GRAPHIC ILLUSTRATIONS OF VARIATION. 



21 



aj?e 3 to 4 grains per plant. Calculated in jield per acre, 3 jj^ranis 

 per plant make a much greater jield tlian that of the ordinaiy held 

 plot. In reading these graphic statements, based on general facts, 

 this difference must be recognized in passing from the yields in grams 

 of the plants in the plant-breeding nursery to the yields in busliels 

 per acre of varieties grown in field plots. 



The double horizontal line, at 23 bushels per acre, serves to indi- 

 cate the standard average yield of tlie varieties used as a basis from 

 which superior varieties of hard spring wheats rise under the breed- 

 ing experiments. Since Fife wheat yielded 22 Inishels per acre, as 

 averaged for several years in tlie first field trials, and Blue Stem 24- 

 bushels, the mean is taken as 23 bushels. In improving the yield of 

 either of tliese wheats those plants are chosen whose vertical-yield 

 lines rank them in the upper curve, as at o — o, plants which vavy 

 toward a l)etter yield. 



In figure is shown, in addition to the Fife and Blue Stem curves 



Fic. !l. -Yields, in grams, of 111(1 plants of Fife, Blue Stem, and a hybrid between the two. Tlie 

 yield of the Blue Stem is indicated by the broken line, that of the Fife ])y the solid line, and 

 that of the hybrid by the line marked — .c--. The best plants which should be used in further 

 experiments are found near the end of the upper curves at the right and are indicated by o—o. 



of figure 8, a curve marked throughout by ;/•, representing a hybrid 

 between the Fife and Blue Stem wheats. Tliis curve has more slant 

 througliout its central straighter portion, and its upward curves goes 

 higher, while the curve at the other end goes lower. This formal 

 diagram has been so made that the center or average line is the 

 same height as the mean between the parents of the hybi-id. I1ie 

 mean liere has little significance, since we do not expect to form a 

 variety by using all of the hybrid progeny, but by using only the 

 exceptional plants whicli rise into the upper curve. 



Tlie im|)ortant features in tlie hybrid are greater number of indi- 

 vi<luals from among which superior t)lants may be selected in the 

 upper curve, and theii- large yields. Tluise are more promising i)lants 

 to use as mother plants in seeking varieties with better yields tlian 

 are any ol" the plants raiii<iiig in Ihc upper (mii-ncs in tlie jiarent varie- 

 ties. There has been created a jjower for gi-eatcvr yield. A"reac- 

 lioii'Mias been produced jiiid 1h<' conslancy of habit of the Fife and 

 Blue Stem wheals 1o yield oid\ 22 and 2-f bushels, respectively, has 



