46 FERTILIZATION 



of all spermatozoa except those of Equisetum arvense to distinguish 

 between cis and trans unsaturated dicarboxylic acids. The sperm- 

 atozoa of Isoetes japonica A. Braun react towards the trans, but are 

 indifferent to the cis, configuration ; while those of Sahinia natans 

 AUioni, Osmiindajavanica Blume, Pityrogramma sulphurea (Schwartz) 

 and Pteridium aquilimim, which, with the exception of Sahinia 

 natans, are ferns, are attracted by the cis but not the trans forms. 

 Unfortunately the specificity of the reaction is not so clear in fern 

 spermatozoa as it is in those of Equisetum arvense. If, for example, 

 we examine the structure of those acids which attract the sperm- 

 atozoa of Pteridium aquilinum (Table 8), there is evidently no single 



TABLE 8 

 Organic acids zvhich attract bracken spermatozoa 



configuration which stimulates their perception mechanisms, as 

 there seems to be in Equisetum arvense. In spite of these difficulties, 

 some predictions can be made from Table 8. Other substituted 

 and unsaturated 4-carbon m-dicarboxylic acids will probably 

 attract bracken spermatozoa. As an OH group on C^ or C^ 

 appears to be essential if saturated acids are used — succinic and 

 malonic acids do not attract — chloromalic acid would be an in- 

 teresting compound to try. It is, however, difficult to predict the 

 effect of chloromalic acid in view of the reputed behaviour of 

 Sahinia natans to succinic and monobromosuccinic acids (Table 9). 

 In addition to their responses to organic acids, fern spermatozoa 

 are reported to be attracted by gradients of many other substances, 

 of which calcium, strontium, lithium, morphine and yohimbine 

 are a few examples (Shibata, 1911). These claims must be accepted 

 with reserve until the experiments have been repeated under care- 



