CH. ix HEREDITY 151 



plotted from those plants only which had all germinated 

 on the same day, being as exactly comparable as they 

 could possibly be, show a very definite segregation of the 

 two parental first-internode -lengths from one another, and 

 from a central mode. Whether the segregation is simply 

 in a ratio of 1:2:1, or whether it is more complex, we 

 cannot pretend to say. It is clear, however, that in com- 

 mencing to analyse the height of our hybrids, we have 

 first to take into account the internode length. 



This alone is not sufficient for us. If internode-length 

 was the only factor, then the height of the Upland would 

 be about half that of the Egyptian, whereas the early 

 height of the former is rather greater than that of the 

 latter. Again, on June 18th, in Fig. 56, the same Upland 

 strain whose internodes we measured is equal in height to 

 the same Sultani, both being grown side by side on the 

 breeding-plot. Hence the growth-rates must have been 

 much the same, and the Upland merely produced more 

 internodes in the same time. Comparing these heights 

 with that of the F a on June 18th, we find that the 

 latter is nearly twice as tall. The internodes are no 

 longer, so that the great height of the F! must be due to 

 a greater growth-rate. 



The rate of growth, due to imperfectly known and 

 constitutional causes, is thus the second factor in the height 

 of the stem. 



From these typical figures for June 20th, one hundred 

 days after sowing, we might declare that tallness was 

 dominant over shortness, whatever the components might 

 be. If we follow the height-curve on to the end of 

 September (Fig. 56) we shall see that this sweeping 

 assertion would also be untrue, for the growth-rate of the 

 F! has slowed down, though not as much as that of the 

 Upland, while the Sultani parent is still growing 

 steadily. 



Hence the third factor in height of stem is the amount 



