284 D. F. JONES. 



and would also not be surprising to find that the phenomenon is 

 greater in maize than in any other species for the reason that in 

 this plant the pollen tubes have a larger distance to traverse to 

 accomplish fertilization than in any other form known to the 

 writer. 



The stigmatic hairs of maize are scattered along a filamentous 

 style which continues to grow until fertilization takes place. 

 The structure withers and dries shortly after poll'nation takes 

 place. Pollen may adhere at any point along the filament. 

 The total length of style through which the pollen tube grows is 

 normally from 10 to 20 centimeters but in extreme cases may 

 be as great as 50 or more. It will be worth while to see whether 

 or not the selective effect is more pronounced when the styles 

 are long than when short. Such an experiment can be easily 

 carried out and would give some indications as to whether the 

 handicap is placed during pollen tube growth or after the sperm 

 nuclei are brought to the egg. 



The lessened ability of moderately different types to fertilize is 

 in line with the impossibility of effecting unions between widely 

 separated forms. In such cases the prevention of fertilization 

 is sometimes due to mechanical difficulties in the way of bringing 

 sperm cells to the eggs but even when this is accomplished there 

 still exists a firm barrier which prevents the passage through the 

 egg membrane. The differential effect demonstrated in maize 

 may be simply a reduced manifestation of this phenomenon. 



It is possible that the experiments on anaphyllaxis may throw 

 some light on this problem. It is known that foreign proteins 

 when injected in animal tissues may have a toxic effect and 

 excite an extreme irritability so that in repeated doses they may 

 cause markedly injurious results. By this means it is possible 

 to distinguish between proteins of very slight differences in 

 composition. Since the differences in protoplasmic substances 

 between the types in which a selective action is shown seem to 

 be small there may be some relation between the two phenomena. 

 However, the male gametophyte growing upon the stigmas is, in 



pollen tubes which start to grow down the style only one tube in every case in 

 nearly 100 observations was seen to reach the ovary cavity. This indicates that 

 the differential fertilizing power is determined by the rate of pollen tube growth 

 and not after the sperm nuclei have been brought to the egg. 



