154 THE COTTON PLANT IN EGYPT CHAP. 



of not less than 0*622. The initial growth-rate of the 

 F 2 plants therefore ranges at least as high as that of the 

 F! and at least as low as that of either parent. (See also 

 Fig. 57.) 



C. Over and above this incomprehensible factor of 

 " growth-rate," which needs the repetition of such experi- 

 mental work as* we have described under " the Indi- 

 vidual " on every plant of a large F 2 before we can 

 interpret it, we have also the change in growth-rate, 

 affecting the height of the plants after the end of June, 

 or earlier, which we have endeavoured to interpret in 

 terms of specific liability to thermotoxy. On classifying 

 the individual growth curves in a family of 179 F 2 plants, 

 we find that they were grouped as follows : 



Continuous growth, like Egyptian parent 52 



Doubtfully continuous 29 



Doubtfully like F, 27 



Semi-continuous, like F, 30 



Doubtfully like Fj, being semi-discontinuous ... 30 



Like American parent 9 



Thus a small percentage of plants reappears in F 2 with 

 the high thermotoxic susceptibility of the American 

 Upland parent. It might be pointed out that the figures 

 could be fitted to a 9:3:3:1 ratio, two pairs of allelo- 

 morphs being involved, namely, presence and partial 

 absence of " ^-production " and of " ^-removal " ! 



There does not appear to be any necessary correlation 

 between any of these factors in their distribution amongst 

 individuals. A plant with a low initial growth-rate may 

 have the habit of continuous growth and short internodes : 

 strains of such plants have been bred out to F 4 and have 

 remained pure. Such plants are short in the stem. 

 Shorter still are those plants which have the discontinuous 

 growth-habit, with short internodes and a low growth-rate. 

 At the other extreme we meet plants with a high growth- 

 rate, long internodes and continuous growth-habit. Con- 



