10 ESTABLISHMENT OF VARIETIES IN COLEUS 



ism must be judged. It is not a question of the appearance of parental 

 characters in hybrid progeny, but of the purity of those segregations. 



Mendel himself greatly modified the conception, gained from his 

 studies with peas, that an entire quality is represented by a single 

 unit. In Phaseolus he crossed P. nanus having white flowers with 

 P. multiflorus having purple flowers. The F 2 generation of 31 plants 

 had flowers ranging from white to pale violet and purple red of various 

 grades. Only one had white flowers like those of P. nanus. He sug- 

 gests that the color of flowers (and seeds as well) in P. multiflorus 

 "is a combination of two or more entirely independent colors' ' (Mendel, 

 p. 367). To Mendel, therefore, should be given credit for the con- 

 ception of multiple factors, later developed especially by Nilsson-Ehle 

 (1909) and by East (19106). 



Very soon after the so-called rediscovery of Mendel's law for the 

 behavior of certain characters of Pisum in hybridization, it was noted 

 that new qualities frequently appear in the F 2 generation, as Mendel 

 found was the case in beans. The presence-and-absence theory 

 developed by Bateson and Punnett (1905) attempted to account for 

 this increased variability giving the appearance of new types. This 

 gave chance for dihybrid ratios from what was apparently a simple 

 pair of contrasting characters, the " absences" segregating out into 

 expressed characters new at least to the immediate ancestry. 



The next important modification of the general view was the appli- 

 cation of Mendel's idea of what we now call multiple factors. Nilsson- 

 Ehle (1908 and 1909) found that certain crosses in cereals gave in the 

 F 2 progeny large numbers of one parental type and relatively few of 

 the other type. For example, the apparently simple cross of white- 

 kerneled wheat (Predel) with a red-kerneled sort (Swedish Velvet 

 Chaff) gave an F 2 progeny of about 63 red-kerneled plants to 1 white. 

 Nilsson-Ehle' s explanation assumes that the red character in the 

 Swedish Velvet Chaff is due to three independent factors which are 

 each of equal value and that any one can produce the same effect as all 

 three. We note that the variability of the F 2 generation was not 

 increased over that of parents, but that the ratio showed almost com- 

 plete appearance of one character. 



East (19106) applied the term to the same sort of phenomena as 

 Mendel did, i. e., to increased variability. The apparently simple cross 

 of white with yellow corn gave in the F 2 a generation with few white but 

 with a large number of yellow kernels. The latter, which were of all 

 gradations of intensity, were grouped into two classes by East. 



It should be noted that such a group of " factors" having among 

 themselves different values but working together to produce a single 

 character are not necessarily independent in the fertilizations of the 

 variety concerned. Inside the variety they go together. When certain 

 crosses are made they separate. This is but another way of saying 



