120 Plant Genetics 



that, in spite of the complexity of material and in spite 

 of lack of control, the facts of inheritance are being 

 uncovered when the consistent results are considered. 

 It also assures us that an unexpected result may occur 

 now and then and that it is not necessary to explain it 

 by trying to make it consistent with other results. A 

 single illustration will make this clear. Suppose that a 

 species is persistently parthenogenetic, as some are 

 known to be, and still there is pollination. If one were 

 conducting breeding experiments with such a plant he 

 would discover that inheritance was of a sort called 

 maternal inheritance, that is, inheritance determined 

 by the ovule parent and not affected by the pollen 

 parent. In explaining this situation it would not be 

 necessary to have recourse to CORRENS' idea of diseased 

 cytoplasm; one would suspect parthenogenesis and then 

 look for it. 



Self-sterility, which is an important and growing 

 subject, to be developed later, is related in one way or 

 another to apogamy. Mention has been made of the 

 stimulating effect of the mere presence of a pollen tube 

 in inducing an unfertilized egg to germinate without 

 sperms being discharged or even formed. An increas- 

 ing number of plants are found to behave in this way, 

 especially in certain families. Notably in some culti- 

 vated races of apples and grapes self-sterility has been 

 developed, that is, the pollen grains are impotent upon 

 their own stigmas. In such cases the fruit develops 

 just as if fertilization had taken place; in many cases 

 seeds develop within the fruit and even embryos in the 

 seeds. Strangely enough if " foreign" pollen is used 

 it may not be impotent but results in fertilization. All 



