96 THE COTTON PLANT IN EGYPT CHAP. 



environment, for later buds on old stems may develop 

 leaves of juvenile form. 



As regards the method of observation, we take three 

 adult leaves from every plant during August September. 

 Each one is laid under a graduated glass plate, from which 

 its length, and two angles, are read. From these we can 

 complete any other constituents of the triangle thus 

 recorded. This triangle gives us the form of the central 

 segment, which is limited lengthways by the petiole and 

 the central tip, and laterally by the sinus at which the 

 leaf-margin bends sharply back on the lateral seg- 

 ment. 



The position of this sinus is the essential feature. On 

 examining growing leaves we find that the sinus moves 

 outwards along a constant angle as the leaf enlarges, at a 

 rate which is directly proportional to the growth of the 

 leaf in length, consequently the sinus-angle relatively to 

 the mid-rib, either from tip (Z. 4 ) or origin (Z. 8 ) shows the 

 minimum fluctuation. More fluctuation is necessarily 

 shown by the length of the line from petiole to sinus pro- 

 portionately with the total length, but both are specific. 

 If we neglect such additional factors as curvature of the 

 segment-margin, plication of the lamina, &c., this method 

 of expression is comprehensive. A check on the result is 

 obtained by plotting sinus-positions in a pure strain, 

 relatively to the tip or to the base. The resulting diagram 

 resembles the side view of a choked shot-gun discharge, 

 passing along the angle-lines, the centre of the pellets 

 defining the mean sinus-length. The P.E. of these angles 

 in Egyptian strains is about 3'0 per cent. 



The number of leaf-segments, or rather the development 

 of the second pair of lateral segments, has been mentioned 

 above as dependent on ontogeny. It is fairly definite 

 under given conditions, same strains refusing to produce 

 the extra pair. 



Perhaps it is worth while to mention that such presum- 



