Pfeffer ascertained 

 that the weakest solution of malic acid which perceptibly 

 affected the antherozoids was one of O'OOi per cent, the 

 antherozoids being in water. When the antherozoids are 

 swimming in dilute solution of malic acid or of a malate, the 

 solution in the capillary tube must be relatively much higher 

 in order that the antherozoids may be attracted to it. Thus 

 when the antheroids were swimming in a solution containing 

 0*0005 per cent, of malic acid, the weakest attractive solution 

 in the tube had to contain 0*015 per cent, of malic acid ; and 

 when the liquid in which the antherozoids were swimming 

 contained 0*01 per cent, of malic acid, the weakest solution 

 which attracted them to the tube was one containing 0*3 per 

 cent, malic acid. Clearly the presence of malic acid in the 

 liquid diminished the irritability of the antherozoids. As 

 the strength of the solution is increased, the attraction which 

 it exercises on the antherozoids also increases, but the more 

 highly concentrated solution comes to exercise a repellent 

 effect in virtue, simply, of its concentration. But it appears 

 that a strong solution of a malate exercises a specific repellent 

 effect quite independently of that due to its concentration ; 

 that an increase of the stimulus beyond a certain point induces 

 a reversed result. Pfeffer did not determine exactly the 

 concentration at which this reversal takes place, but he 

 found that a solution of a malate, containing 10 per cent., 

 or even 5 per cent, of malic acid had a marked repellent 

 effect, whereas a solution containing 15-5 per cent, of nitre, 

 and 0*5 of malic acid was attractive. 



Further, a strongly acid or an alkaline reaction exercises 

 a repellent influence on antherozoids. A solution containing 

 0*01 per cent, of free malic acid attracted apparently to the 

 same extent as one containing the same proportion of acid in 

 combination with sodium ; a solution, however, which con- 

 tains 0*1 per cent, of free acid repelled, whereas one con- 

 taining that proportion of acid combined with sodium strongly 



