THR GENUS ROSA 279 



may carry the full unreduced chromosome complement. What 

 is more natural than to assume that, spurred on by the stimulus 

 of heterozygosis, this cell is capable of development just as if 

 it had been normally fertilised? Even were the pollen 

 functionally active the position here is unaltered ; the only 

 difference would be that aponiixis would be facultative. 



Thus an apomictical race would come into being capable 

 of little variation and therefore faithfully reproducing its kind, 

 like the apomictical Rosa rnbigmosa mentioned previously, 

 the seeds of which have been planted and from them young 

 shrubs grown. 



If, however, the cross was between roses of less remote 

 relationship, then in the maturation division of their Fi 

 hybrids dislocation more or less serious of the ordinary 

 mechanism may occur, some chromosomes finding mates, 

 others vanishing and so on exactly as described above. 

 Hence, whilst many microspores will collapse, others will 

 possess nuclei carrying normal or nearly normal chromosome 

 numbers. These latter will be perfectly active, and like the 

 similar sperm of the autumnata-filigrammai'ia hybrids differing 

 but little in power from those of a pure species. If events in 

 the female apparatus follow a parallel course the mature 

 embryo sac will, in many cases, contain the egg-cell and 

 egg-apparatus complete and prepared for the advent of the 

 male nucleus. Fertilisation is thus possible, and apomixis 

 unnecessary. If, however, the chromosomes are the heredity 

 bearers, and their distribution in the micro- and mega-spore 

 development anomalous, then the potentialities of the fertilised 

 egg-cells will vary greatly, and thus account for the huge 

 range of variation seen more especially in the fertilised roses. 



Should this be the correct explanation of the genesis and 

 continuance of apomixis in the roses, then it offers an 

 adequate explanation of constant hybrids in the plant 

 kingdom, and also of the multiplication of "species" in such 

 critical genera as Hieracimn, Rubus, Mentha and SalLx. 



