stout: pollinations in cichorium intybus 447 



entiated as a whole from those of another form. Yet the differ- 

 entiation, anatomical and physiological, between the two sex 

 organs of a single flower or a plant as a whole is such that there is 

 too great a difference for self-fertilization. The reciprocal differ- 

 entiation, however, between sex organs of different forms is 

 such that the two sex organs of the different plants are as a rule 

 more compatible than are those on the same plant. Jost's con- 

 ception of an influence of differences in the relative concentration 

 of individual stuffs in organs of the same plant or of different plants 

 is an attempt to analyze the physiological conditions of such 

 differentiation. 



CONCLUSION 



Sex differentiation giving male and female plants, male and 

 female organs, and male and female gametes, involves various 

 obvious grades of anatomical and functional difference. This 

 differentiation is what makes sex obvious and has led to an un- 

 warranted emphasis of the view that the compatibility of sexes 

 and the fusion of sex cells depends on such differences. The 

 manifold limitations of cross-fertilization in the higher plants 

 show that anatomical differences are to a large degree superficial, 

 that there can be no doubt that constitutional dissimilarities are 

 responsible for the great number of incompatibilities existing 

 between individuals, and that the most fundamental principle of 

 sexual fertility is that a marked degree of similarity in constitution 

 is necessary. 



The presence of self-sterility and intra-form sterility in such di- 

 morphic and trimorphic species as Liniim grandiflorum and Lythrum 

 Salicaria might seem to indicate that the grade of visible differ- 

 entiation between the sex organs and sex cells of a single individual 

 has been carried so far that there is sufficient constitutional dis- 

 similarity to give even self-incompatibility. But the presence of 

 both self- and cross-sterility within a non-dimorphic species or 

 strain as in Cichorium Intybus shows that incompatibility is funda- 

 mentally independent of visible differentiation. 



On the whole, the evidence favors the doctrine that for success- 

 ful fertilization the element of similarity in cell organization and 

 in the physical, chemical, idioplasmic, and structural properties of 

 all the cells and tissues involved is more important than any 

 dissimilarity which can be associated with sex differentiation. 



