Popular Science Monthly/Volume 80/April 1912/The Red Sunflower
|THE RED SUNFLOWER|
By Professor T. D. A. COCKERELL
UNIVERSITY OF COLORADO
ONCE upon a time, in England, a certain bishop visited a Sunday-school. Being asked to question the children, he inquired of a small and timid boy, "Who made the world?" Completely rattled, the child made no answer. The bishop asked a second time, and, again getting no result, exclaimed in some wrath, "Is it possible, my dear boy, that you don't know who made the world?" The youngster burst into tears, and declared earnestly, "Please, sir, I didn't do it, indeed I didn't do it!"
I have found this story useful in the discussions on the genesis of the red sunflower. The first question people ask is even more direct than that of the bishop, "How did you do it?" The answer is, "We did not do it, but—the author of the world is also the author of the red sunflower."
My wife and I have lived in New Mexico and Colorado for many years, and have seen, I suppose, millions of sunflowers. So far as we know, we had never seen a red one until 1910, nor had any one else, excepting some horticultural forms of a kind quite different from the ordinary plant. The summer before last, in Boulder, Colorado, there was a group of sunflowers growing by the road about a hundred yards from our house. Having occasion to cross the road, my wife noticed what she supposed to be a large butterfly, presumably one of the silverspots, on a sunflower head. Glancing at the plant a little later, she was surprised to note that the butterfly had not moved, and approaching nearer to satisfy her curiosity, found the red sunflower. There was no butterfly; the red appearance was due to the rays, which were deeply suffused with a fine chestnut red. There was only one plant, and this close to the road, where hundreds of people passed daily. Already some one had picked a head as a curiosity. Evidently it was not safe to leave the plant where it was; it would almost surely be destroyed. We had had no experience in transplanting sunflowers, especially in full bloom, so we tried first with one of the common kind. Succeeding well, we carefully dug up the red one, and removed it to our garden. Here, on bended knees, it was carefully tended, and a contrivance was arranged to protect it at first from the direct rays of the sun. It soon recovered from the shock, and proceeded to bloom and go to seed in a normal way.
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 |
|Branched habit||branched habit|
|Small heads||small heads|
|(2) Coronatus||Annuus ("Russian")|
|Red rays||yellow rays|
|Dark disc||yellow disc|
|Branched habit||unbranched habit|
|Small heads||large heads.|
We later found a plant of lenticularis showing a little red on the rays, and of course used this in a cross.
Could we predict the result of these crosses? Yes, to some extent. Could we regain the red as it was before the cross? Yes, no doubt, but in order to explain how, it is necessary to digress.
During the sixties, Gregor Mendel, Prälat at Brünn in Moravia, experimented with plants, especially garden peas. He was the first to appreciate the necessity of following up crosses for several successive generations, tabulating the results in each case, and ascertaining the numerical proportions of the differing forms resulting. He also took pains to consider the different sets of characters separately, treating them statistically as if they were different organisms. Working in this way, Mendel discovered that when two varieties are crossed the resulting hybrid is frequently not intermediate, but resembles one or the other parent. In other cases, when the hybrid, as a whole, seems intermediate, the several characters are nevertheless found to correspond with those of one or the other parent. When this sort of thing occurs, the character which comes uppermost in the cross is said to be dominant, the one which remains latent or hidden is called recessive. Inasmuch as fertilization results from the fusion of the germ-cells of the two parents, it is evident that each individual hybrid must contain material derived from both, although only the characters of one parent may be visible. Now Mendel found that when hybrids obtained as described were crossed together in the next generation he got, in simple cases, three of the "dominant" type to one of the "recessive." Of course the proportions would not be always thus, but whenever the number of cases was large they approximated so closely to the three-to-one ratio, that he became convinced that this was no accident. A simple theory was formulated, according to which the results arose from the chance combination of the elements in the germ cells. We may now make this clearer by a diagram in which D stands for the character which is dominant, E for that which is recessive.
First cross, DD ER
This is written DD and RR, not simply D and R, because we are supposing that each individual is pure for the character involved, that is, has received D or R from each parent.
First filial generation DR DR DE DE, as many as there may happen to be. These are written DE because each gets D from one parent (which has nothing else to give) and of course E from the other. Now in the next generation each parent contributes, not its whole "D," but one or the other, according to the laws of chance. Accordingly, DR DR may produce a DD, or a DR, or a RR, and as a matter of fact, they do so. Why should there be any particular numerical proportion? If we put black and white balls in a bag, and draw them out in pairs at random, the chances are equal that we shall get two alike, or two different. It is so with our crosses. The cases in which we get two alike may be of two kinds, both black or both white, or in the case of the crosses, both D or both R. The cases in which we get two different are necessarily alike, black and white, or D with R. Hence, according to the law of chance, we expect in the third generation the following:
1. Both alike, DD and RR.
2. Not alike, DR and RD, which are the same.
Now we have seen that because of dominance R does not show when D is present, so that a DR looks like a DD. Consequently, of the above four cases, three show the dominant character, and one (RR) shows the recessive. The whole diagram may now be reconstructed:
1. DD RR (original cross).
2. DR DR DR DR (first filial generation).
3. DD DR RD X RR (second filial generation, or grandchildren). How can this be confirmed? Obviously, if the facts are as here given, the DD and the RR of the third line are now pure, in spite of the fact that the DD had an RR grandparent and a DR parent, and the RR a similarly complicated ancestry. Take a number of these pure types, now called "extracted recessives" and "extracted dominants," and breed them separately, the DDs with DDs, and the RRs with RRs, and they will breed true, and their descendents will forever remain true, unless contaminated by a cross, or some new variation arises. The DRs, however, when bred together, will again produce the "three-to-one" results, just like their parents. Consequently, it is possible to extract a pure strain out of an impure one, a fact of tremendous scientific and practical importance.Mendel's results were published in Brünn in 1866, but attracted little or no attention. They never became known to Darwin, who would have immediately perceived their importance. In 1884, when Mendel died, no one had the slightest idea that his name would ever be familiar to scientific workers, though Mendel himself used to say "Meine Zeit
wird schon kommen!" In 1900 three European workers almost simultaneously discovered Mendel's paper, and to-day "Mendelism" is talked of everywhere, and books are written upon it.
Mendel himself could hardly have foreseen the wide application of his theory. It has been applied to animals and plants with like success, and scarcely a month passes without the publication of new "Mendelian results." In practical breeding, it has opened up a new era, and it would he difficult to overestimate the importance of the results certain to be obtained within the next fifty years. It has even been found that Mendel's discovery lights up the way toward the improvement of the human race, and it may well be that many of us now living will see the day when Mendelian questions will enter the field of practical politics.
As experimental work progressed, it was found that many complications arose, so that it was often difficult to interpret the results. Without going into these matters in detail, we must note that frequently the first cross "DR" is not like either parent, that is to say, dominance is not complete. Indeed, experienced breeders say that they can
nearly always tell an impure form from a pure individual, although at first sight they may look alike. Critics of Mendelism were at first inclined to think that the failure of strict "dominance" invalidated the theory, but this is by no means the case, as the elements separate out in the third generation, just as before. The only difference is that as one can tell the DD from the DR, in the third generation we have three visible types instead of two, in the proportions 1, 1, 2.
Now to return to the sunflower; the first thing we were anxious to know was, will the red be dominant? From analogy with other cases, we thought it would. Being impatient, we obtained permission from our friend Mr. Knudsen to grow a few plants during the winter of 1910-11 in his greenhouse. They grew to enormous size (being the Russian 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. I have drawn the "pure" red very dark, on the supposition that it will be visibly more highly colored than anything yet seen, although this may not prove to be the case. If it is distinguishable, we shall then permanently isolate the red without further trouble; if it is no different from the impure reds, it will only be possible to separate the pure strain by noting the results of numerous crosses made at random.
Last year we made various new crosses, especially with the great double "chrysanthemum-flowered" variety obtained from Dreer of Philadelphia. If we can get this red, as we doubtless can, it will be a striking plant, though perhaps less attractive than the single kinds. In tabulating the characters crossed, I enumerated not only the ray color, but also the color of the disc, the size of the heads, and the manner of growth. In the coronatus X lenticularis cross, everything except ray color is the same on both sides, so there is nothing to be noted. In the coronatus X Russian cross, it is quite otherwise. We find that dark disc is uniformly dominant over yellow; the size of the heads in the cross is greater that that of lenticularis, but much less than that of the Russian; and the manner of growth is intermediate, at first simple like the Russian, but eventually branching at the top. It is evident that there is some correlation between the manner of growth and the size of the heads, as a plant could not well support more than one big head of the Russian type. A certain incompatibility between the two varieties seems to be indicated by a number of monstrous (fasciated) plants.
The accompanying diagram shows the Russian coronatus cross in relation to growth form, and in the third line the expected outcome in 1912.
In Dreer's Catalogue of 1911 appears the following:
This is a garden variety of the small Heliantlus debilis, which we understand originated in Italy. We purchased seed, which produced good plants, but showing hardly any red, and that of a dingy color. We hear from others that this variety has been a great disappointment, but are told that the originator is still working on it. In any event, it is an entirely different plant from ours.
A famous discovery somewhat parallel to that of the red sunflower is that of the Shirley poppy, which is described in Bailey's "Cyclopedia of American Horticulture" as "the loveliest of all poppies" and "one of the finest contributions to floriculture ever made by an amateur." The Rev. W. Wilks, of Shirley in England, gives the following account of his discovery and development of this poppy. This was written without any knowledge of Mendelism, and can not at once be reduced to Mendelian terms. It is evident, however, that the Shirley is a minus variation (loss of black pigment), and may be expected to behave as a recessive.
It is certain that many more good variations would be discovered if trained people were everywhere on the lookout for them, and it must be remembered that among the cereals, for example, a good new strain will not be a conspicuous object like a red sunflower. There is here a fascinating field for amateurs, with possibilities of vastly increasing the wealth of mankind, or adding beauty to his gardens. Aside, however, from the discovery of new things, there is an almost unlimited field open for the crossing of known varieties, and their recombination along Mendelian lines. Any one who has a garden may do this work, and if nothing else comes of it, it will certainly give much pleasure and an insight into some of the most interesting biological problems of the day.
- ↑ The matter is complicated by the fact that the "recessive" condition may result from the simple absence of the dominant factor; or one factor, when present, may inhibit or else hide a second. For the latter class of cases the terms epistatic and hypostatic have been proposed by Bateson.