450 Tricotylous Races. 



cnnial form ; and therefore both of them afford material 

 for comparing the harvest of the same plant as produced 

 in successive years. Here again I failed to find differences 

 of any significance. I have given above the values de- 

 rived in 1892 and 1893 from a single plant of Silcne, 

 which flowered in isolation ; they were 3 and 4% for 

 these two years. In ScropJudaria I made a series of ob- 

 servations at the beginning of my cultures when the hered- 

 itarv values were still small, and repeated them in the 

 period of 1896-1899, when they had become higher (15% 

 and more). In these years, 1897, 1898 and 1899 six 

 plants gave the following values, the bracketed number 

 referring to the second year: A 22 (25), B 25 (17), 

 C 22 (17), D 23 (25), E 27 (25), F 23 (22). Ob- 

 viously these figures do not justify a conclusion as to any 

 diminution or increase in the ratio in which tricotyls are 

 produced. 



The result of all of these experiments is such as to 

 justify my practice of limiting the individual harvest to 

 the quantity of seed necessary for sowing. 



§ 8. THE INFLUENCE OF EXTERNAL CONDITIONS ON 



TRICOTYLY. 



Which seeds in a fruit produce aberrant seedlings? 

 This question is at once one of the most simple and one 

 of the most difficult presented by experimental breeding. 

 If some day we could succeed in solving it and thereby 

 make a control of this process possible, much light would 

 be thrown on a whole series of phenomena connected 

 with the origin of races. 



In dealing with this question we are thrown back 

 on Qgg cells and pollen grains and the problem at once 



