156 THE COTTON PLANT IN EGYPT chap. 



members in 1910. Most of the plants chosen for this 

 purpose were — for subjective reasons — short in the stem. 

 Plotting the aggregate height of all the normal F3 plants 

 in 1910 against the same date in 1909, we see the same 

 modes reappearing, in spite of the differences of soil and 

 season (Fig. 58). These modes become less markedly 

 coincident amongst the taller F3 plants, but the general 

 result indicates that such modes are due to definite 

 constitutional causes, inherent in the plants themselves. 



The form of the leaf.* — The component characters of 

 leaf-form appear to be the length of the mid-rib {L) the 

 distance from sinus to petiole {S), and the angle of the 

 sinus from the petiole relatively to the mid-rib (/s). 



Two other components which we have not examined in 

 detail for more than one family are, the angle made by the 

 first lateral vein with the mid-rib, and the presence or 

 absence of a second lateral vein and lobe. 



The three first components are concerned with the form 

 of the central segment alone, but even in this respect we 

 find more problems than we can solve. 



Taking the length (X) first, we have already seen that 

 it is correlated with height of stem, in some obscure 

 manner. King Upland, with a mean leaf-length of 75 mm., 

 crossed with Charara, the mean leaf-length of which was 

 about 135 mm., gave an F, with leaves rather shorter than 

 Charara, and this by selfing produced an Fj which ranged 

 from 70 to 195 mm., with slight indications of modal 

 grouping; most of the plants lay between the parental 

 measurements (Fig. 59). This curve was dissected in 

 various ways, thus, the plants with the "continuous" 

 growth-habit had larger leaves than the rest, but although 

 none of these plants had extremely small leaves, yet 

 moderately small leaves were found as a detached group. 

 Further dissection of this group never succeeded in 



* See Leake, H. M. (1) (3). 



