26 Colour and other Characters in the Potato 



i.e. 1 : 3'6. The families are illustrated in Plates XIII, XIV, XV, 

 XVI, XVII, XVIII, XIX, XX. 



In the third experiment "Congo" was crossed by " Reading Russet." 

 Only four F^ plants survived, and the tubers of these, Plate XII, are 

 elongated, but here again the numbers are not large enough to draw 

 conclusions from. 



The dominant character of length in the tubers has been isolated 

 or identified in the potato G, and is represented by a very elongated 

 kidney ; in B, where it is more pyriform ; and in " Congo," where the 

 ends of the tubers are blunted and the tuber has a cylindrical 

 appearance. 



It is not improbable, as was suggested earlier, that the allelomorphic 

 pair to the character manifested in the " round " potato is length of 

 axis, and that the kidney and cylindrical shapes, though inseparable 

 with respect to length, are dependent on other factors governing shape 

 besides that governing the length of the main axis. 



The dominance of the long potato tuber over the short is analogous 

 to the dominance of the giant over the dwarf plant, as Mendel showed 

 in the Pea Family. This dominance probably rests on the same ana- 

 tomical basis, viz. the respective length and number of internodes 

 involved. Tubers are borne on underground stems, called stolons, and 

 the eyes may be regarded as buds or nodes, so that the number of eyes 

 present may represent the number of internodes condensed into the 

 length of a tuber. A study of the tubers from this point of view is 

 not yet complete, but it is quite clear that as a general rule the 

 " round," i.e. short axis potatoes, have less eyes than the long axis ones, 

 i.e. they represent fewer internodal lengths. 



It has already been shown that the dominance of length is not 

 equal in degree : sometimes the heterozygote is of the most attenuated 

 form, but more often an intermediate shape is assumed varying from 

 kidney to pebble and oval. The ordinary kidney of fair breadth is 

 probably always an heterozygote. 



The Variations in the Shape of Tubers. The amount of variation 

 has already been indicated in the case of the " round " potato ; in the 

 "long" it is rather less. If "(7" and "Congo" be taken as pure "longs," 

 then, accepting the typical well-grown tuber of each sort, it is apparent 

 that they are as to their proportion between length and breadth much 

 the same, and the form is fairly uniform. 



By far the greatest variation in shape, both amongst the indi- 

 vidual members of the same family and the several tubers of the 



