So far good: but now a difficulty arose. On looking up the literature, we found that Dr. G. H. Shull had experimented with sunflowers, and had found them invariably self-sterile. That is to say, no sunflower would produce seed with its own pollen, even though it came from a different head. Here was a dilemma; there was, so far as we could tell, only one plant of the red sunflower in the world, and this could not be self-fertilized! The only thing to do was to make crosses with ordinary sunflowers, and see what would come of it. It must now be explained that, aside from the color of the rays, there are many kinds of sunflowers. Putting aside the perennial species and annuals like the small "cucumber-leafed" Helianthus debilis, there are several types closely related to the "common or garden" sunflower, Helianthus annuus. All are called Helianthus, which is simply Greek for sunflower. The red sunflower found at Boulder belongs to the prairie species, called by some botanists Helianthus lenticularis, by others simply a variety of Helianthus annuus. It is perfectly fertile with the garden strains of annuus, but has a number of marked characteristics. Less robust than the cultivated forms, it branches very freely and produces numerous relatively small heads of flowers. The center, or disc, which is yellow in the big "Russian" sunflower, is "black," or strictly speaking a dark purplish-red.
The red sunflower was crossed with the Russian, the wild lenticularis, and with a plant which we took to be a cross, Russian and lenticularis. All the crosses were made by Mrs. Cockerell, who has in fact done all the work on the red sunflower. The accompanying illustration shows some of the heads covered with bags to protect them from the bees and birds and save the seeds. Crosses could be made either way, that is, using "red" pollen on the other sorts, or other pollen on the red. It was necessary in each case to "bag" the head before it came into flower, and to watch very carefully whenever the bags were off for pollination or inspection. The bees are tireless in visiting sunflowers, and scarcely a moment seems to pass during the warm part of the day when an unprotected head is not visited. A single bee might easily spoil an experiment by bringing unaccounted-for pollen, while later small finches were present in flocks to eat the seeds. All this work was time consuming and laborious, but there is no other way if exact results are desired. Taking the principal characters, as cited above, we may tabulate the two main crosses as follows. The name coronatus, now used for the red sunflower, was proposed in Science, 1910, and was suggested by a certain resemblance to the sun in eclipse, showing the corona. The sign X signifies a cross.
|Red rays||yellow rays|
|Dark disc||dark disc|