THE MODIFICATIONS OF THE CHROMOSOME CYCLE 



67 



the divergence which has taken place during the evolution of the two 

 species. The diploid hybrid may be represented as AB, and the tetra- 

 ploid derived from it as A ABB', it is spoken of as an allotetraploid, in 

 distinction from an autotetraploid AAAA in which all four sets of 

 chromosomes are identical. 



I. The Appearance of Polyploids 



A mere doubling of the chromosome number, to give an autopoly- 

 ploid, does not usually have any great effect on the phenotype. In a 



Fig. 27. Allopolyploids in Mosses.— The drawings show the shape of the para- 

 physes in polyploid hybrids (gametophytes obtained by regeneration) between 

 Physcomitrium pi ri forme and Funaria hygrometrica. The chromosome sets are 

 indicated by P and H; thus PgH is a tetraploid with three piriforme sets and one 

 hygrometrica set. They have all, except the pure H polyploids, cytoplasm derived 

 from piriforme. Note the "balance" between the P and H sets, with some 



dominance of H. 



(After Wettste in.) 



