38 MUTANTS AND HYBRIDS OF THE OENOTHERAS. 



distance from each other of one meter, thus allowing each specimen 

 sufficient space to express its characteristic physiological nature 

 unmodified by complex interrelations with other plants. 



The stature of Onagra nanella. — The character which most strik- 

 ingly differentiates 0. nanella from its parent-form, O. lamarck- 

 iana, is that from which it has so appropriately received its name. 

 The great difference in height between these two species may be seen 

 in the photograph (PI. XIX, fig. 1), which represents a typical speci- 

 men of each. The results of measuring 23 specimens of O. nanella 

 and 34 of 0. lamarckiana are seriated in the form of curves of equal 

 area in fig. 3. It will be observed in this figure that not only are the 

 two curves quite distinct, but that they are separated by a wide gap. 

 The discontinuity is tremendously in excess of the probable errors, 

 the distance between the means of the two curves being more than 

 forty times the sum of the probable errors of the means. The heights 

 of Onagra nanella group themselves about the mean value 22.81 ±1 .02 

 cm., with a range from 7 cm. to 35 cm., and those of O. lamarckiana 

 about the mean value 88.68 ± 0.55 cm., with a range from 77 cm. to 

 96 cm. The other constants present quite as interesting differences. 

 Although the mean height of O. nanella is only one-fourth as great as 

 that of 0. lamarckiana, the standard deviation, <r, is considerably 

 greater, being 7.26 ± 0.72 cm. in the former and only 4.76 ± 0.39 cm. 

 in the latter. As the mean and the standard deviation are combined 



in the formula ^^, to form the coefficient of variability, the latter 



constant shows an even more remarkable difference between these two 

 forms than do the means. The coefficient of variability in the height 

 of Onagra nanella is notably high, 31.84 ± 3.16 per cent., while that 

 in O. lamarckiana is quite as notably low, 5.37 ± 0.44 per cent. This 

 is particularly interesting, as will be seen later, in its agreement with 

 the fact that in most of the characters chosen for this study the 

 mutant is significantly more variable than the parent-form. 



The branching habit of Onagra rubrinervis. — The nature of the 

 branching of O. rubrinervis as compared with that of 0. lamarck- 

 iana is such as to give it a very characteristic aspect. On the average 

 about twice as many of the axillary buds develop branches and these 

 branches have a greater average length. In both species there is a 

 whorl of long lateral branches surrounding the base of the main axis. 

 In . rubrinervis the central axis bears numerous branches roughly 

 correlated with their position on the axis in such a way as to give the 

 plant as a whole a distinctly conical form. In 0. lamarckiana, on the 

 other hand, the branches arising from the central axis are less numer- 



