310 DouhJeness in Stocks 



an Fi crossbred, bred as above, but from which nevertheless doubles 

 would be obtained if a further sowing was made. The remaining 

 exception was a family of 83 singles, but even this total constitutes no 

 very strong case for the genuineness of the exception, seeing that in 

 another case a result of 40 s. 2 d. (see above) was actually observed. 

 It represents, it is true, a greater excess of singles than was recorded in 

 any other family of the same parentage, but much stress cannot be laid 

 upon this point, since among the mixed F„ families obtained when one 

 of the sap-coloured forms was used as the double-throwing parent in 

 similar matings, we find a case where the proportion of singles was as 

 high as 30 : 1 (the actual numbers were 60 s. 2 d.). An equally high 

 proportion might presumably be obtained with the sulphur-white ; so 

 that even in this last case it is quite possible that doubles would have 

 occurred in a larger sowing. Another possibility is worth noting in 

 this connection. The plant from which the F^ family of 33 singles was 

 derived was one of 46 obtained from pure-breeding creams which had 

 been fertilised with the pollen of sulphur-whites. Tlie other 4.5 all 

 yielded a mixed offspring of singles and doubles. Now the strain of 

 sulphur-white used in this experiment evidently did not contain the 

 colour factor C found in the ordinary pure white glabrous race', for the 

 mating witli the cream pruduced offspring which were all cream, and, 

 as we should expect under these circumstances, all glabrous. Thus the 

 F^ plants obtained from crossmg the cream with the sulphur-white are 

 indistinguishable in appearance from .^i plants derived from the same 

 cream parent by .se^/"-fertilisation. Where F^ shows reversion in colour 

 and surface character we know that we are dealing with a genuine 

 crossbred, but in this case we have no such proof It is in fact within 

 the bounds of possibility that the F^ plant which produced the 33 

 singles, although supposed to be a crossbred, may in reality have been 

 a pure-bred resulting from accidental self-fertilisation. 



To sum up the evidence in Tegard to these two double-throwing 

 forms, red and sulphur-white : 



Experiments carried through 6 generations showed that the 149 

 individuals tested were all throwing doubles. It therefore seems 

 beyond doubt that both forms are genuinely eversporting — that in 



' As stated in the Evolution Reports one of the two factors C and It which are essential 

 to the production of sap-colour is found in the pure white race, the other in the cream. 

 As white is there represented as containing C and cream as containing R, it will be 

 convenient to retain the same formulae here (see Report IV, p. 36). For a fuller account 

 of the constitution of the sulphur-white, see p. 370 of the present account. 



