136 Plant Genetics 



cross-pollinated. DARWIN uncovered this situation and 

 concluded that there is a general tendency among 

 plants to secure cross-pollination. The significance of 

 this universal tendency seems to be to secure variation. 



Some ascribe another value to cross-pollination. 

 Hybrid vigor is a well established phenomenon, which 

 will be discussed later. Continued inbreeding fre- 

 quently results in degeneracy, such inbreeding races 

 being spoken of as " running out." It is a well known 

 fact that to cross an inbred race with some other race 

 results in a kind of rejuvenescence, a regaining of the 

 original vigor that was present before inbreeding began. 

 Even in the case of two normally vigorous races a cross 

 will often produce a hybrid more vigorous than either 

 parent. Of course, one might regard hybrid vigor 

 itself as a device to secure crossing. 



In any event, it seems to be safe to conclude that 

 cross-pollination is of advantage to plants, and of all 

 mechanisms to secure cross-pollination, self-sterility 

 may be regarded as the culmination. If "own" pollen 

 cannot produce viable seed in a plant cross-pollination 

 becomes a necessity. Inbreeding cannot continue in a 

 race that has become or is becoming self-sterile. 



This raises the question why is "own" pollen not 

 effective where foreign pollen is? There are well 

 known cases in which pollen is not effective on stigmas 

 of the same race but quite effective on those of another 

 race. Although the biological advantage of such be- 

 havior is evident, this does not explain the mechanism 

 involved. 



The theories offered in the explanation of self-sterility 

 are very numerous. Some years ago an attempt was 



