ON TWIN HYBRIDS. 473 



All these numerous elementary species agree witii one another in 

 a most interesting character. Their anthers touch the stigma, open 

 themselves in the bud and produce fertilization before the flower 

 opens. The process is almost entirely finished before the insects are 

 admitted. Crosses seem to be very rare in nature, although they 

 do occur, since I collected the hybrid of 0. muricata and 0. biennis 

 in the dunes near Amsterdam, and even more than once. A simple 

 means of pure self-fertilization may be derived from this pollination 

 within the bud. I cut the buds of our 0. biennis one or two hours 

 before opening, cutting through the middle of the tube, and the pods 

 developed as strongly and produced as many good seeds as those 

 whose flowers were allowed to be visited by bees. The flowers of 

 0. Lamarckiana, 0. Hookeri, and other large-flowered species, on the 

 contrary, are not fertilized in the buds, but are in need of the help 

 of insects (moths, bees, and bumble-bees). 



The specific hybrids of this group of Onagra differ in a very con- 

 spicuous way from ordinary hybrids in so far as the reciprocal forms 

 often are not identical, but differ widely from one another i). Al- 

 though I have observed this fact in numerous cases, it is not a general 

 rule. The hybrid 0. Lamarckiana x gigas is identical with 0. gigas 

 X Lamarckiana. In the same way the species of the subgenus 

 Euoenothera crossed with those of Onagra yield uniform hybrids. 

 I crossed the 0. Sellowii with 0. biennis, 0. muricata and 0. La- 

 marckiana and observed their reciprocal hybrids to be identical. 



Within the group of the Onagra's, however, the reciprocal hybrids 

 are in most cases different and with a few exceptions are more similar 

 to their father than to their mother. They are, as it is called, patro- 

 clinous. So it is, for instance, with the reciprocal hybrids of 0. muri- 

 cata and 0. biennis in their subspecies from our European dunes. 

 The 0. muricata x biennis looks like 0. biennis and might be mistaken 

 for it, since it is broad-leaved and large-flowered. The biennis x 

 muricata, on the other hand, is narrow-leaved and small-flowered 

 like its male parent. During the summer of 1907 I cultivated the 

 first and second generations of the latter type and found them uni- 

 form, but in almost all respects different from the muricata x 

 biennis. Especially the flowers are of a deep yellow and strongly 

 scented, while those of 0. muricata x biennis are pale and faintly 

 scented. 



This difference between the reciprocal hybrids is also clearly 



t) Die Mutationstheorie 2:471. 



