42 



MUTANTS AND HYBRIDS OF THE OENOTHERAS. 



145 

 150 



Fig. 6. Variation in leaf-length of Onagra 

 lamarnkiana and O. rubrinervis. O. 

 lamarckiana : R.,49to 137 mm.; M., 96.85 

 0. 42 mm. ; <r, 13.08 0.30 mm. ; C. V., 

 18.60 0.31 per cent. O. rubrinervis : It., 

 58 to 142 mm. ; M., 97.99 0.84 mm. ; (r,12.37 

 0.24 mm. ; G. V., 12.62 0.24 per cent . 



lama?'ckiana is noticeably thicker 

 than that of 0. rubrinervis. So- 

 called qualitative differences are 

 usually compounds of several meas- 

 urable characters, any one of which 

 is a wholly unsatisfactory measure of 

 the quality, while only one, two, or 

 several at most of these measurable 

 characters can be dealt with mathe- 

 matically at a time. Leaf- form is 

 a character of this kind. Not only 

 is the relation of length to breadth 

 important, but the relative posi- 

 tion of the widest part of the leaf, 

 the angles of apex and base, and 

 indeed the curvature of the mar- 

 gin at every point from petiole to 

 apex, enter as essential features of 

 leaf- form, and no tangible mathe- 

 matical expression can be devised 

 to represent it. Confining ourselves 

 to a single measurable character, 

 such as leaf length or breadth, 

 there is still another difficulty which 

 must be met. The leaf is a differen- 

 tiated organ and there is no exact 

 homology between any two leaves. 

 In a plant which has but few leaves 

 this is strikingly evident, and no 

 leaf on a stem which has ten leaves 

 corresponds exactly to any leaf on 

 another plant of the same species 

 which carries but nine leaves. In 

 plants with numerous leaves, as 

 in the various species of Onagra, 

 the degree of differentiation between 

 adjacent leaves is so slight that 

 they may be treated as homotypic 

 without appreciable error, provided 

 as nearly as possible the corre- 

 sponding parts of the various speci- 

 mens are used as the source of 

 leaves for the study. 



