Genetic Factors 



441 



abundant in perennial herbs than in annuals or woody plants (Fig. 

 19-12). The basic chromosome numbers, however, are significantly 

 higher in woody genera. 



Much work on polyploidy has been done by students of ecology and 

 plant distribution ( Miintzing, 1936 ) , though most of this has little direct 

 concern with morphogenesis. In general, polyploids can endure extremes 

 of climate better than diploids. The distribution of polyploids as to 

 latitude has been discussed and the literature reviewed by Love and Love 

 (1949). 



Polyploids are of importance for evolution not only in matters of selec- 

 tion and distribution but from the fact that by their means sterile 

 hybrids can become fertile and genetic lines, separated by incom- 



35-- 



1 



EL 



n 



tl WOODY 



■ HERBACEOUS 



U WOODY b HERBACEOUS 



nln hi 



m. 



Lii 



LtiL 



m. 



5 & 7 8 9 



BASIC HAPLOID NUMBER 



Fig. 19-12. Distribution of basic chromosome numbers in herbaceous and woody 

 genera of dicotyledons. Dotted lines indicate genera known only tentatively. Herba- 

 ceous types tend to have smaller numbers. ( From Stebbins. ) 



patibilities, thus be brought together. Many species will hybridize but 

 the offspring are usually sterile. If their chromosome number is doubled 

 (to form an allopolyploid), as sometimes happens in nature, fertility 

 is restored since there are now pairs of homologous chromosomes and 

 normal meiosis can take place. 



Somatic Polyploidy ( Polysomaty ) . Polyploidy is concerned with an 

 important aspect of differentiation, for investigation has shown that many 

 somatic cells, particularly the larger or physiologically more active 

 ones, are polyploid and often to a rather high degree. This condition is 

 termed polysomaty. In some cases, such as the meristematic region of 

 the root of spinach (Gentcheff and Gustafsson, 1939; Berger, 1941), 

 chromosomes of certain of the dividing cells, particularly in the periblem, 

 are twice, or sometimes four times, the normal diploid number. This seems 

 to be the result of an additional doubling in the prophase before the 



