No. 568] X CAPSELLA BURS A-P AST ORIS A I,'A( II XO I DI\ A 209 



elongated primary lobes, a marked secondary lobe, in the 

 i list al axil of the primary lobe and a well-marked terminal 

 lobe; rhomboidea, with leaves divided to the midrib, with 

 an unelongated primary lobe, with an incision in the distal 

 margin setting off a secondary lobe and a corresponding 

 incision on the proximal margin of the primary lobe, set- 

 ting off, in well-grown specimens, a terminal portion of 

 each lateral lobe, generally of rhomboidal form; i otitis, 

 with the elongated primary lobe of heteris, but with a 

 sinus which usually does not reach the midrib, terminal 

 lobe clear cut; simplex, with lateral lobes obtuse, never 

 attenuated, the incisions being shallow and never reach- 

 ing the midrib. 



Shull recognized here the presence and absence of two 

 factors, one (A) responsible for the sharp primary lobe 

 of heteris and the attenuation of the lobes in tenuis, while 

 the other (B) is responsible for the division of the leaf 

 to the midrib, the definite terminal lobe and the second- 

 ary lobes. On this basis Shull was able to represent the 

 biotypes by conventional Mendelian symbols, thus: 

 heteris, AH; rhomboidea, aB; tenuis, Ab; simplex, ab. 



That this conventional presentation gives us a reliable 

 working basis, my experiments have shown most satis- 

 factorily. With tiie aid of these symbols I have been able 

 to solve the origin of Capsella arachnoidea, the experi- 

 ments showing that, without question, forms presenting 

 the spider-like appearance of the rosets typical of this 

 plant are of hybrid origin. 



The Zygotic Constitution of 4,108.6 

 The problem to be solved was that of the zygotic con- 

 stitution of the original parent, the plant which in my 

 notes is recorded as 4,108.6. Among its offspring neither 

 heteris nor tenuis made their appearance, while both 

 rhomboidea (aB) and simplex (ab) were met with. Hence 

 the parent was homozvgotic for (a), but heterozvgotic 

 for (B). Therefore, its zygotic constitution, in part", must 

 have been aaBb. 

 Besides rhomboidea and simplex there appeared two 



