1925] Mann : Chromosome Number and Individuality in the Genus Crepis 309 



1924) and Matthiola (Frost and Mann, 1924). Both of these tetrasomic 

 types are even feebler than the trisomic plants, and hence would have 

 little chance of survival under unfavorable environmental conditions. 

 The possibilities of species-hybridization as a source of differences 

 in chromosome number within a genus are still less known. It might 

 be argued with some plausibility that if a tetrasomic condition is 

 unbalancing and associated with lessened viability, even less in the 

 way of stability and viability should be expected of organisms having 

 a pair of chromosomes from another species added to a complete specific 

 complex. The Drosophila workers have found, however (Morgan, 

 1922), that a similar genie structure characterizes the chromosomes of 

 several species of that genus, and if this is true of Crepis, one method may 

 be as probable as the other. It has been shown (Collins and Mann, 

 1923) that new types with more chromosomes than either species 

 possesses are formed when the Fi C. setosaXC. capillaris is backcrossed 

 to setosa. It is only through further work on such types that the 

 question of stability can be answered. The theoretical and practical 

 vdlue of such work is self-evident. 



While the little work that has so far been done on tetrasomic plants 

 tends to show that they would be expected to be somewhat unstable 

 genetically, tetraploid plants, e. g., Oenothera gigas, breed true. That 

 Crepis biennis may be an octaploid from a five-pair species is indicated 

 by the following experimental evidence : 



1. In the Fi C. setosaXC. biennis the twenty pairs of chromosomes 

 from biennis form ten pairs. 



2. In the backcross of this Fi to biennis the thirty chromosomes 

 from C. biennis form fifteen pairs. 



The great size and vigor which distinguish it from the other species 

 studied also indicate that it is polyploid. The evidence from chromo- 

 some measurements indicates strongly that Crepis biennis is the only 

 one of the twenty species discussed in this paper that could owe its 

 origin to polyploidy. 



It would seem possible that, if the whole complex of one species were 

 added to that of another by segregation following species-hybridization, 

 zygotes formed by the union of two such gametes might be expected to 

 give stable races differing in chromosome number from other species 

 of the genus. There is no evidence that such a procedure has occurred 

 in any of the species of Crepis discussed above. 



There is at present little evidence that whole chromosomes can be 

 lost and the resulting organisms be expected to give rise to new species. 

 Genetical and cytological results on Drosophila (Bridges, 1921) indicate 



