More Thoughts About Hybridizing
By G. G. Nearing
At Mr. Leach's suggestion, our mock feud is now discontinued. Some of our friends thought it undignified, for actually, of course, we are the best of friends. Both of us enjoyed the comparatively uninhibited sparring which, we trust, called attention to some very real differences of opinion on rhododendron breeding. A more restrained discussion of these opinions is still in order.
Mendel, whose work I studied rather carefully about 1912, started the classic method of obtaining variations, by crossing two species, then selfing the offspring (in the case of peas), or again crossing two of the original offspring together. This can result in the redistribution of the characters of the original parents in various combinations.
When two species are crossed, the immediate result is a rather uniform lot, and you either have what you are looking for, as in the case of Loderi, or you have made the first step toward it, as in most other instances. So far Mendel and the geneticists and the rest of us must all agree.
Now comes the first exception. Nobody knows what a species is. Mendel was working mostly not with species, but with strains or varieties found in variable cultivated plants. Later investigations showed that whether a strain, variety or species was used, the results would be the same, provided the race was stable and uniform. But suppose you take a named species like Rhododendron haematodes, which is most unstable, varying in habit, flower color and other characters, such as leaf indumentum, so perplexingly that botanists don't know whether they have one species or a dozen, and with characteristic generosity (or what is it?) have given us the dozen, and still left us an extremely variable group within each of the dozen. In this case your first cross, say with R. catawbiense, will not be uniform.
The practical hybridizes will not gnash his teeth at this point. He doesn't want uniformity in rhododendrons, because he is not going to propagate by seeds any fine form he originates. He is going to make a clone of it. Neither is he going to wail because Mendel has deserted him. Not knowing where this catawbiense x haematodes belongs, because he doesn't know what R. haematodes really is, he can't fit the hybrid into any genetic formula.
Mr. Leach would inbreed his plant of R. haematodes before crossing it with R. catawbiense. Well, he is a young man. Perhaps there are shortcuts which we older fellows can take. Some day when I have time, and have learned a little more, I will sit down and write a book about these shortcuts.
But first let us examine the obsession for inbreeding which now possesses the genetic fraternity. Inbreeding is the first step in a process which has already enriched our country by many billions of dollars, and has placed our agriculture on a new high plane. The second step is cross breeding, the result, hybrid vigor. It has worked wonders with corn, so Mr. Leach and Mr. Morrison are trying it on rhododendrons. Will they get a billion dollars? Conservative by temperament, perhaps, I'll make either or both of them a generous offer for their results, sight unseen. I'll offer them two cents. No, I hedge. I'll make it one cent.
You see, I have within sight of my window an excellent example of hybrid vigor, obtained from Joseph B. Gable, a cross between R. catawbiense, a fairly stable species. and Gable's R. discolor, which takes its own pollen freely and is evidently an inbred plant. The progeny is awe-inspiring. If it grew much faster, I should have visions of needing to remove the neighboring oak trees to make room for it. It is stiff and straight and fat and ugly, the trunk showing like a broomstick because the branches grow so far before they produce any leaves. Happily the flowers wasted on this unsightly plant are not outstanding. Whether its hardiness is superior to what would be expected from the two hardy parents, might be revealing to know. I value it highly for future breeding, not because of its vigor, which is a curse to it, but because of its parentage.
A breeder will do well to look at any first generation hybrid with X-ray eyes, to see, not what it is, but what both its parents were. Even hybrid vigor will not spoil the usefulness of across between two good species, for if properly handled, a later generation can bring out the qualities of the original species minus the hybrid vigor.
Hybrid vigor means money in corn because it increases the number of pounds of grain per acre which can be reproduced. There may be some form of vigor which will mean money in rhododendrons, possibly superior hardiness, but from a number of examples I have seen, it is not likely to bring beauty, and in horticulture of ornamentals, beauty is money.
The form of vigor which may bring beauty is likely to be found in triploids. Now that we have a preliminary study of rhododendron chromosomes presented in print by the Royal Horticultural Society, we may profit from genetics, not by shackling ourselves with its supposed laws, which still change with considerable frequency, but by studying its findings for suggestions. The chromosome counts made so far are utterly inadequate to use as a foundation for binding formulae, but a mere reading of the list proved most illuminating for me. Counts were made on two specimens of R. fastigiatum, one diploid, the other tetraploid. In case you haven't had time to follow up these genetic terms, the diploid is the normal number, tetraploid double the normal. It is usually difficult to cross a diploid and a tetraploid, but when the cross is made, it will usually result in a triploid. The triploid will probably be sterile, but is likely to produce great numbers of flowers, and may have a form of vigor which will increase hardiness without loss of beauty.
This may explain 'Ramapo', a cross between fastigiatum, which may have been tetraploid, and carolinianum, supposed to be diploid. 'Ramapo' is sterile, vigorous, hardy and floriferous, probably a triploid. Many hybrids between species of different series prove sterile. That between racemosum and keiskei commonly bears no stamens whatever, and sets very few seeds, but lacks hardiness.
Why can't we have more triploids? Because of their sterility it would be unnecessary to remove the faded flowers, which could not set many seeds, if any. Think what that would mean in man-hours of labor. Well, triploids can't be planned, though they can be invited incidentally by trying to make crosses between diploids and tetraploids. The counts so far do not indicate anything but diploids in the large leaved species, those which come to mind first when we speak of rhododendrons. However there may be and probably are polyploids among the hybrids, though most of them cross so freely that diploids are undoubtedly far more numerous. Further scientific investigation can clear up the point. However, don't try it yourself unless you have a very modern, high-power microscope, superb technique, and a great deal of time.
Returning now to the idea of crossing two species together, then selfing or crossing the F-1 generation ("shaking Mendel's hat" as I like to style it) to secure the recombination of the various characters of the two parents, I don't know of any two species which contain all the good qualities I am looking for in a hybrid. I want three or four species to complicate the situation, giving an almost infinite number of possible combinations.
It so happens that of the seed I am sowing this season, aside from the species themselves, of which I enjoy building up a collection, there are about equal numbers of the types (a x b) x (c x d), (a x b) x c, and (a x b) x complicated hybrid. A very few were the simple (a x b). This does not represent my convictions in the matter, but is the result of circumstances. When I have pollen to use, and do not wish to waste it, I make sure that somewhere in the cross there will be a parent completely hardy, and that there could be combinations of the other parents to produce a fine plant. It is not often that I have more than a half dozen choices, for the blooming season begins under glass in March or April, and ends outdoors in July, and I have only so many budded plants. Most conspicuous in this program is the use of complicated hybrids, and I want to say more about that because it concerns chiefly the hybrid 'Kettledrum'.
There is a sort of tradition, which I think came to me from Gable, that 'Kettledrum', 'Atrosanguineum' and 'Charles Dickens' were grown from seeds originating in the same capsule. The flowers of all three look almost identical, with many of the characteristics of R. thomsonii, while the leaves of 'Atrosanguineum' are strongly suggestive of R. arboreum. We know that Anthony Waterer had both species to work with. All three are so hardy that R. catawbiense must have figured in their parentage. After studying all three for a quarter of a century, and watching their offspring, I can see no evidence of any other species which Waterer could have possessed.
'Charles Dickens' was exhibited in 1865 and awarded a first class certificate, which in my opinion it still deserves. 'Kettledrum' is not quite so deep a red, and a little too open in growth. 'Atrosanguineum' falls a trifle short in hardiness, will not endure full sun, but its deep color and the readiness with which its cutting root, make it the best of the three for our use, since 'Charles Dickens' roots hardly at all.
If these hybrids were bred from the three species named, and no others, then one of the probable parents was 'Russelianum', recorded in 1831, a cross between catawbiense and arboreum. 'Red Admiral' (arboreum x thomsonii) is attributed to J. C. Williams, without date, but as he lived until a few years ago, he could hardly have contributed anything to Anthony Waterer's work. The other parent could well have been R. thomsonii, as there is no record of any other cross among these three species. The parentage (catawbiense x arboreum) x thomsonii is not very complicated, and accepting the not improbable hypothesis that this is the breeding of all three hybrids, I am using all three as extensively as my yet small plants will permit. Of the three, 'Kettledrum' has been most accommodating in producing a good supply of flowers every year. 'Atrosanguineum' has entered into many of Gable's hybrids, and I am using also his 'Atrier', which according to my calculations, should be ((catawbiense x arboreum) x thomsonii) x griersonianum.
I must confess that the known results of my own breeding among the elepidotes, as they have survived the flood and their treatment at alien hands, does not offer much evidence in support of my breeding theories, outside the Ridgewood hybrids. I have, however, the entire experience of Joseph B. Gable to fall back on, both from observation of his plants and from his many letters. Reviewing what I have learned, common sense tells me that the best course to follow is the one that led to my best success.
The Ridgewood hybrids are certainly an extraordinary lot. I forget how many seedlings there were, but not over 300, virtually all hardy as plants, though some do not carry their buds too well in severe winters. Of 34 which Mr. Leach saw in bloom, he asked for cuttings of 4, and I believe eventually took more than that. At least two others are worth naming and propagating. I have two from another source, which I am propagating, and Hardgrove has 'Beatrice Pierce', which unfortunately does not propagate well, and another which does.
All these were (decorum X griffithianum) X 'Kettledrum', 'Charles Dickens' and 'Chas. Bagley'. Mr. Leach explains away the unusual results of this cross more easily than I can.
R. decorum kills outright the first winter in this climate, while R. griffithianum is still tenderer. The cross killed to the ground the winter after it had seeded, its first in the open, but when in midsummer it put up a shoot from the root, it was repotted, and wintered thereafter in a pit. The fact that with three hardy hybrids it gave all hardy seedlings, indicates that either this was an exceptional parent, or if these same hybrids were crossed with other similar crosses between tender species, like hardy results could be expected.
The breeding of this plant of decorum X griffithianum is open to suspicion. I cannot remember where I got the plant, but no doubt it came from Gable, either as a plant or as seed from Magor in England. Certainly decorum is one of the parents, because its influence is clearly apparent in nearly all the offspring. With equal certainty I can say that its foliage was nothing like decorum, but neither was it what could be expected from a cross with griffithianum. The fact that it flowered copiously when about two feet high would further remove the likelihood of griffithianum blood.
Used subsequently with 'Kettledrum' it gave another fine deep rose form of which I have rooted cuttings, and with 'Purpureum Elegans' a pale violet with very large flowers which I have not seen, but which Gable describes in glowing terms. I do not remember what became of the original parent plant and do not know whether it is still alive. It went under the name 'Dorothea', according to the British rule. If it has a present owner, I should like to secure cuttings of it.
I do not believe there is any rule in genetics which would forbid this advice to breeders. When you have made a successful cross, or have positive knowledge of how one was made, try as many similar crosses as you can think of with both parents separately, and try the same parents again. I am not scoffing at scientific formulae for all my life I have studied science in many of its forms and phases, and have made small contributions to it here and there, but I think the imagination of an artist will prove more potent in this field than the findings of a microscope.