knowledge of heredity in pisum. 107 



6. — Habit of Growth. 



The characters included under this heading require further 

 stud3^ Mendel showed conclusively that the very tall habit 

 is dominant to the very dwarf. But dwarf plants vary very 

 much among themselves in habit and tall ones probably not 

 much less, whilst other plants exist which cannot be placed in 

 either category. The study of these characters is rendered 

 difficult by the great individual fluctuation which plants of 

 the same strain exhibit, and (what is perhaps to a large extent 

 the same thing) its great susceptibility to the influence of 

 external conditions. 



The difference between ordinarj'^ tall and dwarf depends 

 much more upon the length of the separate intemodes than 

 upon the number of these, and the distinction is readily 

 recognizable in quite young seedlings. Considerable differences 

 in the number of internodes also exist however, but the 

 inheritance of these differences has never been adequately 

 studied. In the case of some crosses indications have been 

 observed of phenomena which appear complex, but which 

 might doubtless be elucidated by adequate numerical treat- 

 ment. 



7. — Duration. 



This is another feature in which so many fluctuations occur 

 that it is doubtful whether a complete study can profitably 

 be made of it. It is only mentioned here for the purpose 

 of recalling the circumstance that a marked correlation or 

 coupling has been observed between the time of flowering and 

 the colour of the flowers, &c., in F2 from a cross between a 

 white and a coloured strain (13). This is the only case known 

 in the pea of coupling between characters having no obvious 

 relation to one another. 



General Remarks. 



Mendel's seven pairs of allelomorphs have now been multi- 

 plied with certainty to 13, whilst indications are to be observed 

 of others, which are less definite in their manifestations, but 

 will probably be found to amount to a still greater number. 



