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Previous Chapter : X. HEREDITY AND SEX : page 164.
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CHAPTER XI
DOUBLE FLOWERS.
Miscellaneous Cases. Recessive and Dominant Doubling - "Hose-in-Hose" Flowers - The Special Case of Double Stocks.
The inheritance of doubling in flowers has been only studied with success in a few instances. In one of these, however, that of Stocks (Matthiola), a feature of such great physiological significance has been discovered that we may be sure the subject will before long assume considerable importance. A special chapter, though a brief one, must be devoted to it.
Doubling, the multiplication, that is to say, of the conspicuous parts of flowers, especially the petals, may occur as the result of a number of various and apparently quite distinct physiological processes. The different sorts of doubling have been often described in the treatises on plant teratology*. In the commonest kind the stamens are bodily transformed into petals, as a manifestation of that phenomenon which I have called Homoeotic† variation, viz. the transformation of a part into the likeness of another with which it stands in a series. This is the kind of doubling which occurs so conspicuously in the Rose, Ranunculus, Godetia, &c. In other cases doubleness is
* A good general account of the phenomena will be found in Masters' Teratology, 1869. For a more minute description see K. Goebel, "Beiträge zur Kenntniss geffüllter Blüthen," Pringsheim Jahrbücher, XVII. 1886.
Materials for the Study of Variation, 1894, p. 85.
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attained by an actual multiplication or division of the petals, the stamens and other parts remaining apparently unchanged. Of this some of the double Fuchsias, Hyacinths and several Liliaceae afford very clear examples. These two processes, however, very often, in fact most usually, occur in combination with each other, and it is not generally possible to distinguish how much of the change in number is due to the one process and how much to the other.
In some flowers, especially those with gamopetalous corollas, the reduplication can occur in such a way that the corolla is simply repeated, two or more corollas standing in the place of one, but sometimes when two corollas are thus formed it may be seen that the outermost is in reality formed by a homoeotic variation of the sepals into the likeness of the petals. The most familiar examples of this "hose-in-hose" arrangement are known in Primula, Campanula and Mimulus. Another kind of doubling is due to what is termed proliferation or prolification (Masters) of the floral envelopes. The best illustration of this is the common double Arabis albida, in which the corolla and calyx are repeated tier above tier on an elongation of the axis. The different forms of increase in the number of the petals may commonly occur in varying degrees of perfection, and many grades of doubling are often to be seen on the same individual plant. There is also evidence that, in certain cases at least, high feeding and generous cultivation greatly promote the doubling. In some forms too it is known that the amount of doubling undergoes a change with the age of the plant, showing what has been called "Periodicity*," the most extensively doubled flowers appearing on the strongest stems and at the height of the flowering period.
To students of genetics the interest of the doubling of flowers arises partly from the fact that it is a character the heredity of which can be readily investigated, but especially from the obvious suggestion that the phenomenon is or at least may be associated with disturbance in the sexuality of the plants.
* See Correns, JB. Wiss. Bot. XLI. 1905, p. 465, de Vries, Ber. Deut. bot. Ges, XVII. 1899, p. 45.
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In many fully double flowers fertility is obviously impaired through the conversion of the reproductive organs into vegetative parts. It must not however be assumed that the sterility so often accompanying doubling is solely due to this comparatively definite circumstance. In the hose-in-hose Campanula, which has the sepals petaloid, the well-formed anthers contain plenty of pollen (some may be petalodic), but the female organs are in some way influenced by the variation of the sepals so that they are in some strains sterile. In the double Arabis albida mentioned above there are neither male nor female organs, but the tierupon-tier structure, which is here the form the doubleness assumes, plainly shows that something more than a simple homoeosis of the stamens and carpets has occurred.
In the Stock two kinds of doubling occur which are of quite distinct nature. The ordinary double Stocks much used in gardens are fully double, possessing an immense number of petals but no sexual organs, male or female*. Such doubles must therefore, as we shall see, always be bred from singles, a phenomenon which furnishes one of the most curious problems that the study of heredity has to elucidate. Besides these real doubles, plants are occasionally seen with one or two extra petals, but the experience of breeders makes it probable that these plants are not more prone than ordinary singles to produce the real doubles.
In Petunia another problem is presented. The doubles have an immense mass of petals, apparently formed at the expense of the stamens. A few anthers are nevertheless formed which contain good pollen. The female organs on the contrary are abortive and the double flowers set no seed†.
The case of Begonia offers some points of interest. The plants are of course monoecious and it is only the male flowers which are double as a rule. When the plants flower for their first season the doubleness is frequently extreme, no anthers being formed. But in their second season such plants bear male flowers which contain good anthers, especially, according to the experience of practical breeders,
* Goebel states that rudiments of anthers are very rarely produced.
† See Vilmorin, Fleurs de pleine Terre, 1886, p. 669, and various horticultural authorities.
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if the plants are starved*. The female flowers are generally normal, but sometimes extra petals are formed in them also. Such female flowers have singular malformations which have been often described, the most noticeable being an opening of the ovary which causes the ovules to lie freely exposed.
Of the hereditary descent of doubleness we know little, but from what is known it appears that several distinct systems are followed.
In Primula Sinensis doubleness is an ordinary recessive, singleness being completely dominant. The doubleness of Primula is of a very unusual kind. It may be primarily a petalody of the anthers, but I have never fully satisfied myself of this, nor do I know any critical observations on the subject. When a large collection of Primulas is examined, strains can usually be found, the members of which exhibit the lower degrees of petalody in some or all of their flowers. In the fully double flowers there is a complete series of petals inside the normal ones and arising from them. These are formed as images of the outer petals, so placed that their inner surfaces correspond with the outer surfaces of the normal petals, and the two adjacent surfaces are structurally both alike inner surfaces. The bizarre colour of double Primulas is due to this circumstance. Inside the corolla the stamens stand properly formed, and in their normal relations. The appearance of these fully double flowery is strongly suggestive of a delamination in the petals themselves ; but as in those partially double flowers which have one or two extra petals imperfectly formed and showing their staminal origin, the petaloid tissue similarly faces outwards, it becomes impossible to distinguish any boundary between the two phenomena. The morphological problems which these facts create must be left to the expert botanist.
The segregation of the single character from the fully double is, to some families, clearly quite sharp, and doubles always breed true when fertilised inter se. The inheritance of the partial or petalodic doubles has been investigated by Mr R. P. Gregory, and his experiments show that when such a strain is crossed with a pure single, F1 is single, and in F2 singles, partial petalodics, and some doubles
* For information on this point I am obliged to Mr Leonard Sutton and to Mr Wootten.
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appear, but owing to the great fluctuation in the degree of doubleness exact counts are impossible.
In Carnation, where the doubling is of the ordinary kind, due to petalody combined with sub-division of parts, there is some reason for suspecting doubleness to be a dominant. Everyone who has grown Carnations from seed is aware that a proportion, often considerable, of the seedlings come single. It is most improbable that any large number of these can owe their singleness to cross-fertilisation with single plants, for breeders would not keep such plants wittingly. Till critical experiments are made, however, the point cannot be regarded as certain beyond question.
In Poppies, on the contrary, the dominance of the single type appears to be quite complete. In the annual Larkspur (Delphinium consolida) also, from the fact that pink doubles exposed to the pollen of blue singles produce among their seedlings some blue singles*, there is no doubt that the single type is dominant. Instances of this sort could probably be multiplied without difficulty, and from what we know of plant-breeding in general, there is no practical doubt that doubling of the ordinary type is usually recessive to singleness.
The Hose-in-Hose or Calycanthemous Campanula and Mimulus.
The inheritance of this well-known variation or monstrosity was studied by Correns (76). The calyx, as described above, is petaloid in many degrees, varying from a condition in which the sepals are still partially green ups to the full development of a second corolla. In Correns experience this variation in the calyx is accompanied by as marked diminution in the fertility of the female organs. This in Campanula persicifolia amounted to total sterility, though in C. media a few seeds were set.
The abnormal character proved to be a partial and somewhat irregular dominant, considerable fluctuation occurring on the individual plants. Owing to complete failure of F1 to set seed F2 could not be raised, but when the norm type was fertilised with pollen from F1 a mixture of the two
* I have observed this in my own garden.
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forms resulted*. The Campanulas are plants well suited for this kind of experiment and it is to be hoped their genetic properties may be fully explored. Interesting experiments, for example, could be made with the white double form of persicifolia known in horticulture as C. Moerheimii. In it there is extreme doubling, apparently of the ordinary kind, due to petalody of stamens and splitting of the parts.
In the hose-in-hose Mimulus Correns found also that that variety is an irregular dominant. Here however there was no sign of sterility in the female organs.
Double Stocks (Matthiola).
Elaborate experiments on the heredity of doubleness in Stocks have been made by Miss E. R. Saunders, and though the research must still be regarded as in an incipient stage, some facts of quite unusual interest have been discovered.
Single Stocks in general breed true to singleness. As a rarity an extra petal may appear, but there is no evidence to connect such an appearance with the extreme and most definite kind of doubling characteristic of the ordinary double Stocks of horticulture. In double Stocks both carpets and stamens are wholly absent so far as our observations have gone, though, according to Goebel, rudimentary anthers are sometimes formed. From time immemorial these doubles have been bred entirely as the offspring of special strains of single Stocks which are maintained for that purpose. Since all doubles are absolutely sterile the succession is represented thus : -
[Table not reproduced in this version]
* Correns states that seed sold for "hose-in-hose" or "cup-and-saucer" Campanula gives a majority of plants showing the variation. The sterility of the ovules is perhaps an accompaniment of the petalody when strongly pronounced. Campanula media, with considerable though not complete calycanthemy, that I have examined had no obvious reduction in fertility.
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The numerical proportion in which the doubles appear cannot be confidently stated, but it is certain that this proportion differs greatly in different strains. There is some reason for supposing the ratio 9 doubles : 7 singles to be the common one in some families, but the experience of breeders points clearly to the fact that the proportion doubles may be much higher than this in specially good strains, while in others the proportion again may be much lower. Generally speaking, however, we have the certain fact that plenty of strains consist of singles which throw a great excess of doubles.
Many writers have recommended special cultural devices for increasing the output of doubles. Starvation, drying the plants off, keeping the seeds from the lower ends of the pods, are among the expedients advised. I do not know that any of these suggestions have been properly tested ; and it would be rash to deny that they may have some, effect. On the other hand it is practically certain that horticultural bad treatment will not cause a double Stock to produce stamens or carpels. Even in the weakest flowers on the doubles, which often may be so reduced as to have only 4 or 5 petals, no sexual organs are formed.
The morphological nature of the doubleness of Stocks is by no means clear. When such flowers are examined it is seen that the 4 sepals are normal. Above them the floral axis is continued for some length, and on it are set the crowded petals in imbricated fashion. There is no repetition of the sepals as in the case of Arabis albida, and the variation cannot readily be described as a strobilisation.
Since the doubles are totally sterile, the problem of the hereditary transmission of the condition must be investigated by making crosses with the single-flowered parents which produce these doubles. As the doubleness appears in the offspring of these singles the condition is evidently present either in the male cells of such flowers or in the female cells, or is produced by the combination (in fertilisation) of the factors with which these cells are endowed.
Miss Saunders has made many crosses between different strains of double-throwing singles, and as was to be expected, such crosses have given a mixture of doubles and singles just as either parent would have done if self-fertilised.
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The numbers are irregular and obviously need a further analysis.
The crosses which are instructive are those which were made between the double-throwers and the pure single strains. Whichever way the cross is made, F1 is always single. When F2 is raised from these plants the curious fact appears that the result differs according to the way in which the original cross was made. When the cross is in the form pure single f x double-throwing m, all* the F1 plants give a mixture of doubles and singles in F2 ; but when the reciprocal cross is made, namely double-thrower used as f x pure single used as m, it is found that the F1 plants are of two kinds, (1) those which throw doubles mixed with singles in F2, and (2) those which throw only singles in F2.
The conclusion to be drawn is evidently that the pollen-cells of the double-throwers are all (perhaps nearly all) bearing the double character, but that the egg-cells of these same plants are of two kinds, those which bear singleness and those which bear doubleness (see 22, pp. 5 and 36).
For the first time therefore we have a proof that, in a hermaphrodite form, there may be a substantial difference between the factors borne by the male and female cells of the same plant. It is the existence of this instance which leads me to hazard the suggestion introduced in the discussion of the facts of sex-inheritance in Bryony, that the female cells of Bryonia alba may be of two kinds and male cells of one kind (see p. 168).
This discovery, though it may prove to be the clue to the problem of the double Stocks, leaves the main difficulty still unsolved. We have to find the scheme whereby it comes to pass that the doubles, though bred from singles, are nevertheless as a rule in the majority, and the difficulty of offering a plausible suggestion is not diminished by a knowledge of the fact that the male cells may be all double, for this fact implies that a majority of the egg-cells must also be bearing doubleness, and ultimately that part of the singleness must have, as it were, disappeared in gametogenesis. We are at present quite unable to offer any
* No exception has yet been met with. There are however some reasons for anticipating that exceptions may exist.
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symmetrical or probable scheme of character-distribution by which such a phenomenon can be represented.
The various difficulties of the case culminate in the fact which seems well established, that though the double-throwers generally give 7 singles : 9 doubles - with certain departures from this ratio which are probably not fortuitous - yet F1 raised from single x double-thrower, and from the reciprocal cross, always on self-fertilisation gives 3 singles : 1 double*.
* The relation of white to cream-colour in the strain known as "Sulphur" is approximately the same as that of single to double described above. "Sulphurs" are single whites throwing singles all white, and doubles which are usually creams with whites as exceptions. The pollen-grains of "Sulphur" all bear cream, while of the ovules some are creams and some whites.
Next Chapter : XII. EVIDENCE AS TO MENDELIAN INHERITANCE IN MAN : page 205.
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