88 THE PSYCHIC LIFE 



emits a mucilaginous substance, which must contain 

 malic acid or a malate, for these compounds are the 

 particular excitatory substance of the fern-spermato- 

 zoids. Thanks to a drop of dew that falls upon the 

 prothallium, the spermatozoids swim around and ap- 

 proach the female ovule, which attracts them by acting 

 upon them with the malic acid. 



A confirmation of this hypothesis is primarily the 

 fact, that all substances tested, with the exception of 

 malic acid and malates, remained completely inactive; 

 another proof is, that malic acid is found in prothal- 

 lium -decoctions of the Pteris serrulata and of the Ad- 

 iantum capillus veneris; another proof still, is the cir- 

 cumstance that malic acid is largely diffused through- 

 out the vegetable kingdom. 



The author has made, in this connection, a series 

 of very curious experiments upon the degree of con- 

 centration necessary to attract the spermatozoids. 

 The lower limit at which attraction begins, is found in 

 a solution containing malic acid in the proportion of 

 one to 1000 parts. This the author has designated 

 by a favorite word of the Germans: Reiz-Schwelle, or, 

 in other words, the threshold of excitation. 



When the solution in the watch-crystal contains 

 one part malic acid to every thousand parts, in order to 

 make the spermatozoids pass from the watch-crystal 

 into the tube, the solution held in the tube must be 

 thirty times as strong, or 30 x i-iooo = 3-100. If the 

 liquid in the watch-crystal contains one part malic 

 acid to every hundred parts, similarly the solution of 

 the tube must be thirty times as strong, that is to say 

 three tenths. 



The author justly compares the result of these ex- 

 periments with the law laid down by Weber, which M. 



