462 



THE AMERICAN NATURALIST [Vol. LV 



cell received both .halves of one motlier-cell chromosome, while 

 the other daugrhter cell received both halves of the homolof^ous 

 chrornosOTne. Such a variation, then (Mnller. 1920. p. 459), 

 permits an almost certain decision 'l)et\veen factor mutation and 

 ''mitotic irregularity."^ 



A ")>ud variation" apparently involviufr two linked factors 

 was observed in a culture of Matthiola annua, at Iliverside, Cali- 

 fornia, in 1916-17. Unfortunately the factorial relations are 

 not entirely clear, but the case seems decidedly significant never- 

 theless. 



The plant in question occurred among ])rogeny of a "slender" 

 parent {27^h-i]-S-6 : Frost, 1919). The slender type (S') is one 

 of several alx't-raiit forms evidently dependent on factors linke<i 

 with the factor ''for" single (normal) flowers (D). Slender 

 parents have given (Frost. 1919) on the average about 32.5 ± 2.0 

 per cent, of slender progeny, most of the rest l)einu- "SnowHake" 

 (normal). The constitution of the slender -single parent men- 

 tioned appears to have been S'D/s'd. Both .S" and D (or a 

 factor completely linked with D) appear to !«■ inipt'i-fn-tly re- 

 cessive for a lethal effect; no functional pollen ciii'fics />.■ and 

 S'S' zygotes appear to be non-viable, while *S".s' z\ i:.,t( s aiv >(»iue- 

 what weak and probably are selectively eliminaTcd hcl'di't' germi- 



like Snowflak(>, hut ihr r-M -^l-wWv. Wlini matinv thi^ plant 

 had produced from oiir side of rlic main stem at least three 



yielding 



