Nature 102(2550): 25-26 (Sept 12, 1918)
Hybrid Sunflowers
University of Colorado, Boulder, Colorado, August 6.

IN crossing the different species and varieties of Helianthus some peculiar results have been obtained. The crosses referred to have all been made by my wife at Boulder, Colorado, and the results may, be classified as follows:—

(1) The varieties of Helianthus annuus (including H. lenticularis, regarded by some botanists as a distinct species) when crossed together produce plants which are as fertile as the parents. In some of the mongrel varieties there is, however, a marked deficiency of pollen.

(2) The annual species of sunflowers (typical Helianthus) crossed together are quite fertile, but the hybrids are themselves nearly sterile. H. annuus has been crossed with three species, H. argophyllus, H. petiolaris, and H. cucumerifolius.

(3) The annual species can rarely be crossed with the perennial, and when this occurs the offspring closely resemble one or the other parent species. One such hybrid was recorded in the "Standard Cyclopedia of Horticulture" (vol. vi., 1917, p. 3281) as between H. pumilus and H. annuus. Renewed study of the living plants this year convinces me that this is an error; the perennial parent was, in fact, H. subrhomboideus. Both species occur here, and Mrs. Cockerell, at the time of making the cross, did not distinguish between them. Morphologically they are especially distinguished by the fact that H. subrhomboideus has underground migratory branches, by means of which it spreads, while H. pumilus is strictly stationary, reproducing only by seed. The hybrid, which closely resembles H. subrhomboideus (though this was the pollen parent), but is much larger, with larger broad leaves, has small or short underground branches, but, nevertheless, is stationary. That is to say, the migrators are present, but the plant does not spread by them in all directions as do the true migratory forms. Comparing the details of structure, I found that the ray-florets of the hybrid were quite without pistils, whereas these were well developed (though not functional) in the H. subrhomboideus. However, further investigation showed, to my surprise, that some heads of the wild H. subrhomboideus had the ray-florets wholly without pistils. The involucral bracts of the hybrid are more distinctly pointed than those of H. subrhomboideus. In other cases attempts to cross annuals with perennials have resulted in total failure, as has happened when crossing H. annuus on H. pumilus, in attempts to repeat the cross described above, which was erroneously interpreted. In other cases seeds were obtained from the pollen of perennials used on annuals, and the resulting plants were indistinguishable from the annual parent. Seeds received from Mr. L. Sutton from England, representing the F2 of a cross between the red H. annuus and the perennial H. rigidus, also gave plants entirely of the annuus type.

Babcock and Clausen, in their recent (1918) admirable work, "Genetics in Relation to Agriculture," have (chap. xii.) discussed those remarkable cases in which the F2 generation of a cross gives plants resembling the original species crossed, with greater or less fertility. A very ingenious and plausible explanation is given. Collins and Kempton recently found that in crossing two distinct genera of grasses, Tripsacum and Euchlaena, they obtained plants agreeing with the pollen parent, the Euchlaena. They call this patrogenesis (Journal of Heredity, vol. vii., No. 3, 1916). One of the explanations offered by them is that the male nucleus may have developed in the ovary to the complete exclusion of the female, "representing in a way the counterpart of parthenogenesis." It appears quite possible that in some hybrids, and perhaps other heterozygous forms, particular pairs of homologous determiners do not both function or develop, so that in respect to certain characters the organism is simplex, not in the sense of the old "presence and absence theory," but in the sense of not being a hybrid at all in respect to particular features.