Page:Popular Science Monthly Volume 80.djvu/383

Rh 1910-11 in his greenhouse. They grew to enormous size (being the Russian $$\times$$ coronatus cross), but when at length they flowered, all the rays were pure yellow! This was indeed disappointing, though we thought we could readily get the red back in the next generation. In the meanwhile, the greater part of the seed was sown in our garden, though some was sent to the English naturalist Dr. A. R. Wallace, who successfully raised the plants. During the summer we went east, but before we left, we noted with hope that some of the young plants showed a great deal of purple in the stems. On our return early in August, a gorgeous sight met our eyes. The sunflowers were in full bloom, and about half were splendidly red! The reds were by no means uniform, as the accompanying figure shows, some having a ring of red, while others were suffused with red all over, and others showed only a little of the color. Indeed on a single plant there is great variation, and often heads on a genuinely "red" plant may have wholly yellow rays. This results from the fact that the red is produced as the end-result of a chemical process, which seems to be completed only under favorable conditions. Thus a "yellow" head on a red plant differs fundamentally from a true yellow in its make-up, but resembles it, owing to what may be called a lack of opportunity. The controlling factors are not well understood, but even in the case of the original plant, the last small heads of the season were almost entirely yellow-rayed.

I have said that about half of our sunflowers were of the red type. It was a matter of chance that the four grown in the greenhouse were all yellow. But how can we reconcile these results with Mendel's law? All were crossed with red: if red is dominant, then all should be red; if it is recessive, none should. The explanation is, no doubt, that the original plant was a DR, not a DD. This could come about without the existence of earlier red plants, by a variation occurring in a germ-cell, which mated, of course, with one which was normal. Consequently, the original plant, though it may have had no red parent, was in fact a hybrid (or more correctly, mongrel), and we have not yet seen a "pure" red.

The accompanying diagram represents the supposed course of events. The first line (1910) shows the original cross made by us. When YY meets RY, two combinations are possible, and are equally probable, namely, YY (yellow) and RY (red). The result observed in 1911 thus follows naturally. The third line shows what may be obtained in 1912. If the yellows are mated, we get only yellows. We have a few of these already in bud, from seed gathered from the greenhouse plants. If we cross the reds with reds (as has been done in large numbers) we must expect one fourth pure yellows, one half impure reds like the parents (I have drawn only one to save space), and one fourth "pure" reds.