USDA Yearbook of Agriculture (1937)
Improvement of Flowers by Breeding
S. L. Emsweller, Principal Horticulturist,
Philip Brierly, Associate Pathologist,
D. V. Lumsden, Associate Physiologist,
F. L. Mulford, Associate Horticulturist,
Division of Fruit and Vegetable Crops and Disseases, Bureau of Plant Industry

SNAPDRAGON

The early history of the cultivated snapdragon (Antirrhinum majus L.) is not known. Some records indicate that it was first grown in Italy, whence it spread to the remainder of Europe. At the present time the species is found growing wild in many spots along the Mediterranean coast west of Italy. In one of the earliest published accounts, in 1578, there were described several color variations and two distinctive leaf types, the narrow and the broad. A little later, five varieties, white, purple, blush, yellow, and variable, were known. Soon after this a double-flowered form and one with variegated leaves appeared. At the beginning of the nineteenth century several striped and spotted varieties were listed by various growers. By 1824 a wide range of colors existed from rich orange and yellow to white, with the same types in reds and purples as well as many bicolor forms.

The first-named botanical varieties of Antirrhinum majus did not appear until about 1830 or 1835. Among these were reticulatum, youngii, and carophylloides. In 1844 a deep blood-red double-flowered type appeared. The earlier doubles had ranged from white to rose. In the next few decades countless horticultural varieties were introduced each year and were sold as high as $1 per plant. At this time propagation was entirely by cuttings. In 1850, George Parsons, of Brighton, England, introduced a variety that was a distinct change in the arrangement of the colors of the flower. It was white with a deep rose band in the form of an edging to the petals. It was distributed by E. C. Henderson & Sons, in 1852, under the name of Hendersonii.

The snapdragon did not come into its own until the latter half of the nineteenth century. All the earlier varieties had been propagated only by cuttings, and very little had been done with seedlings. The climate of Scotland and England was very favorable for the production of snapdragons of fine quality, and the flower soon became very popular. The demand for newer and improved varieties stimulated the growing of seedlings, and hundreds of new types were soon developed.

At the present time snapdragons are roughly divided into two groups — (1) florists' varieties and (2) types for outdoor culture. Some varieties, of course, may be placed in both groups. In England and Europe generally, the chief interest is in the outdoor sorts, while in the United States the forcing or greenhouse types are the most important.

The snapdragon flower is so constructed that smaller insects find it very difficult to gain entrance to the nectaries. It requires considerable effort even on the part of a large bumblebee to open the two parts of the corolla. The flower is, nevertheless, frequently visited by large bees, and undoubtedly considerable cross-pollination occurs. Because of this situation, varieties of snapdragons grown for seed should be separated a considerable distance from others. This is especially important for florist varieties, where it is essential that strains be true for type and color.

The practice of growing snapdragons from seed did not become very general until early in the present century. In England and France, seed had been offered by various seedsmen for some time. Most of the old standard varieties, however, were still increased solely by cuttings. In this country, nearly all florists propagated entirely in this manner. As a result many local varieties arose but were not widely distributed. In 1913, snapdragon rust suddenly appeared in the vicinity of Chicago. It had been known in California and along the Pacific coast since 1896. Within a few years after its appearance the disease spread to all sections of this country, to Mexico, and to Canada. The result was almost disastrous to greenhouse snapdragons and practically eliminated propagation by cuttings. The moist conditions and warmth in the cutting bench were also the optimum conditions for snapdragon rust. This situation, and the fact that the disease was not seed borne, stimulated development of better seed-propagated strains. Within a short time a great number had appeared, and today the total list includes many hundreds of varieties.

Most of the present-day varieties have probably arisen from chance crosses made when the commercial seed crop was produced. Even though the seedsman rigidly removes all off-colors and types, the seed produced will usually contain some crosses made by bees. The florist or grower then finds offtype plants the next year. In addition to this source of contamination it is also very probable that some strains usually contain a few hybrids, which carry recessive characters that do not show up until the next year.

Following the widespread distribution of rust, the popularity of the snapdragon began to wane in this country. The florists were able to control the disease under glass, to some extent, by careful attention to watering and maintaining a temperature unfavorable to its development. Plantings outdoors, however, continued to suffer, and the snapdragon began slowly to disappear from parks and home gardens. In California the growing of snapdragon seed was a rather precarious undertaking. In some years a fair crop might be secured, but on the average the yields were very poor. Many attempts to control the disease by spraying were on the whole unsuccessful.

Figure 22.—The way in which rust resistance is inherited in the snapdragon: A, Flower and leaf of the susceptible variety; B, those of a resistant plant; C, resistant hybrid resulting from cross‑pollination of A and B; D, E, F, G, flower and leaf from each of four plants descended from the resistant hybrid C; this second hybrid generation had a ratio of 3 resistant plants to 1 susceptible.

In 1922 E. B. Mains, then at Purdue University, found two snapdragon plants that showed some resistance to rust. In 1927 he distributed seed to several investigators. Continued selections and self-pollinations in descendants of these strains finally resulted in the development of highly resistant strains of snapdragons. The inheritance of this resistance has been studied by Emsweller and Jones (117), White (545), and Mains (327). In all cases resistance was due to a dominant gene. The inheritance of resistance is shown in figure 22. At the upper left are shown a flower and leaf of a susceptible variety; in the upper right a flower and leaf of a resistant plant; directly between and below, a flower and leaf from the hybrid resulting from a cross between susceptible and resistant. Since resistance is dominant, the first-generation hybrid is completely free from rust. At the bottom are shown flowers and leaves from four of the secondgeneration plants. Three are resistant and one is susceptible. This count of 3 to 1 is the typical Mendelian ratio when a single pair of contrasting genes is involved. The actual figures from such a cross made in California were as follows: All of the 562 first-generation hybrid plants were completely resistant. One of them was self-pollinated and 550 second-generation plants were grown from the seed. In this large population 405 plants were resistant and 145 were susceptible. A perfect 3:1 ratio would have been 413 resistant to 137 susceptible. The ratio actually secured was off just eight plants. Such a small deviation is not significant, and the ratio secured undoubtedly represents what is called a simple monogenic segregation. Resistance was also found in other Antirrhinum species imported from western Europe by the Division of Plant Exploration and Introduction, but these were not used in the breeding of the rust-resistant strains mentioned.

Recently in several localities in California, some of the supposedly resistant plants have again succumbed to rust. Such a situation is not unusual, being common in grain varieties bred for resistance to certain strains of the cereal rusts. It is possible that the condition in California is caused by a new strain or physiological form of the rust organism. Since the rust parasite on the snapdragon is itself a small plant, it is not unusual that it should produce a new strain able to attack otherwise resistant plants. If this should prove to be the situation, the production of rust-resistant strains of snapdragons will be more difficult in those sections where more than one physiological race of rust occurs.

The genetics of the snapdragon has been extensively studied by a large number of workers. The inheritance of color is very complex. According to Miss Wheldale, magenta is in general the most dominant color and yellow the most recessive. By this is meant that magenta is dominant to practically all other colors, while yellow is usually recessive to all others. This explains why seedsmen find that yellow varieties and strains are usually very easy to purify. Since yellow is recessive, plants can exhibit this color only when pure for it. In the same investigations, crimson was dominant to bronze, bronze to yellow-tinged bronze, magenta to rose doreé. Delilah forms, in which corolla lips are colored and tube is colorless, were recessive to the corresponding nondelilah. For example, crimson was dominant to crimson delilah.