Journal of Heredity 6: 456-461 (1915)
DOUBLE SEEDING PETUNIAS
MRS. MYRTLE SHEPHERD FRANCIS
Crossing and Selection Result in Production from Single and Imperfect Double Types of Four Strains That Are Double and Produce Seed-Methods of Operation-Curious Variations Observed1
1Read before the twelfth annual meeting of the American Genetic Association, at Berkeley, California, August 6, 1915.
|FEMALE PARENT OF THE CROSS|
|Petunia grandiflora, shown above, is single and has perfect reproductive organs, producing several hundred seeds from each flower. So far as she can learn it had not been used in commercial production of double-flowering petunias until Mrs. Francis began to work with it, all breeding have previously been down with various forms of Petunia hybrida. (Fig. 6.)|
|DOUBLE FORM OF HYBRID PETUNIA|
|This was used as male parent in Mrs. Francis' crosses. The first double petunia appeared in 1855, and since then they have been steadily produced, but as the doubles produced pollen but could not set seed, it was necessary to create them by a new cross each year, since they could not reproduce themselves. Doubles had to be crossed on singles, the latter then bearing seed which produced a small proportion of double flowers. Mrs. Francis undertook to produce a strain of doubles that would not have to be crossed on singles this way each year, but would be capable of bearing seed. For this purpose she crossed the above hybrid form with the giantflowering form shown in Figure 6. The result has been successful: not all the double flowers produce seeds, but enough of them do to make the culture commercially profitable. (Fig. 7.)|
|ONE RESULT OF FIVE YEARS OF BREEDING|
|By crossing the two forms shown in the preceding photographs, one single and producing seed, the other double and producing no seed, Mrs. Francis was able to get forms like this, which are not only double but produce seed as well. They are, therefore, a great improvement over the previously known double forms, from the gardener's point of view. In spite of this, reproductivity has been only a secondary object of the breeding, most attention having been given to improving the shape and color along lines that would be likely to appeal to the lover of this very variable member of the potato family. (Fig. 8.)|
BEFORE presenting the subject of my double petunia that reproduces itself, I want to state that I claim no scientific attainments, that while the scientific aspect of the work has been of deep interest to me my chief aim has been to produce the finest strains of double petunias to be had in the market and to make those strains reproduce themselves. Competent authority assures me that my work has been successful.
Data regarding the crossing of both single and double petunias is exceedingly difficult to obtain as most of it is scattered about in horticultural reports and magazines.
The first single petunia was found by Commerson in Argentina, on the banks of La Plata River, and sent by him to Jussieu, who named it Petunia nyctaginaflora, introducing it into France in 1823. This plant had an upright habit with thick sticky leaves and long-tubed fragrant white flowers. The second species was sent by Tweedie from Buenos Ayres to the Glasgow Botanical Gardens in 1831. This plant had a decumbent habit, small violet purple flowers and short tube and was named Petunia violacea. From these two species all varieties of petunias have been bred. They have been freely crossed with each other; hence the garden varieties now go under the general name of Petunia hybrida Hort.
While the nyctaginaflora type is quite common, the true violacea form is seldom seen, proving that the nyctaginaflora species was the dominant factor in the early crosses. Even today most varieties revert to that form when left to themselves.
For convenience sake I shall loosely divide the single varieties now under various names into two classes: those with upright habit, long-tubed flowers with small reproductive organs, slender style and filament adherent low down in the corolla tube and wide range of colors with satiny texture, as hybridas, representing P. nyctaginaflora; and the varieties with the decumbent habit, large leaves, flowers with short tube, large reproductive organs, thick style and filament adherent high up in the corolla tube and limited range of colors, as representing P. violacea.
The first double petunia appeared in a private garden in France in 1855 and from this, so far as I have been able to learn, have all other doubles been obtained by artificial fecundation.
METHOD OF OPERATION
For the benefit of those in this audience who may be unfamiliar with the method by which double petunias are obtained I will explain, that the double is an imperfect flower and the single is a perfect flower. The unbroken anthers (the pollen-bearing organs) of a single flower are removed, the flower is then covered with gauze or paper until the stigma is ready, the pollen is then applied from a double flower by means of a camel's hair brush and the covering replaced, to prevent the possibility of insect fertilization.
Such a procedure, however, is entirely too laborious for commercial work. I have never used the coverings but remove the anthers and pollenize at once from a nearby flower, double and single plants being grown in adjoining plots.
From the size of anthers and stigma, colors and habit of growth it would seem that the hybrida had been universally used for both male and female parents, until recent years.
Though advised otherwise, in my early work I chose the form known as grandiflora as the female parent for my doubles, probably because the flowers were easier to work with. Later when an ideal had formed itself in my mind the grandiflora seemed more likely to give the desired results.
Many doubles have rudimentary organs of reproduction, but in my first work in 1901 I noticed this and formed the habit of examining each bloom carefully before picking it to pollenize with.
The first perfect double bloom was found on a hybrida plant in 1910. This plant had delicately fluted flowers with cream colored pollen and, when pollenized with another flower from the same plant, matured a capsule of seed. The stamens of this flower were many, rising directly through the center, the filaments being bound together by a band or collar, while the ovary sat upon a torus. The ovary of the single form sits directly upon the calyx.
From this capsule of seed thirty-seven plants were raised. No records were kept until 1911, but as near as I can remember about 75% were double, both single and double being of the hybrida type. None of these plants gave many perfect flowers though all were examined for reproductive organs and some seven or eight matured seed.
That season among our regular doubles appeared a semi-double of steel blue and white which bore all perfect flowers, and on an inferior double red was found a capsule of seed which had matured without hand pollenizing.
From these three distinct types 660 plants resulted in 1912, 85% double and 22% seeding slightly.
The petunia is perhaps the most variable flower under cultivation but its fluctuations have a certain regularity. In this generation, the three types being planted together, the wildest con-fusion prevailed. In it appeared for the first time the true grandiflora, represented by three plants of deep magenta color with steel-blue pollen. Their doubling was of an entirely different nature,--all extra petals were adherent to the corolla tube instead of the usual mass of petals and stamens which generally fill the center of the flowers. Nearly all blooms on these plants were perfect, though they did not all mature seed.
In this planting were also some very small inferior doubles of dingy purple flowers, which were perfect with the same manner of doubling and which matured several capsules of seed without pollenizing. Both extremes have the same form and both are fertile.
From the grandiflora crossed by the hybrida double and some seed of the hybridas also we raised in 1912, 510 plants, 73% in double, 25% seeding. Many of the flowers showed great variety of color, beauty and size.
In 1913 we got 187 plants with 73% double but 33% seeding. This year marked a decided change, the grandiflora heretofore recessive became the dominant type with blooms of extraordinary size, while its seeding capacity had increased 8%. Both beauty and reproductiveness had developed to such an extent that for our stock seed I crossed a perfect double with a perfect double for the first time, but disaster overtook me for our seed beds with our entire stock of seedlings were washed out by the floods of 1914.
Replanting from our selling stock yielded 918 plants, 85% double, 42% seeding. 1915 produced 567 plants, 90% double, 40% seeding. While the increase of doubles has been quite steady the seeding percentage has not increased so rapidly due to the use of a plant that carried singleness in its pollen but with other qualities which I wished to preserve.
I have not yet made the reciprocal cross again but expect to do so this season, as many of the fine large flowers are perfect, seeding as freely as singles when pollenized.
Doubtless my work would have been done on entirely different lines had my knowledge been greater in the beginning. During the last five years I have bred four distinct strains of double seeding petunias, steadily increasing doubleness, lengthing the stems, and giving greater delicacy to the texture and colors. In all my work those qualities have had precedence over reproductiveness.
Many interesting and curious variations have been observed, in one of which the whole flower becomes petalous. Some of the finest flowers are pistillate, reversing the old form, others have anthers containing no pollen, while some almost single blooms have malformed reproductive organs, still others are perfect but infertile, etc.
Lavender and steel blue seem to be the best seed producers and I am quite sure that blue pollen is more productive of fertility in doubles than yellow, which continues to give about 25% seeding plants. This may be due to the contracted throat which seems to accompany this pollen.
Of volunteers which appear each season the doubles predominate. I have never found one with other than blue pollen.
The small pointed capsule of the containing about 250 seeds has developed with the flower, one capsule often producing as many as 450 seeds. The dehiscence in singles is in twos but in these doubles it is often in threes and fours.
In conclusion I quote from De Vries' Species and Varieties: "Hays has repeatedly insisted upon the principle of the choice of the most favorable variety for the experiments in improving races. He asserts that half the battle is won in choosing the variety which is to serve as a foundation stock, while the other half depends upon the selection of parent plants within that variety." I blindly striving to realize my ideal unconsciously chose the most favorable variety and the right parents in that variety for what I desired to produce, and if the entire stock of these strains should be lost, with my present knowledge I could consciously choose the right variety and the right seed parents in that variety and other strains of seed producing double petunias could be developed.