STUDIES ON POLYPLOIDY 



I. CYTOLOGICAL INVESTIGATIONS ON 



TRIPLOIDY IN CREPIS 



BY 



M. NAVASHIN 



CONTENTS 



PAGE 



Introduction 377 



Acknowledgments 378 



Occurrence, morphology, and cytology of triploids 378 



Cytogenetic behavior of triploids 380 



Discussion and conclusions 383 



Summary 393 



Literature cited 395 



INTRODUCTION 



Among chromosomal variations triploidy deserves first attention 

 for several reasons. In the majority of cases it becomes a point of 

 departure for further alterations. Except for tetraploidy, it repre- 

 sents the most widespread type of chromosomal variation, at least 

 in Crcpis. The majority of other chromosomal aberrations occur in 

 the progeny of triploid individuals and are therefore only conse- 

 quences of triploidy. Moreover, being unbalanced, a triploid indi- 

 vidual is incapable of sexual reproduction without segregation, and 

 this unbalance is the very source of new chromosomal combinations 

 in subsequent generations, some of which cannot originate in any 

 other way. 



Among these derivatives from triploids should be mentioned the 

 higher grades of polyploidy which are to be observed in the progeny 

 of a triploid individual, as will be shown later. Some of them may 

 be balanced and constant, the others in their turn may become sources 

 of further chromosomal variation. 



These various chromosomal variations, including the higher grades 

 of polyploidy, produce profound changes in the biological properties 

 of the individual, affecting, as will be seen, the viability and the tempo 

 of development, the conditions of pollination, the relations as to inter- 

 specific crossability, etc. The influence of chromosomal variation on 

 the size of organs and on fertility is well known. 



