58 EIGHTEENTH REPORT. 



fact tliat some crosses between nearly related plants never pro- 

 duce seeds may be due to the fact that the different nuclear hor- 

 mones produced by the chromosomes of male and female origin 

 within the zygote nucleus are so opposite in their effects on cyto- 

 l»]asni that the latter can not develop further and so no embryo 

 is formed. 



Department of Botany, 



Michigan Agricultural College, East Lansing, 



