250 THE THEORY OF THE GENE 



There are several ways of testing the validity of this 

 hypothesis. The tests have been made and the hypothesis 

 confirmed. This convincing analysis, the result of carefully 

 planned genetic experiments, is a contribution of the first 

 rank to a problem that has baffled students of fertiliza- 

 tion for seventy-five years and more. The solution is not 

 only a keen genetic analysis of the case but gives an in- 

 sight into the physiological reactions between the haploid 

 pollen tube and the diploid tissue of the female. It has been 

 shown by direct observation that the rate of growth of the 

 pollen tube in the tissue of the female is consistent with 

 the view that a differential rate of growth is actually pres- 

 ent. The nature of the relation is not known at present 

 but may reasonably be assumed to be chemical. It is 

 possible that the same or a similar chemical reaction with 

 its genetic basis may account for the self-sterility ob- 

 served in the union of the genetically unlike mycelia of 

 the lower forms. If this can be established the genetic 

 problem has to do primarily with self -sterility factors 

 that are probably Mendelian genes. It may seem of doubt- 

 ful value to identify these factors with sex factors which 

 conventionally at least apply to somatic differences in 

 dioecious types or to those with separate sexes. It is true 

 that amongst these differences are those concerned with 

 producing eggs and sperm whose main function is to unite 

 with each other, but, as generally understood, these func- 

 tions are less conspicuous than those appertaining to the 

 bodily constitution of male and female individuals. 



