98 



FUNDAMENTALS OF CYTOLOGY 



gametes it produces. In plants having such an alternation of gameto- 

 ])hyti(' and sporophytic generations in the hfe cycle, therefore, this 

 alternation is typically, though not invariai)ly, correlated with an alterna- 

 tion of monoploidy and diploidy in the nuclei. 



In subsequent chapters we shall describe in detail the cytological 

 features of such typical cycles and review other cycles of quite different 

 types. We shall also deal with cases in which the nuclei show an alterna- 

 tion not of one and tw^o genomes, but rather of two and four, or three and 

 six, or four and eight. Such a condition, which is known as polyploidy, 

 is largely responsible for the high chromosome numbers observed in many 

 organisms, especially among plants. Because of the frequent occurrence 



^ Crepis pulcherrima 



4€ 



Crepis parviflora 



^D, 







^i Crepis virens 



/?, A 1^^ iEz Crep/s rhoeadifolia 



fl B C D E 



I ll 



C D E 



tll/ll/ 



Fig. 71. — The chi-omosomes of four species of Crepis. Genome at right; diploid comple- 

 ment from root cell at left. {Crepis virens = C. capillaris.) {After M. Navashin.) 



of polyploidy here and there among organisms, it is advisable to use 

 the general terms gametic number and zygotic number for the reduced and 

 unreduced chromosome numbers in life cycles. 



Nuclei with different numbers of genomes tend to have different 

 numbers of nucleoli. This is because a given genome commonly includes 

 but one chromosome with a nucleolus organizer. As a result, the nuclei 

 in ordinary tissues often show as many nucleoli as there are genomes, 

 although the correlation is disturbed by the tendency of the fluid nucleoli 

 to fuse if they come in contact. The character has, however, in many 

 instances been a useful one in estimating the number of genomes 

 present. 



Most known genomes consist of relatively few members: among 

 flowei-ing plants 12, 8, and 7 are the most frequent numbers. Crepis, a 

 genus of composites, has been especially valuable in studies involving 

 chromosome complements because of the small number and distinct 



