63 



Those combinations which lack both of the units for spreading panicle = 

 (& l , a 2 ) will be side-panicled. In the above manner may be explained the 

 "sudden appearance " of a side-oat type in a variety which normally produces 

 a spreading panicle. 



Some very interesting and significant results have followed investiga- 

 . . , ,. 



tions at bvalof into the inheritance of resistance against yellow rust (Puc- ance 



cinia glumarurri) in wheat (52) . As is well known, different sorts and lines wheat 

 of wheat differ considerably in their attitude toward this disease. Some 

 are relatively resistant although not absolutely immune; others possess a 

 low degree of resistance. Between the two extremes again are to be found 

 sorts possessing a varying degree of resistance. Eriksson and Henning 

 recognize five grades of rust ((Mr) and according to Nilsson-Ehle (52 p. 59) 

 a few more classes may still be added. Thus Extra Squarehead II and Kotte 

 are classified under class o, being most resistant; in the second, relatively 

 resistant class, is placed Grenadier; in the third, possessing a lower degree 

 of resistance, belongs Pudel, while in the fourth and most susceptible class, 

 the sort Top Squarehead constitutes a prominent representative. The 

 degree of rust resistance possessed by a given sort is found to be relatively 

 constant from year to year. 



From the crossing between the sorts 0401 X 0705, both classified as 

 Grade 2, eight separate cultures were investigated during the bad rust year, 

 1904. The difference between these plots in regard to their attitude toward 

 rust is recorded as "exceedingly striking." The difference between the most 

 susceptible culture and that which showed greatest resistance was much 

 greater than the difference between the parents themselves. Thus plots 

 2, 5 and 6 were more severely attacked than were either of the parents, 

 while plots 7 and 8 showed somewhat greater resistance than did the parents. 



From the crossing 0319 X 0501 (both sorts of high resistance) there 

 were produced a number of lines some of which, in F 3 proved much more 

 susceptible than either parent. Similarly, in the crossing between 0315 

 (grade 1) and Swedish Velvet Chaff (very resistant) 2 plots out of a total of 96 

 in F 3 proved extremely susceptible, being classed as grade 5, despite the fact 

 that the year was not a particularly bad one for rust. 



It will thus be seen that apparently new forms may arise from crossings 

 between sorts which to all appearances are practically identical in regard to 

 certain characters. The origin of these forms is due simply to the peculiar 

 grouping of definite units already in existence and not to the acquisition 

 of anything actually 'new.' In other words, they constitute different 

 gradations a quantitative hereditary variation the different gradations 

 being "continuous." This fact at once suggests the necessity of making 

 a clear distinction between outer visible characters and inner factors. 



