MUTATION 255 



which may therefore transmit the recessive lethal gene to 

 half the progeny. How far the frequent occurrence of im- 

 perfect pollen grains in many species of plants is due to 

 such factors is still uncertain. 



Belling found that while the Florida velvet bean 

 produces normal pollen grains and ovules, and the Lyon 

 bean, another bean of the same genus, also produces nor- 

 mal gametes, the F^ hybrid contains 50 per cent, abortive 

 pollen grains, and possibly about 50 per cent, of the ovules 

 are abortive. In the second generation (Fo) half of the 

 pollen grains of half of the plants are abortive. The other 

 half of the plants have normal pollen grains. This is the 

 result expected if there are present in one of the species 

 the factors AAbh, and in the other species the factors 

 aaBB, the viable gametes in the F^ generation being 

 those containing Ab, Ba, and the two gametes that die 

 being AB, db. 



Other observers have made records of abortive pollen 

 in hybrids, but without knowing the condition of the 

 pollen in the parents the interpretation of the results is 

 doubtful, for, as Jeffrey has emphasized, abortive pollen 

 is a characteristic of many wild species. There is one 

 fact of capital importance recorded by several botanists, 

 viz., that the degeneration of the germ-ceUs only takes 

 place after the tetrad has been produced, and only in some 

 of the cells of each tetrad. In other words, the lethal 

 effect is not observed until the chromosomes have under- 

 gone reduction. It is obvious that if there is present a 

 recessive lethal for the germ-cells (or for any cells, in 

 fact), it causes no injury in the presence of its normal 

 allelomorph, but kills when the counter-effect of its part- 

 ner is removed. 



Tischler found in a hybrid currant that tetrad fonna- 

 tion was normal, and that the shrinking of the pollen 

 grains occurred afterwards. Geerts found that one-half 

 of the pollen grains of CEnothera Lamarckiana degen- 

 erate, and that half of the embryo sacs abort in the tetrad 



