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In the case of Oenothera muricata x biennis the differentiating units reduce the fertility to a low degree, threatening the offspring with almost complete infertility and extinction. But then we do not know whether these characters are really units, or perhaps only seemingly so and are in reality composed of smaller entities which as yet we are not able to segregate.
Two cases of sudden mutations have come to my knowledge, producing this same anomaly in allied species. One has been already alluded to; it pertains to the common evening-primrose or Oenothera biennis, and one is a species belonging to another genus of the same family, the great hairy willow-herb or Epilobium hirsutum.
O. cruciata has a purple foliage, while biennis and lamarckiana are green, and many of the hybrids may instantly be recognized by their purple color. The curious attribute of the petals is not to be considered simply as a reduction in size. On anatomical inquiry it has been found that these narrow petals bear some characteristics which, on the normal plants, are limited to the calyx.
The parents were the common evening-primrose or Oenothera biennis and of its small-flowered congener, Oenothera muricata. These two forms were distinguished by Linnaeus as different species, but have been considered by subsequent writers as elementary species or so-called systematic varieties of one species designated with the name of the presumably older type, the O. biennis.
I propose to designate both new forms by the varietal name of cruciata, or cruciatum. Oenothera biennis cruciata was found in a native locality of the O. biennis itself. It consisted of only one plant, showing in all its flowers the cruciata marks.
As a further consequence the O. biennis opens on the same evening only one, two or three flowers on the same spike, whereas O. muricata bears often eight or ten or more flowers at a time. In this respect the hybrid is similar to the pistil-parent, and the crowding of the broad flowers at the top of the spikes causes the hybrids to be much more showy than either of the parent types.
It would be worth while to see whether the climate of California, where neither O. lamarckiana nor O. biennis are found wild, would not exactly suit the requirements of the new species rubrinervis and gigas. A fixed hybrid between O. cruciata and O. biennis constituting a species has been in cultivation for many years.
The hybrid has the broad leaves of O. biennis during most of its life and at the time of flowering. Yet small deviations in the direction of the other parent are not wanting, and in winter the leaves of the hybrid rosettes are often much narrower than those of O. biennis, and easily distinguishable from both parents. A third distinction consists in the density of the spike.
But it is very difficult to distinguish between them, and if biennis and hybrid flowers were separated from the plants and thrown together, it is very doubtful whether one would succeed in separating them. The next point is offered by the foliage. The leaves of O. biennis are broad, those of O. muricata narrow.
The blade shows numbers of convexities on either surface, the whole surface being undulated in this manner; it lacks also the brightness of the ordinary evening-primrose or Oenothera biennis. These undulations are lacking or at least very rare on the leaves of the new laevifolia.
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