xv] Variation 28 1 



little prostrate plants shown in Fig. i. Crossed with 

 ordinary tall plants they give a normal Mendelian result, 

 the tall being dominant. The Bush is a distinct type. It 

 is only half or two-thirds the height of the taller, and its 

 habit of growth is peculiar. The stems are erect, thin and 

 wiry, branching profusely, whence the description Bush is 

 derived. Bush again is an ordinary recessive to tall. But 

 Mr Punnett and I found, a good deal to our surprise, that 

 the cross Bush x Cupid gives F^ tall, i.e. reversionary 

 (Fig. 36). Though we cannot venture on any surmise as to 

 the chemical causation of this phenomenon it is obvious 

 that once again the reversionary character, here the tallness, 

 is a compound character due to the meeting of comple- 

 mentary elements. The allelomorphs are 



Dominant. Recessive. 



1. Tallness (T). Dwarfness (/). 



2. Prostrate habit : Erect : branching (/). 



non-branching (P). 



In /% therefore a new type occurs, namely an erect or 

 Bush-like Cupid (Fig. 37), and the /% series is 



9 Tall (TP) : 3 Bush (7 : 3 prostrate Cupid (tP) : i erect 



Cupid 



The " height " of Sweet Peas is evidently therefore a 

 compound character and in part depends on the existence 

 of a factor which suppresses or inhibits that stimulus which 

 otherwise would compel it to branch. A successful analysis 

 of the physiological processes concerned in this series of 

 phenomena would be a significant addition to plant 

 physiology. 



The analysis of compound structural characters may 

 confidently be expected to lead to the recognition of 

 numerous examples comparable with these two. 



With the progress of such analysis other examples will 

 certainly be encountered illustrating those curious inter- 

 relations between the factors spoken of under the names 

 Coupling and Spurious Allelomorphism. In the existence 

 of such phenomena we meet evidence that the central 

 problem of genetics is in part a geometrical one. Perhaps 



