394 University of California Publications in Agricultural Sciences [Vol. 2 



comprises simple and double trisomies (3.0 per cent), triploid tetra- 

 somics (0.3 per cent), and higher grades of polyploidy (4.0 per cent). 

 The cause of these curious ratios is still unknown. 



7. Of the six possible trisomic types in C. capillaris five have 

 actually been found and grown under controlled conditions. These 

 different trisomies differ morphologically from one another ; some of 

 them are usually not less fertile than the triploid plants. 



8. The other two species (C. tectorum and C. dioscoridis) behave 

 in quite a different way. The progeny of triploid tectorum plants 

 consists of a majority of trisomies, the diploid and triploid plants 

 being present in the minority. In C. dioscoridis, on the other hand, 

 the progeny of triploids contains only a small percentage of trisomies 

 and triploids; the great majority of the offspring are diploid. 



9. Crossing of triploid plants with other Crepis species has shown 

 that the triploid condition in the female parent is favorable for inter- 

 specific hybridization. From these crosses certain hybrids have been 

 obtained which could never be obtained heretofore by using normal 

 diploid plants. INIany of the hybrids thus secured were polyploid. 

 These properties of triploids under natural conditions may play a 

 considerable part in species formation ; they also make triploids 

 favorable material for theoretical genetic work as well as for prac- 

 tical breeding. 



10. The formation of diploid gametes (most likely of female ones) 

 should be considered as the original source of triploidy in Crepis. 



11. In a number of instances the occurrence of triploid, tetraploid, 

 and possibly pentaploid and hexaploid eggs has been proved ; these 

 polyploid gametes are perfectly viable and are capable of fertilization. 



12. Triploidy in Crepis may play a considerable although indirect 

 part in formation of new species, for it gives rise in subsequent gen- 

 erations to a number of further chromosomal variations, especially 

 of higher grades of polyploidy. Through change in the rate of devel- 

 opment, a polyploid individual may acquire the ability of withstand- 

 ing different climatic conditions. As a consequence it may penetrate 

 into a new territory where it may become subject to a series of various 

 influences, especially to crossing with other species and forms. ()u 

 the other hand, the climatic barrier may be considered as a po.ssible 

 source of new polyploid forms arising in response to extreme tempera- 

 ture conditions; and at the same time this barrier will provide perfect 

 isolation. 



