372 INTRODUCTION TO CYTOLOGY 



10-chromosome plants with one aspera and two capillaris sets (ACC) are 

 more like capillaris, while 11-chromosome plants with the formula A AC 

 are more like aspera. A triple hybrid involving these two species and C. 

 setosa (ACS) showed certain setosa characters (M. Nawaschin, 19276). 

 Similarly, in hybrids between C. fectorum and C. alpina the characters of 

 the achenes vary according to the numbers of parental chromosome sets 

 involved (M. Nawaschin, 19306). 



Segregation of Parental Chromosome Sets. — It frequently happens 

 that when a hybrid forms spores and gametes, only those which contain 

 a complete chromosome set from one parental species or the other are 

 functional. Or, if the gametes carry various combinations, possibly 

 only those zygotes survive which have two complete sets, be they of one 

 or of both species. In a Crepis tedorum and C. alpina cross like that just 

 mentioned, the only three plants obtained in F2 contained, respectively, 

 two tedorum sets (TT), two alpina sets {A A), and one of each (TA). 

 Of further interest is the fact that the F2 plant with two alpina sets was 

 precisely like the original alpina plant, notwithstanding the fact that its 

 cytoplasm had been derived from tedorum (M. Nawaschin, 1927a). 



In a Crepis hybrid containing two capillaris sets and one tedorum 

 set, the six capillaris chromosomes formed three bivalents at meiosis, 

 leaving the four of tedorum as univalents. Although most or all of the 

 female gametes formed by this hybrid were able to function, only those 

 male gametes with none or all of the tedorum chromosomes were func- 

 tional. F2 plants with one or more tedorum chromosomes were less viable 

 than those with capillaris chromosomes only (Hollingshead, 1930a). 

 These facts all emphasize the importance of "balance" in the chromosome 

 complement, particularly in the pollen. 



A Fertile Tetraploid Obtained through a Sterile Triploid. — In the 

 genus Galeopsis the gametic chromosome numbers of four species are as 

 follows: puhescens, 8; speciosa, 8; tetrahit, 16; bifida, 16. In the progeny 

 of a cross between puhescens and speciosa was an almost completely 

 sterile triploid plant with 24 somatic chromosomes. When this plant 

 was pollinated with puhescens pollen, a single seed was obtained; this 

 grew into a fertile plant with 32 somatic chromosomes, of which 24 were 

 probably contributed by a triploid gamete from the triploid plant and 8 

 by the gamete from puhescens. The plant would not cross with either 

 puhescens or speciosa, although it crossed readily with either tetrahit or 

 bifida. It was morphologically and genetically indistinguishable from G. 

 tetrahit, and it had the same chromosome number. It was therefore called 

 "artificial tetrahit"; and Miintzing (19306) described it as "the first case 

 where a Linnaan species has been synthesized by means of species 

 hybridization and chromosome summation." 



Conclusions. — The observational and experimental evidence reviewed 

 in these chapters indicates that distinct heritable changes in the char- 



