KNOWLEDGE OF HEREDITY IN PISUM. 95 



still exhibit the Mendelian ratio, although three generations 

 of ancestors have shown exclusively the dominant character. 



The conclusion drawn by Mendel (16) from this experimental . 

 result is well known.* A practical demonstration of the 

 correctness of the theory was obtained by crossing the hete- 

 rozygote with the dominant and recessive parents respectively. 

 From such crosses the following numerical results have been 

 obtained : — 



Heterozygote crossed with the Dominant. 



Observer. Round. Wrinkled. 



Mendel (16) . . 192 . . — 



Tschermak (17) .. 38 .. — 



Heterozygote crossed with the Recessive. 



Mendel (16) . . 106 . . 102 



Tschermak (17) .. 20 .. 18 



A further visible difference between a round and a wrinkled 

 pea, in addition to the shape, lies in the colour of the coty- 

 ledons. This difference was first noticed by Hurst (11), and is 

 only visible after removal of the testa. When this has been 

 done, the colour of the wrinkled pea is seen to be some shades 

 paler than that of the round. This distinction in colour is 

 possibly to be associated with the internal difference described 

 below. 



(ii.) Indent and Smooth. — The inheritance of these characters 

 was not observed by Mendel, but was first described by Tscher- 

 mak (19). The indentation, which at first sight appears to be 

 only a mild form of wrinkling, is a totally distinct character. 

 It is, furthermore, a maternal or plant character, being present 

 or absent ia all the seeds of tlie same individual. 



The presence of the indented character is dominant to its 

 absence. 



The effect of the combination of the two pairs of characters, 

 round -wrinkled and indent-non-indent, is as follows. The 

 two pairs segregate independently. The round character is 



* In 1900 Correns (4) applied the term Mendel's Law to this conclusion. 

 The rule is to this effect : Hybrid peas produce gametes, which represent 

 in equal numbers all possible combinations of paired characters, no two 

 members of the same pair appearing in the same gamete. 



