220 



FUNDAMENTALS OF CYTOLOGY 



and separate equationally. All may succeed in reaching the poles iti 

 time to be included in the telophase nuclei, or some may fail and become 

 the nuclei of small extra microspores (Fig. 163). Such irregularity- 

 results in various degrees of sexual sterility. 



The difference in behavior shown by the poppy and rose hybrids is 

 interpreted on the basis of chromosomal homology as follows. The 

 genomes involved in the poppy cross are of five kinds: A B Bi C 

 Ci. The diploid species has in the soma two A genomes only, one being 

 transmitted by each gamete. The decaploid species has all five in the 

 gamete. Hence after a cross the pairing in the hybrid is AA BBi CC\, 

 making 21 bivalents in all. In the roses there are also five genomes, 

 ABODE, but their degree of difference is great enough to preclude 

 synapsis between them. The diploid type furnishes a gamete with A 



Fig. 163. — 1, 2, meiosis in miciosporocytes of an interspecific wheat hybrid. After the 

 bivalents disjoin and pass poleward in division /, the univalents occupy the equator and 

 separate equationally. 3, supernumerary microspores in a wheat-rye hybrid. {After 

 K. H. von Berg.) 



and the decaploid type one with A B C D E, so that in the hybrid only 

 the two A genomes form bivalents. The hypothesis that a differentiation 

 of the fundamental genome of a genus into several kinds has occurred in 

 different strains with the passage of time has the support of other similar 

 cases. From such synaptic behavior, conclusions are drawn regarding 

 the degree of residual homology in these genomes and the degree of rela- 

 tionship of the plants containing them. This method of analysis has been 

 carried out on a very extensive scale with different types of wheat, 

 goat grass (Aegilops) , rye, and their various interspecific and intergeneric 

 hybrids. Here, as in the roses, there are genomes of several types each 

 consisting of seven chromosomes, and these are combined in different 

 ways in the genera, polyploid species, and hybrids. 



Amphidiploidy. — Special consideration should be given to the type of 

 polyploid hybrid having two genomes from each of two species. Such a 

 plant is said to be aniphidiploid, since it is diploid for both parental 

 genomes. (It is also allotetraploid, but a plant combining three genomes 



