Sorting and blending of unit characters, 225 



development at different periods. Soil conditions such as fertility, moi- 

 sture content, etc., probably play a part. The formation of wide spread- 

 ing branches, particularly branches which grow for a considerable 

 distance nearly parallel with, or on the surface of the ground, appears 

 to bear some relation to temperature, especially to the seasonal cold of 

 late summer and autumn. 



The autumnal cold seems also to have a gxeat influence in arresting 

 the full development of petal characters. The petals, are smaller and 

 the form changes. Those of pycnocarpa and pycnella lose the notched 

 character and the angles become more rounded. Those of nutans and 

 nutella become smaller also. In this depauperate condition the petals of 

 the two parents and of the two hybrids are more difficult to distinguish. 

 Depauperate petals may appear also on old plants which are nearly 

 mature. Another variation might be mentioned here, which all students 

 of the evening primroses probably have observed. During the autumn, 

 the flowers, instead of opening at nightfall, tend to open at any time 

 of day, and remain fresh for a longer period. 



The behavior of Oe. nutans and pycnocarpa compared 

 with other wild species of Oenothera. De Yries (30 — 59, 1913) 

 has shown that the production of but one form of hybrid in the first 

 generation of a cross between old, or wild species of Oenothera, of the 

 section Onagra, is the general rule. But the hybrids from reciprocal 

 crosses are often different in type (De Vries, 471, 1903; 30, 1913). 

 The hybrids in these reciprocal crosses often resemble the pollen parent 

 strongly, i. e. they are strongly patroclinous {Oe. biennis X muricata, etc.). 

 The parents of such crosses he terms heterogamous species. A few of 

 the wild species he has shown to be isogamous, i. e. the single hybrids 

 of reciprocal crosses are identical {Oe. hooheri, cockerelli and strigosa, 

 see de Vries, 59, 1913). The behavior of Oe. nutans and pycnocarpa 

 in reciprocal crosses does not conform to either of the types of behavior 

 found by de Vries to be characteristic of the heterogamous or isogamous 

 species which he has studied. So far as the vegetative characters are 

 concerned patrocliny cannot be ascribed to either of the hybrids, for the 

 characters are either blended (in hybrida nutella) or sorted and distributed 

 about equally from the parents to the two segregate hj^brids {pycnella 

 and tortuosa). 



In the segregates, whüe certain characters resemble those of one 

 parent, other characters resemble those of the other parent and it is diffi- 

 cult to say which set of characters dominates. But even if one hybrid 



Induktive Abstammungs- und Vererbungslehre. XVI. 15 



