19i THE THEORY OF THE GENE 



An extensive series of crosses between species of 

 Crepis have been carried out by Babcock and Collins. 

 The chromosomes of these hybrids have also been studied 

 bv Miss Mann (1925). 



Crosses between Crepis setosa with 8 chromosomes 

 (n=4), and C. capillaris with 6 chromosomes (n=3) have 

 been made by Collins and Mann. The hybrid has 7 chro- 

 mosomes. At maturation some of the chromosomes con- 

 jugate and other chromosomes, without dividing, are scat- 

 tered in the pollen mother cells, forming nuclei with 

 from two to six chromosomes. At the second division all 

 the chromosomes divide, at least, those in the larger 

 groups, and pass to opposite poles. The cytoplasm usu- 

 ally divides into four cells, but sometimes into 2, 3, 5, or 6 

 microspores. 



These 7-chromosome hybrids do not give functional 

 pollen, but some of the ovules are functional. When the 

 hybrid was used as pistil parent and fertilized by pollen 

 from one of the parents, five plants were obtained with 8 

 and 7 chromosomes. The maturation stage of one with 8 

 chromosomes was examined. It had 4 bivalents, which 

 divided normally. The plant resembles C. setosa in its 

 characters and has the same type of chromosomes. One 

 of the parental types has been recovered. 



Another cross was made between Crepis biennis with 

 40 chromosomes (n=20) and C. setosa with 8 chromo- 

 somes (n=4) (Fig. 109). The hybrid has 24 chromosomes 

 (20+4). In the maturation of the hybrid, at least 10 

 bivalents are present, and a few univalents. It follows 

 that some of the biennis chromosomes must conjugate 

 with each other, since setosa contributes only 4 chromo- 

 somes. At the ensuing division 2 to 4 chromosomes lag 

 behind the rest, but finally pass, in most cases, to one or 

 the other nucleus. 



The hybrids are fertile. They produce (Fo) plants hav- 



