38 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



resembles an apple, where the similar depression at the junction of the 

 stalk is common. The opposite of the rou7id potato is the long-axised 

 potato, the tubers of which may be either cylindrical, pyriform, or 

 kidney shaped, and such that are pure in respect to the character of 

 length breed true, all the offspring being more or less alike and all 

 possessing the long vertical axis. 



Potatos which are hybrid as regards length of axis are also long in 

 shape, because the long axis is dominant to the short; but the actual 

 shape taken by the tubers is very variable. Many of the kidney potatos 

 on the market are hybrids in respect to length, whilst nearly all the 

 oval, lapstone, and pebble- shaped tubers are also borne by hybrid 

 plants, and such plants when crossed by a round-tubered individual 

 give rise to a family half the members of which bear long tubers and 

 the other half round. As constancy of shape is desirable, and as hybrid 

 individuals vary, according to my observations, more than others, I 

 think it is highly desirable in selecting a new variety that one should 

 be chosen that is known to be pure to length. 



"When tubers are young they are generally of the typical round form, 

 but those that are pure to length often begin their existence as elon- 

 gated tubers, however minute. 



It should be observed that owing to the drought in 1911, when a 

 great many varieties suffered from secondary growth, these secondary 

 outgrowths were all round, even when they were attached to cylindrical 

 tubers. It will be of interest to see whether in 1912 these round out- 

 growths produce, vegetatively, tubers like themselves, or whether they 

 revert to the long forms from which they arose. I think it is quite 

 possible that there are other factors which control the transverse axis 

 of the tuber, but so far their determination has eluded me. 



Tuher Eyes. — Th-e eyes or buds of the tuber may be either deeply 

 inserted or quite superficial, and a single pair of factors controls their 

 disposition. The deep eye is recessive, and breeds true; the superficial 

 eye is dominant. The hybrid is superficial-eyed, but it frequently 

 happens that the dominance is not complete, and it was because in my 

 original observations* I classed all hybrids which showed the slightest 

 trace of depth oi eye as deep that I spoke of the latter as being 

 dominant ; whereas now I am convinced that the superficial eye is far 

 more of a dominant nature than the deep is, and am therefore in agree- 

 ment with East, who pointed out this fact before my paper was 

 published. 



Monstrous over-growths of the " eye-brows " in the tuber are not 

 uncommon. I have examined two cases exhaustively, and do not find 

 the condition to be inherited either sexually or vegetatively. 



Colour of Tuher. — Tubers may be either white-skinned, red-skinned, 

 or black (i.e. deep purple). Mendelian analysis shows that the red coat 

 of tubers is due to at least two distinct factors, the absence of either of 

 which renders the tuber white, whilst the purple tuber is due to the 

 presence of a third factor. 



* Jour. Genetics, 1 (1911), p. 27. 



