BIOTYPES AND HYBRIDS. 33 



lected by J. Marion Shull at Edgewood, New Carlisle, Ohio, May 26-28, 

 1905. This specimen was robust and the lobes of the leaf were lanceolate, 

 with prominent dentations on both proximal and distal margins. 



The seeds were sown June 26, 1905, and produced a progeny of 284 

 individuals, among which there was such a variety of form and aspect as 

 to make them at that time entirely inexplicable, because none of the unit- 

 differences between the forms was then known and there was no possibility 

 of discriminating between minor fluctuations and the distinguishing char- 

 acteristics of the several elementary species. Efforts were made to arrange 

 these plants into groups that would be strictly homogeneous, and in this 

 way no fewer than 25 categories were established. Even these were not 

 sufficient, and finally a considerable number of individuals that could not 

 be classed with any of these were preserved in order that they might be 

 studied at any subsequent time at which it should be desired to work out 

 their variations and their relationship to each other and to the rest of the 

 family. 



Fortunately the distinguishing marks of each of these several groups 

 had been noted with sufficient thoroughness that later, when the differen- 

 tiating characteristics of the several elementary species were better under- 

 stood, it was possible to go back and reclassify the several categories in 

 such a manner as to determine approximately the proportions in which the 

 different elementary species were present. The maze of different forms 

 had proved so baffling at every attempt at a satisfactory classification that 

 the result of the redistribution of the several forms in the light of knowl- 

 edge subsequently gained occasioned much surprise. There were among 

 the 284 individuals composing the family, 114 B. dp. heteris, 47 B. bp. tenuis, 

 53 B. bp. rhomboidea, 16 B. bp. simplex, and 54 which, because of certain 

 deficiencies in the original notes, could not be reclassified with certainty, 

 but nearly all of which were certainly fluctuations of B. bp. heteris. 



It need only be assumed that of these 54 doubtful individuals 46 were 

 B. bp. heteris, 6 B. bp. tenuis, and 2 B. bp. simplex to make the ratios of 

 these several elementary forms agree exactly with a frequently observed 

 Mendelian ratio, 9:3:3:1. Fortunately most of these doubtful specimens 

 were preserved in the herbarium and were thus available for study. Of 51 

 thus preserved, 37 were B. bp. heteris, 9 B. bp. tenuis, 1 B. bp. rhomboidea, 

 and 4 of doubtful affinity, these latter probably belonging to the heteris 

 group also, but representing cases of incomplete dominance. 



The ratio 9:3:3:1 is the normal one for the second generation of 

 typical Mendelian di-hybrids, /. e., hybrids between forms that differ from 

 each other in two unit-characters . All of these different elementary species 

 were supposed to differ from each other by single units until this family 

 was worked out, because in nearly all the other hybrid families the simple 

 ratio of 3 to 1 appeared. 



