Mendel's law of heredity and horticulture. 



33 



of a third unit-factor which only becomes patent in the presence of 

 the other two. From this discovery follow^s the interesting fact that 

 * albino ' sweet peas and stocks (white or cream) may carry certain 

 sap-colour factors which may become patent when, the ' ' albino ' ' is 

 crossed with another " albino " or with a coloured form. Other experi- 

 ments show that various species of plants behave in a similar way. 



For instance, Professor Batesox and Mr. E. P. Gregory crossed 

 Primula shicnsis ' Crimson King ' with Piiniula sinensis stellaia 

 ' Primrose Queen ' (an " albino " form, white with large yellow eye). 

 In the second generation (F2) this cross gave crimson, magenta, rose, 

 tinged white, and pure white forms : each colour a[)i)eared with small- 

 e\'es iind lai'ge-e\es. All these appeared in " sinensis " and " slellala " 

 furuis, there being at least eigliteen distinct forms, some of which bred 

 true in accordance with Mendel's law. 



Pi 



Sweet Peia 



Bush X Cupid 



3oM 



T)atix/rf 







Tall 



9 Ta LL 3 Bush SCupid IErectCupid 



"SoM ^ 3qM T)uj-anf dumy>f 



Fig. 19. — Illustrating the compound nature of tall, bush, and cupid 

 FORMS OF Sweet Peas. (After Bateson, Saunders, and Punnett.) 



In my own experiments I obtained similar colours by crossing 

 crimson and white forms of the ordinary P. sinensis. 



In another of my Primula crosses — palm-leaved " stellata " with red 

 stems and pink flowers, crossed with fern-leaved " sinensis " with green 

 stems and white flowers, I obtained in the second generation (F2) 36 

 distinct forms, of which 34 were new in the sense that they were 

 distinct from the original parents. Of these 34 I found that 14 could 

 be bred true, while the remaining 20 were untixable, being Mendelian 

 hybrids. 



Practical Value of Unit-Factors. 



In view of the fact that a decade has hardly passed since the re- 

 discovery of Mendel's long-lost paper, the progress made in Mendelian 

 studies is indeed remarkable. Thanks mainly to the genius of 



VOL. XXXVI. D 



