24 



JOUENAL OF THE ROYAL HOKTICTTLTURAL SOCIETY. 



one-linlf of them would ho cnrrying t,]ie (Iwavf rnclor. (^(^nsioqiioTiiily , 

 wboii i\\c cM'oss-ln'od \\;is; s(^] f- foH-ilizod foni' clislincli rc^sulls would 

 happen, on tlie avorago willi p(iual frcnpicMicy : — • 



(1) A poUen-cidl carrying tlic (all fat'1(M- might lueet an egg-cell 

 containing the tall fai'lor, giving rise io a pure tall plant. 



(2) A pollen-cell carrying the tall factor might meet an egg-cell 

 containing the d\A'arf factor, giving rise to an impnre tall plant. 



(3) A pollen-cell carrying the dw^arf factor might meet an ogg-ccll 

 containing the tall factor, giving rise to an impure tall plant. 



(4) A pollen-cell carrying the dwarf factor might meet an egg-cell 

 containing the dwarf factor, giving rise to a pure dwarf plant. 



The diagram (fig. 13) on p. 25 will serve to illustrate Mendel's con- 

 ception of germinal segregation. The tall plants are represented by 

 oblongs and the dwarf plants by squares. The germ-cells of each plant 

 are shown as circles. The contained tall factors appear black and the 

 dwarf factors white. 



If the diagram is compared with the previous one, it will be 

 seen that Mendel's conception of germinal segregation fully explains 

 the results obtained in his experiments. In further confirmation 

 of the above interpretation of his results, Mendel made all the possible 

 matings between the three germinal types. As we have already seen 

 (fig. 12), both the extracted pure types bred true, while the impure 

 type selfed gave all three types again. Mendel also mated the impure 

 tails with the pure tails, and these gave, on the average, equal numbers 

 of impure tails and pure tails. He also mated the impure tails with 

 the pure dwarfs, and these gave, on the average, equal numbers of 

 impure tails and pure dwarfs. 



The diagram (fig. 14) on p. 26 will illustrate these matings and their 

 results, and at the same time Mendelian segregation- 



The " Presence and Absence " Method. 



Eecent experiments with many different characters in various plants 

 and animals fully confirm Mendel's results with peas, and show clearly 

 that Mendel's law of heredity is capable of general application. At the 

 same time these results have suggested a more simple method of pre- 

 sentation of the facts of Mendelian segregation, w^iich not only explains 

 later developments better, but puts a different construction on the im- 

 portant phenomenon of Mendelian dominance. It is curious to observe 

 how this striking phenomenon of dominance has proved a stumbling- 

 block to many in their comprehension of the Mendelian principles, and 

 yet seemingly it w^as this phenomenon that enabled Mendel to discover 

 his simple law of segregation. 



There seems to be no doubt that Mendel himself regarded tallness 

 and dwarf ness, for instance, as a definite pair of contrasting characters, 

 which behaved in breeding as "dominant" and "recessive," and 

 Mendel seems to have imagined tliat the definite germinal factors of 

 each contrasting character actually segregated from one another in the 



