MUTANTS AND HYBRIDS OF THE OENOTHERAS. 



43 



The leaves chosen for this comparative study were taken from a 

 point about two-fifths of the distance from the proximal toward the 

 distal end of the long lateral branches mentioned above as forming 

 a whorl about the base of the main axis. The number of such 

 branches borne by each specimen allowed the collection of a suffi- 

 ciently large number of leaves by taking them from a very short sec- 

 tion of each branch, so 

 that the error due to 

 differentiation is insig- 

 nificant compared with 

 the ' 'chance"variation. 

 From 20 to 30 leaves 

 were taken from each 

 of 20 specimens of each 

 species, and the width, 

 and the length from the 

 base of the petiole to 

 the apex, were meas- 

 ured in the fresh con- 

 dition. The length and 

 width were then com- 

 bined for each leaf in 

 the ratio, width -f- 

 length, this being the 

 simplest possible ap- 

 proximation to a sat- 

 isfactory mathematical 

 expression for leaf- 

 form. The curves 

 shown in figs. 6, 7 

 and 8 compare graph- 

 ically the results of 

 these measurements. It 

 will be noted on refer- 

 ence to fig. 6 that the 

 length of the leaves 

 in the two species is 

 almost identical, while 

 fig. 7 shows that with respect to the width of leaf they are quite 

 different, though not discontinuous. In consequence of the approxi- 

 mate identity in leaf-length the ratio representing leaf- form corresponds 

 closely with the leaf-width in the character and degree of overlapping 

 of its curves, as will be seen on comparing figs. 7 and 8. 



Fig. 7. Variation in leaf-width of Onagra rubrinervis and 

 O. lamarckiana. O. rubrinervis: R.,17 to 40mm.; M., 

 29.736 0.098 mm.; <r, 3.589 0.069 mm.; C. V., 12.07 0.23 

 per cent. O. lamarckiana : R., 24 to 50 mm. ; M., 87.617 

 0.137 mm. ; <r, 4.248 0.097 mm. ; C. V., 11.29 0.28 per 

 cent. 



