Sterility of Hybrids 259 



Others become young plantlets, but are too weak to 

 develop any further, and perish during the first weeks 

 after germination, as I have frequently seen, for example 

 after crossings of Oenothera Lamarckiana and 0. miiri- 

 eata. Or only the most vigorous individuals continue to 

 grow, while the weaker ones perish, and this, in diocious 

 plants, sometimes results in the male seedlings perishing 

 while some of the more vigorous female ones develop 

 flowers, as Wichura observed in several willows. Finally 

 there might originate hybrids that grow vigorously, but 

 do not flower at all or only incompletely, or begin too late 

 to do so. There is a whole series of cases between the 

 unsuccessful crossings and the development of hybrids 

 into adult plants. And on the whole this series runs 

 parallel with the increasing systematic relationship. 



If the hybrid has succeeded in reaching the period of 

 flowering, that is, the period of the formation of the sex- 

 ual cells, a new difficulty arises at the moment of the 

 exchange of the units. Whereas, up to that time, the co- 

 operation of the two pronuclei was more or less disturbed, 

 now the gaps become very important. Hence the quite 

 common phenomenon that the production of tgg- and 

 sperm-cells fails more or less completely, that the hybrids 

 either produce no ovules that are capable of being fer- 

 tilized, or no good pollen, or neither. They are more or 

 less or even completely sterile. They either form no seed 

 at all, or only an insufficient quantity. Only where the 

 differences between the parents are quite small, does one 

 succeed in harvesting any seed, and even here frequently 

 only a little. 



How the unpaired characters behave during the ex- 

 change, when they are not numerous enough to make a 

 failure of the entire process, is at present unknown. Ex- 



