That hybrids are commonly sterile is a matter of frequent observa- 

 tion, but the exact cause of this sterility is still largely unknown. It is 

 very probable, as De Vries suggests, that there is a lack of harmony 

 within a part of the cells due to the failure of certain characters to find 

 their mates, — they remain unpaired, disturbing elements in the society 

 of the cell. 



Stimmary. 



1. The majority of the characters of the hybrid plants studied, 

 were intermediate between the corresponding characters of the parents; 

 100 of the 121 characters tabulated were intermediate. 



2. Nicotiana Tabacum, when used as a pistillate parent in a cross 

 with Nicotiana sylvestris, influenced the external or morphological 

 characters of the hybrid to the extent of 64.6%, and the histological 

 characters 61.9%. The staminate parent, N. sylvestris, had influence 

 to the extent of 35.4% and 38.1% respectively. 



3. Nicotiana Tabacum, when used as a pistillate parent in a cross 

 with Nicotiana alata had influence over the external characters of the 

 hybrid to the extent of 36.5%, and over the histological characters, 

 37.2%. The staminate parent, N. alata, had influence to the extent of 

 63.5% and 62.8% respectively. 



4. Raphanus sativus, when used as a pistillate parent in a cross 

 with Brassica oleracea var. caulo-rapa, had influence over the external 

 characters of the hybrid to the extent of 62.2%, and the histological 

 of 54.7%. B. oleracea, var. caulo-rapa had influence to the extent of 

 37.8% and 45.3% respectively. 



5. There is a close relation between external and histological 

 characters. In every hybrid studied, the parent having the higher per 

 cent, of dominance in one set of characters was also dominant in the 

 other set by about the same per cent., the difference being, as can be 

 seen from the figures given above, 2.7%, .7%, and 7.5%. 



6. Neither parent, in the crosses studied, exercised a iiniform 

 influence over any particular characters of the hybrid or over the sum 

 total of characters. 



This work was done in the botanical laboratory of Cornell Uni- 

 versity. I wish to thank Prof. W. W. Rowlee, for advice and criticism 

 during the progress of the work, ar.d I also wish to express my indebt- 

 edness to Dr. H. J. Webber for many helpful suggestions and assistance 

 in various ways. 



