240 THE BEHAVIOUR OF POLYPLOIDS 



section of the Rosace^e (D. and Moffett, 1930 ; Moffett, 1931, a and 

 h ; 1934) in which it is assumed that a chromosome set of 17 (and 

 its multiples) found throughout the section is derived from the set 

 of seven found in other sections of the order (e.g., in Rosa and 

 Rubus) by unequal reduplication, so that the seven chromosomes 

 A to G are present in the proportions 



AAA, BBB, CCC, DD, EE, FF, GG, 



making up 17 in all. 



The evidence, chiefly from Pyrus, is of the following kinds : — 



(i) Occasional qiiadrivahnts are found in the diploid with normally 

 17 pairs. (These might also result from interchange hybridity.) 



(ii) Secondary pairing reaches its maximum in the appearance 

 of seven groups of bivalents, three being groups of three, four being 

 groups of two (Fig. 80). 



(iii) In auto-" triploids " (with 51 chromosomes), autosyndesis 

 occurs in the third set of 17, so that more than 17 pairs may 

 be formed (Nebel, 1929); and multivalents up to 9 chromosomes 

 are formed [v. Fig. 70). 



(iv) The progeny of triploids back-crossed with diploids, instead 

 of showing the elimination of types with intermediate numbers so 

 that most of the seedlings have approximately the diploid number, 

 show the highest frequency with 41 chromosomes, i.e., the secondary 

 diploid number, 34, together with the primary basic number 7, 

 Moffett, 1934. This indicates that the original balance of 7 still 

 has a certain vestigial significance (v. Ch. VII). 



(v) The somatic complement includes four chromosomes of one 

 exceptional type, as is found in tetraploids. 



(vi) Genetic evidence indicates more complex conditions than 

 those found in strictly diploid organisms. 



In the Pomoideae the inference of polyploidy is necessarily the 

 most indirect, since the original change from 7 to 17 by which such 

 an important group arose must have occurred at an epoch more 

 remote than any on which inference can usually be brought to 

 bear. In another group, however, the case is simpler. Dahlia 

 Merckii has 18 haploid chromosomes (Lawrence, 1929), while all 

 other species of Dahlia have multiples of eight. The occurrence of 



