IMPROVEMENT IN THE LEAFY
CRUCIFEROUS VEGETABLES

ROY MAGRUDER, Olericulturist, Division of Fruit and Vegetable Crops and Diseases, Bureau of Plant Industry

[ABSTRACT].  A SINGLE piece of publicly supported plant breeding saved the cabbage-growing industry in many sections of the country and brought an enormous return on the cost. This was the work begun in Wisconsin in 1910 by L. R. Jones, and continued through the cooperation of J. C. Walker and his associates of the United States Department of Agriculture, to develop varieties resistant to the devastating cabbage yellows or fusarium wilt. As a result, there are now yellows-resistant varieties of all the major types of cabbage demanded by the market. Today other diseases are receiving the attention of breeders, as well as the problem of adaptation of varieties to definite regions and the development of types superior in eating quality and in ability to hold up well in storage.

According to most botanists, cabbage, cauliflower, broccoli, green-sprouting broccoli, brussels sprouts, kale, collards, and kohlrabi are very closely related, being horticultural forms of the species Brassica oleracea L. Kohlrabi is assigned by some botanists to B. caulorapa (DC.) Pasq. Wild cabbage (B. oleracea), illustrated in figure 1, from which all these forms or varieties are supposed. to have arisen, is still found growing wild along coastal regions of Europe and northern Africa. Its use by man as food antedates written history, and it is believed to have been in rather common use for more than 4,000 years.


Figure 1.—A wild cabbage plant.

Just when or where the various forms or varieties of cabbage first appeared or were developed it is difficult to say with any degree of certainty because of the lack of written records.  Kale and collards (var. viridis L.) are probably the oldest type, and the present wrinkled or ruffled type was men-tioned by Theophrastus in 350 B. C. and described and illustrated by Dodonaeusin 1559. Red cabbage or kale was also known to Theophrastus, and Pliny describes heading cabbage (var. capitata L.) and includes in his list of types a savoyed or blistered-leaf type which is thought to be the ancestor of our present savoy cabbages. The name savoy indicates that much of the early developmental work occurred in the locality of Savoy in southeastern France. Cauliflower and broccoli (var. botrytis L.) are believed to be more recent additions to our types and to have been developed from the green-sprouting broccoli. Kohlrabi (Brassica oleracea var. gongylodes L., or, if considered a species, B. caulorapa) probably was brought north to the low coastal countries of Europe by way of Prague and Vienna. The two most important varieties of today are White Vienna and Purple Vienna. Brussels sprouts (var. gemmifera DC.) were not mentioned by the early writers prior to 1759, but by 1793 this vegetable was an article of international commerce, and its origin is generally ascribed to Belgium.

THE PART PLAYED BY SEEDSMEN IN THE DEVELOPMENT OF CABBAGE

Varieties of vegetables as we speak of them today in our seed catalogs hardly existed before the last of the eighteenth century either in the United States or in Europe. This date corresponds with the rapid rise of seed growing as a business venture in Europe, and no doubt the competition between seedsmen for something new to offer their customers was, then as now, the incentive that led to the rapid introduction of varieties.

The early named varieties were groups of plants with a few common characteristics but many variations. The more observant, careful, and critical growers were always looking for improvements, and when plants were found that seemed superior, these were selected for seed propagation. By continued selection of superior or distinctive types suitable to the particular locality in which the grower lived, many local strains or horticultural varieties—as distinct from botanical varieties—were developed. The local seed seller disposed of any surplus the grower might have, and, as the industry developed, an opportunity was afforded for the general distribution and trial of many varieties over a wide range of conditions. If the variety performed well in its new location, it soon attained major importance.  If it was unsuitable in general yet possessed some special merit, it probably was the starting point for a new variety, which was developed as the result of continued selection for the especial point of merit.  Thus the early development of all our crops was the work of observant and critical growers who through superior skill in selection and seed growing gradually became the local sources of superior seed. The early seedsmen were merchants, not growers or breeders of seed.

   The types or varieties of cabbage were developed mainly by the people of the Netherlands, Denmark, Germany, France, and England.  When brought to this country by the early settlers, they were not always suited to our climate, which generally was hotter and drier than the climate of the north European countries. The uncertainty of supply and the cost of imported seed forced the isolated grower to attempt to produce seed for himself. Starting with imported seed, he proceeded to select the individual plants that best suited his needs and grow seed from them. If he wanted something earlier maturing, later maturing, with less outer leaves, more rounded head shape, smaller size, more heat-resistance, or what not, he selected toward that goal and in many instances was successful.

The enterprising mail-order seed dealer was largely responsible for the location and introduction to the public of many of these locally developed strains, sometimes with the growers’ name affixed but many times with a name selected by the introducer.  The high prices paid for these types stimulated seed growers in their effort to find new and superior varieties or strains. Some of the growers soon found they could make more money by growing seed than from the market crop and devoted their energies to seed production. Most of the early cabbage-seed growers were located on Long Island, N. Y., because of the favorable growing conditions there.

At the beginning of the nineteenth century most of the cabbage grown on Long Island belonged to three types: Early York, an early maturing variety with elliptical heads; Flat Dutch, a large midseason variety with flat heads; and Red Dutch, a late, hard, round-headed variety. The Flat Dutch type was more productive and in greater demand on the market than the other two types, and most of the American varieties or strains introduced during the nineteenth century were selections from the Flat Dutch type. An objective of major importance to all growers of cabbage was that every plant, should produce a marketable head, and this characteristic is emphasized in the names of some of such varieties as All Head and Surehead.  Selections were also made for earlier and later maturing strains of the Flat Dutch type in order to extend the marketing season over a longer period. Some of these selections for difference in maturity resulted in differences in head shape and size and in a rather wide range in type when compared with the parent variety. The early growers were also interested in securing cabbages resistant to disease, and the Houser and Bugner varieties are the results of the efforts of two men along this line. As the seed business became more highly competitive and the growers more critical, the matter of attaining uniformity of type became an object of considerable attention and effort. At the present time there are available very uniform stocks of varieties that cover the entire range of season of maturity and are satisfactory in head size and shape.

A list of the cabbage varieties of known American origin with the year of introduction and other information is given in table 1.

It is also of interest to note which of our present-day important varieties are foreign introductions. In the list of what may be regarded as the nine principal American varieties of cabbage we find Early Jersey Wakefield, Copenhagen Market, Early Winningstadt, Glory of Enkhuizen, Late Flat Dutch, and Danish Ballhead to be the names of orginal importations, although the present strains of the first and of the last two varieties show a decided improvement in uniformity of type, which may be attributed in part at least to the efforts of American seedsmen. Other varieties of lesser importance that are known to be importations by American seedsmen are Golden Acre, Resistant Detroit, Early York, San Francisco Market, Volga or Early Stonehead, and almost all of the red and savoy varieties.

Table 1.—Cabbage varieties of known American origin
Variety nameProducerHow producedParental varietyIntroducerYear introduced
All Head EarlyMr. Strong, Long Island, N. Y.SelectionFlat DutchW. Altee Burpee & Co.1891
All Seasons (The Vandergaw)Mr. Vandergaw, Long Island, N.Y.James JH. Gregory1886
Bugner (Bugner Wonderful)John Bugner, Prairie View, Ill.Cross and selectionUnknownUnknownAbout 1890
Charleston or Large WakefieldJohn M. Brill, Hempstead, N.Y., or Francis Brill, Riverhead, NYSelectionJersey WakefieldF. W. Bolgiano & Co.About 1866
Ferry’s HollanderD.M. Ferry & CoDanish BallheadD. M. Ferry & Co.1900
Fottler’s Early Drumhead or Brunswick Short StemJohn Fottler, Roxbury, Mass.Frucker BraunschweigerA. Schlegel1866
HouserGeorge Houser, Harrisburg, Pa.Stein’s Flat DutchHolmes Seed Co1897
Kraut King or Victor Flat DutchUnknownCross and selectionFottler’s Brunswick X Short Stemmed Danish RoundUnknownBefore 1900
Newark Early Flat Dutch, Early Flat Dutch, or Early SummerFather of Francis BrillFrench Ox Heart x Large Flat DutchBefore 1880
SuccessionAbraham Van Seclen, Jamaica, N.Y.SelectionEarly SummerPeter Henderson & Co1888
MasonJohn Mason, Marblehead, Mass.Scotch DrumheadUnknownBefore 1863
Stone MasonJohn Stone, Marblehead, Mass.Mason
Improved American SavoyUnknownLarge SavoyBefore 1870
Ferry's Round DutchFerry-Morse Seed Co.Early Dwarf DutchFerry-Morse Seed Co.1933
Gill's Oregon BallheadGill Bros. Seed CoDanish BallheadGill Bros. Seed Co.1918
Harris’ BallheadJoseph Harris & CoJoseph Harris & Co.1932
Midseason MarketD.M. Ferry Seed Co.Low Late AmagerD.M. Ferry Seed Co.1921

BREEDING WORK BY PUBLIC AGENCIES

The severe losses caused in the cabbage-growing sections of Wisconsin by cabbage "yellows” or fusarium wilt led Jones (6), of the Wisconsin Agricultural Experiment Station, to start selection work in 1910 toward the development of yellows-resistant varieties. The work was continued through the cooperation of J. C. Walker, of the United States Department of Agriculture, and his associates (3, 7, 8, 29, 30, 31, 32, 33) to the point where we now have yellows-resistant varieties of all the major types. A good example of a yellows-resistant cabbage is illustrated in figure 2. (See the appendix for the list of 10 varieties and years of introduction.) This one piece of publicly supported plant improvement work has saved the cabbage-growing industry in many sections of the country and resulted in an enormous return on the cost.


Figure 2.—Comparison of yellows-resistant and yellows-susceptible breeding lines of cabbage on heavily infected land. Rows a and c, resistant progenies; row b, susceptible.

Workers at the Iowa Agricultural Experiment Station (14) have also produced an early yellows-resistant variety of cabbage, Iacope, by selection on disease-infested soil from the Copenhagen Market variety, and introduced it in 1922. It has since been very largely replaced by earlier and more uniform strains from the Wisconsin work. In 1926 C. E. Myers, of the Pennsylvania station, released a strain of Ballhead called Penn State Ballhead, which was produced by pedigree selection for uniformity and solidity of head and for large yields under Pennsylvania conditions. The work of J. C. Miller, at the Louisiana station, has resulted in the production of a strain called Louisiana Copenhagen, which is earlier, slightly smaller, and shorter-cored and has harder heads under Louisiana conditions than commercial strains of Copenhagen Market, from which it was produced by inbreeding and selection.  As a result of the inbreeding, hydridization, and selection work of C.H. Myers and W. I. Fisher at the New York (Cornell) station, nine new varieties or improved strains of cabbage have been introduced.  (See table 4 of the appendix for list and special characters of each.)  Several other State experiment stations and the United States Department of Agriculture have cabbage-improvement programs underway from which no introductions have yet been forthcoming (see table 6 of the appendix).

PRESENT TREND IN CABBAGE IMPROVEMENT

The major emphasis in cabbage breeding is now upon the development of strains that are resistant to diseases other than yellows, strains that are particularly well adapted to a definite locality, or strains with superior eating or storage quality. There is need for an early-maturing, round-headed, winter-hardy, nonbolting variety of cabbage that can supplant Jersey Wakefield for wintering-over in the South Atlantic coastal region. Our best early round-headed varieties are usually either killed by the cold in this region or produce seedstalks when planted in the fall. Work on this problem is well under way, and it is hoped that within a few years a strain or variety will be available that will combine the hardiness and nonbolting of Jersey or Charleston Wakefield with the more productive and more desirable head characteristics of Golden Acre or Copenhagen Market. Except for sauerkraut manufacture, there is a decided preference for heads of small to medium size, very hard, with mild or sweet flavor and crisp or succulent texture. The development of strains especially adapted to cultural conditions in the large production centers in various sections of the country is another problem on which cabbage breeders are working.

USUAL BREEDING METHOD FOR BIENNIAL BRASSICAS

In improving cabbage or other Brassicas by selection, commercial seedsmen commonly select several plants possessing the desired characteristics and store them over winter either in cold storage or by burying them in the field under alternate layers of soil and straw or other coarse litter. As soon as the ground can be prepared in the spring, the plants are transplanted to their permanent location. In the case of cabbage, vertical cuts are made on four sides of the head to enable the seedstalk to push through. The mature seed from each plant is saved separately, and the plants that result from it are planted in a separate row the next year. At harvest time only the plants having the desirable characteristics from the most uniformly desirable row (which of course comes from a single head) are selected for storage and seed production the following year. The breeding block and the fields for increasing seed should be at least one-fourth mile distant from any other varieties of Brassicas in order to prevent crossing.

Selfing (applying the pollen of the flower to its own stigma and preventing pollination by other plants) is the most rapid method of securing uniformity in type, but because of the reduction in vigor usually caused by inbreeding the Brassicas, the large proportion of self-sterility present in these plants, and the special equipment and large amount of hand labor required, this method has not come into widespread use by commercial breeders. Various methods of surmounting some of these difficulties are discussed in the section on Developments in Breeding Technique.

AMERICAN IMPROVEMENTS IN OTHER BRASSICAS

No plant-improvement work with kohlrabi has been or is being done by any of the American seed growers or the State or Federal research institutions. Cauliflower, likewise, has not been worked with successfully, because of the difficulties attendant upon seed production in this country.

Broccoli seed, however, can be grown successfully in California, and the Ferry-Morse Seed Co. has developed a number of strains that differ in their ability to make marketable heads at different periods through the winter and early spring months. The names November, Christmas, February, March Early, March Late, and April indicate the seasons at which the heads mature most successfully. Green sprouting broccoli, a rather recently revived introduction from Italy, has been much improved in uniformity of type and productiveness by several seed growers.ψ By proper manipulation it can be easily grown as an annual to produce seed the first year.

Long Island Improved, a half-dwarf strain of brussels sprouts selected and improved by early Long Island growers and seedsmen, is the only important variety of this vegetable listed in many American seed catalogs. Very recently the Gill Bros. Seed Co. of Portland, Oreg., has introduced a dark-green strain called Oregon Special, and a medium-green taller strain called Half Moon Bay. The latter probably developed among the growers in the Half Moon Bay section of California.

Improvement work on kale by seedsmen has been limited to fixing the type or selecting more uniform strains. In 1936 the Virginia Truck Experiment Station released a strain of kale and named it V. T. E. S. Scotch. It has blue-green, heavily curled leaves and is more cold-resistant and more uniform than commercial stocks of this Dwarf Blue Curled Scotch type.

Louisiana Sweet is the name of a uniform, shorter petioled, solid green-colored strain of collards introduced in 1934 by the Louisiana Agricultural Experiment Station as the result of several years’ inbreeding and selection work in the Georgia collard variety.

DEVELOPMENTS IN BREEDING TECHNIQUE

As a result of numerous experiments by workers with cabbage and related crops, improvements in technique have been made that greatly facilitate breeding and improvement work. In the work for early-maturing yellows-resistant varieties of cabbage great difficulty was experienced in keeping the plants over winter in storage because of decay and rots that developed during the long storage period. It was discovered that cutting the roots or pulling them loose on one side of the plant and then on the other side at a later date would delay the maturity of the plant until late in the fall. With greenhouse space available, it was possible to transplant the selected plants into large pots, which could then be moved into the greenhouse before freezing weather. When grown at low temperatures until the seedstalk started to elongate, these plants would bloom during early spring and produce seed for sowing in May. This made it possible to treat the usually biennial or perennial cabbage as an annual and greatly speeded up the work. It was also more convenient and easier to do crossing and self-pollination in the greenhouse than in the field. It did not conflict with other field work in the late spring or early summer.  Figure 3 illustrates the use of a greenhouse for pollination work.


Figure 3.—Artificial control of cross- and self-pollination of cabbage in the greenhouse.  The use of the greenhouse in winter makes it possible to obtain a new generation each year instead of every 2 years.

It was also determined that testing for resistance to yellows could be done as well in the greenhouse under the proper temperature conditions (68° to 77.5° F.) in disease-infested soil as in the field.  The plants that proved to be resistant could then be grown to maturity in the greenhouse, and those selected for propagation would produce seed during the winter and early spring without any period in storage, where the plants might be lost through disease or decay.

In breeding for shortness of stem or core, compactness of head, and superior eating quality, it is necessary to remove the head for examination and testing. When cabbages were grown in Louisiana as a fall crop, it was found that the axillary sprouts would develop after the head was cut, and, if the weather was cold enough during December, January, and February, the plants would produce good crops of seed in time to sow for the next fall crop. If the plants were to be moved into a greenhouse the transplanting was delayed until the lateral sprouts had made a good compact growth.

In some seasons the field-grown plants failed to produce seedstalks and instead produced small heads from the lateral sprouts. Difficulty was sometimes experienced in getting all of the plants to make seedstalks when the material was grown in the greenhouse during the winter. A series of experiments at Cornell University showed that a rest period of approximately 2 months’ duration at about 40° F. was required for the subsequent formation of seedstalks. This period could be spent in the fall either in storage or in the greenhouse, and if the temperature was then raised to 70° and maintained there, ripe seed could be produced for sowing in May. Plants that were not given the cold treatment when grown in greenhouses at a temperature of 60° to 70° produced no seedstalks, indicating that a period of low temperature is necessary for the subsequent formation of seedstalks.  Increasing the length of day by the use of 5 hours’ electric illumination at the end of the daylight period did not cause the appearance of seedstalks in plants grown continuously at the warm temperature (60° to 70°) or hasten their appearance in the cold-treated plants.

In genetic work the use of pure-breeding or. homozygous strains is advantageous. Various degrees of self-sterility have been encountered when inbreeding members of the cabbage family to produce such strains. Numerous lines more or less self-fertile have been isolated, but the importance of starting with large numbers of individuals should be emphasized in any program that calls for inbreeding. Experiments to determine the proper time to pollinate cabbage have shown that better seed production results when pollinations are made several days before or several days after the flower firstopens.  Lines or families that produce practically no seed when pollinated with their own pollen after the flowers open may produce good crops of seed when pollinated from 1 to 5 days before the flower normally opens. Hand-pollination in the bud stage is effected by separating the surrounding sepals with the points of a pair of tweezers and applying the pollen from a mature anther to the exposed stigma. It is not necessary to remove the sepals, and in fact they may be helpful in preventing drying out of the pistils.

It has been well established that not only are there various degrees of self-sterility or self-incompatibility but also there are various degrees of cross-incompatibility among plants of related or unrelated origin. Careful hand pollinations are necessary to determine the exact fertility relations among strains or lines, but the facts when established are useful in working out a breeding program or in the production of hybrid seed on a commercial scale, as pointed out by Pearson (22). By planting in alternate rows strains that are self- incompatible but cross-fertile, hybrid seed will result through the action of insects in carrying the pollen from one strain to the other.  Bud pollination of a few flower clusters of each strain results in enough seed to perpetuate the strains for later crops. Bees have been found to be very effective agents in the cross transfer of pollen, and by enclosing the individuals or groups of plants under cheesecloth cages the bees may be used in working out the problem of obtaining desirable crosses between different strains or increasing the seed of a number of desirable crosses for preliminary commercial tests (23).

When incompatibilities are encountered, it may be possible to continue the improvement work by following a method of alternating selfings with mass increase. The first step is the production by bud pollination of as large progenies as possible of the desirable individuals. Most of the commercially important characters are quantitative in inheritance and large numbers of plants are necessary to produce enough individuals of the desired type. A number of desirable individuals of the same type are selected from the best line or lines and each lot or group is grown in an isolated location, where the plants are open-pollinated. The seed from all the plants of each lot or group are lumped together and sown. Plants of the desired type are selected from the best lots of this planting and bud-pollinated. The seed from each bud-pollinated plant is saved separately and selections for massing are made only from the best lines.  In a few years it will be possible to eliminate all but one best line, which, when uniform for the desired characters, may be increased for commercial use.

Propagation of new plants from the axillary buds or sprouts of cabbage has been followed by commercial growers of cabbage when it was desirable to increase the variety or individual as rapidly as possible. Only recently, however, has it been demonstrated (5) that vegetative propagation from the head or curd of heading broccoli was possible. The most satisfactory material was from pieces of the curd with scale leaves attached. When placed in a propagating house maintained at 55° F. during the night and with low humidity. and plenty of ventilation, these developed roots in 20 days, and in 40 days elongation of the floral axis had taken place.

By transplanting to the field in late spring the shortened stems of plants that had produced a crop of seed in the greenhouse, Miller (16) was able to force the development of new heads from lateral buds during the summer and by subsequent cold treatment to produce another crop of seed the following spring. By thus manipulating the environment in which the plants were grown he was able to maintain the cabbage plant as a perennial and yet produce a crop of seed annually.

Pearson (22) at the California Experiment Station worked out a rapid and ingenious method for determining the solidity of the cabbage head by determining the apparent specific gravity or density.

CYTOLOGY AND GENETICS OF LEAFY TYPES OF BRASSICA

Cytological investigations of a number of workers have shown that the wild cabbage found along the seacoasts of Europe, various varieties of heading cabbage, kohlrabi, kale, collards, cauliflower, heading broccoli, green sprouting broccoli, and brussels sprouts all have nine pairs of chromosomes (n=9). No significant differences have been reported in size or form among the chromosome sets of any of these forms of botanical or horticultural varieties. Hybrids among any of these forms are usually highly fertile, although sterility may sometimes occur, as is pointed out in the section on genetic studies and in the article on root crops (turnips and rutabagas). Botanical varieties of Brassica oleracea have been successfully crossed with radish (Raphanus sativus L.) (see the article on root crops), and rarely with other species of Brassica having different chromosome numbers, the resulting hybrids usually being entirely or highly self-sterile.

The wide diversity of form and function in Brassica oleracea would seem to make it an ideal species for genetic analysis, and except for several circumstances, our knowledge might be much more extensive than it is today. Much of the early work cannot be considered dependable because open-pollinated varieties were used. The presence of self-sterility has discouraged many workers from attempting to secure inbred lines with which critical work could be undertaken.  Many of the horticultural or botanical varieties require 2 years’ time to complete the life cycle and relatively large areas of land. Provision must also be made to prevent cross-pollination by insects.  Transference of pollen from anther to pistil must be done by hand or by insects enclosed in the isolation chamber, and extensive use of the backcross is almost prohibitive because of the large amount of hand labor required to get a sufficiently large number of seeds. In spite of these difficulties a number of studies have been made on the inheritance of various characters in the cabbage family. The more important contributions are briefly reviewed below under sectional headings indicating the plant character studied.

In view of the wide range of materials used it is not surprising that the workers report different results with what appear to be the same characters. It is obvious that critical genetic work on the Brassicas has only begun.

LEAF COLOR

Kristofferson (12, pt. I) reports on the inheritance of leaf color in various Brassica oleracea botanical and horticultural varieties. He tentatively assumes the interaction of five factors, each with the following effect: A produces no color alone, but with B produces the dark red violet midrib; B causes the light-red midrib; C under favorable conditions is able to produce a very faint pink color but with A produces the dark-violet midribs ef kale; D causes the general dark- red color of red cabbage; E is concerned with the distribution of the dark red violet color.

The factorial composition of the material with which he worked and its phenotypic appearance with regard to leaf color is given as follows:
          Red cabbage, AbcDe, dark red violet midrib and blade.
          Kale, Abcde, green midrib and blade.
          Cabbage, aBCdE, light-red midrib and green blade.
          Brussels sprouts, aBCdE, light-red midrib and green blade.
          Broccoli, abCdE, green midrib and blade.

The evidence presented in support of this hypothesis is far from conclusive, and the author himself in 1927 (12, pt. II) concludes: “For a firm establishing of the factorial basis of the total dark red violet color it may be necessary to grow the F3 generation.” He also states in this later paper that the factor D “shows any effect only when both factors for violet, A and C, are present,” which is evidently at variance with the scheme proposed in the first paper.

Other workers (1, 25, 28) have found a single factor difference between red (purple) and green foliage. Pease (26) and Moldenhawer (17) conclude that two complementary factors are concerned in the inheritance of color in a purple kohlrabi X Green Savoy hybrid, for in the F2 they obtained 9 purples : 7 greens.

C. H. Myers, of Cornell University, has isolated a type designated as magenta that is more nearly red than the so-called red cabbage, which he calls purple. Sunred is also a new foliage color name for a genetic type that shows reddish purple on the stem and midribs and on the edges of leaves on mature plants exposed to sunlight. Genes controlling purple and magenta, magenta and suncolor, and sun color and green are reported to be allelomorphs respectively. Crosses between magenta and sun color gave a purple F1, and 9 purple : 3 sun color : 4 magenta in the F2).  Working with related families, Magruder (13) obtained an F2of 9 purple: 3 magenta : 3 sun color: 1 green from across between a magenta and sun color, indicating the interaction of two independent genes in the production of the purple type studied. There was only asingle factor difference between sun color and green and between magenta and green. Kwan (11) used different families of the Cornell material and in a cross of purple X sun red the F2 approximated 15 purple : 1 sun red, suggesting duplicate factors responsible for the purple. The same purple crossed with green gave an F2 of 9 purple : 3 sun red : 4 green, indicating that this purple was not the same type as that used by Magruder.

In a review of the inheritance of leaf color it is obvious that a standard nomenclature should be used or detailed descriptions given in terms of one of the recognized color dictionaries. Free exchange of genetic color types among investigators would also facilitate a complete analysis of color.

OTHER LEAF CHARACTERISTICS

In a cross between wide blade (cabbage) X narrow blade (kohlrabi), Pease (25) found the F1 to approach more nearly the broad type, and in the F2 the narrow type constituted about one-fourth of the total. Kristofferson (12), however, in a cross between broad. (cabbage) and narrow (kale) found that the F1 resembled kale and most of the F2 plants had more or less intermediate type leaves, but parental types were also obtained.

A type of leaf in which outgrowths of a leafy nature called “asparagodes” occur along the midrib and larger veins at right angles to the plane of the blade was found by Pease (26) to be dominant to the normal leaf type and due to a single factor. Detjen (4) reports a similar character in his material but believes its expression is due to multiple factors.

The curliness of kale was found to depend on the action of several genes. The F1 of a cabbage X kale cross is intermediate in curliness, and the F2 shows continuous variation between the parental types.  Malinowski (14) assumed three polymeric genes, Allgayer. (1) four, and Pease (26) and Detjen (4) an indeterminate number. Kristofferson (12), however, reports a red cabbage X kale cross in which the F2 was relatively uniform and in which neither parental type appeared.  Kwan (11) crossed wrinkled (savoyed) and smooth-leaved cabbages and concluded that the wrinkled condition is due to the complementary action of two factors. There was no evidence of linkage of either of the factors for wrinkled leaves with either of the complementary factors for purple foliage color.

Contrasting entire with lyrate leaf shape, Pease (25) found entire to be due to a single dominant gene.

Petioled type of leaf as contrasted with sessile was found to be due to a single dominant gene by Pease (25); but Allgayer (7) postulated the action of three genes after his study of a cross between red cabbage and kale. Detjen (4) found the F1 of a cross between winged (sessile) and petiolate to be fully winged, which is the reverse of the condition described by Pease (25). He concludes that in his material "clean petiolate head leaves are governed by a recessive factor which may he one of a multiple series.” Environment has a marked effect on the expression of this character and makes a study of it very difficult.

Counts of the number of leaves below the mature head showed that in the F1 the number of leaves was generally that of the parent with the smaller number, but Pearson (24) concludes that “according to the evidence, the number of leaves is probably governed by modifying factors.”

HEIGHT OF PLANT

Tallness of plants is dominant to dwarfness and is due to a single gene according to Pease (25), Malinowski (14), and Allgayer (7). Kristofferson (12, pt. II) found continuous variation in plant height between the parental types in the F2 and concludes that numerous genes are involved. Detjen (4) also concludes that length of stem is dependent upon multiple factors for its expression. Kwan (11) obtained plants taller than his tall parent and shorter than the short parent in the F2 generation, with the F1 showing marked increase in height over the tallest parent. He says:

     The data suggest that the inheritance of plant height can probably be explained on the assumption that a series of dominant independent cumulative factors favorable for growth are concerned, and that each parent strain carried only part of these favorable factors.

No estimate of the number of factors was made, but the normal distribution of the F2 population indicated that the factors concerned were of equal value. There was no evidence of linkage between plant height or plant color or foliage surface.

HEAD CHARACTERISTICS

Most investigators have found the F1 from crosses between cabbage and any nonheading oleracea (except gemmifera) to show a slight heading tendency, and the F2 to exhibit continuous variation with recovery of both parental types, true heads being in the minority.  Malinowski (14) and Allgayer (1) consider that heading depends on the action of three pairs of genes. Pease (25) attributes it to duplicate genes, while Detjen (4), working with related headless and heading types of cabbage, found heading to be “fully dominant among related plants, or else the heading factor in headless strains is prevented by one or more factors from clearly manifesting itself.”

In crosses between inbred lines Pearson (24) concludes that “head shape, in general, is controlled by many factors, of no definite dominance." Crosses between long- and flat-headed strains of Copenhagen Market showed an intermediate shape. "There is some slight evidence that certain head factors are complementary to each other, since one group of crosses produced flatter heads than the parents.”  Detjen (4) concludes that ‘head form is not governed by single factors but may depend on a combination of several to many factors.”

In crosses between inbred lines Pearson (24) found some of the F1 lines to exceed either parent in weight; in others the F1 equaled the larger parent; and in crosses between closely related lines no increase in size resulted in the F1 generation. Detjen (4) records several crosses in which plant size of the F1 showed size equal to or greater than the larger parent.

Crossing brussels sprouts (which have axillary heads but no terminal head) with cabbage (the reverse of brussels sprouts) results in an F1 that closely resembles the brussels sprouts parent but has a head at the top. In the F2 there is continuous variation in the tightness of the axillary buds, with a tendency for most of the plants to have loose heads or buds. Kristofferson (12) believes the formation of axillary heads is governed by many factors that are independent of factors for the formation of the terminal head.

The formation of axillary shoots is due to a single factor and is recessive to nonformation, according to Allgayer (1).

In crosses among inbred strains differing in the relative length of the core, Pearson (24) found from his F1 results that "no dominance is shown in the inheritance of penetration of the core into the head.” Hybrids between short- and long-core types have been intermediate in length.

SEASON OF MATURITY

By utilizing inbred lines and F1 hybrids among them, Pearson (18) concluded that the season of maturity is—
dependent in part upon genetic factors, and that hybridization with resulting increase in vigor is not necessarily accompanied by an earlier time of maturity; likewise that environmental differences do not affect all strains in the same way, and that for a definite test, replications together with check rows are very necessary.

Detjen (4) says that "heading is found to be governed by a multiple factor for season, which fact explains the many seasonal strains.”  Rasmusson (27) noted an early maturity of the F1 from crosses among varieties of the same season of maturity. In crosses among early- and late-maturing varieties he notes the F1 as being only a little later than the earliest parent.

SWOLLEN STEM; BOLTING; COROLLA COLOR

The swollen stem or bulb of kohlrabi is incompletely dominant to unswollen stems (as in cabbage) in the F1, and in the F2 there is continuous gradation between the parental forms indicating the presence of several genes for bulbing in the kohlrabi. Pease (26) presents the clearest data in support of three factors, B1, B2, and B3, of which the first two are major factors and the third a modifying factor. In homozygous condition B1 and B2 result in bulb, and when either or both are in heterozygous condition the presence of B3 converts the stalk into “semibulb” condition. From crosses among related lines of cabbage differing in their stem diameter, Detjen (4) concludes that stem diameter is hereditary and dependent on many factors.

Certain varieties are known as "bolters" because when sown in the fall they produce seedstalks instead of heads the following spring.  Sutton (28) crossed a bolting and a nonbolting strain of cabbage and found the F1 to be nonbolting and the F2 to contain approximately 3 nonbolting : 1 bolting. Detjen (4), working with strains of cabbage from the Volga variety, found bolting to behave as a monogenic dominant over nonbolting or biennial habit, although he recognizes that bolting is influenced by other genes for time of maturity and heading-

White corolla color was found by Pearson (19) and Kakizaki (10) to be due to a single gene and dominant to the yellow corolla color.

SELF- AND CROSS-INCOMPATIBILITY OR STERILITY

According to Kakizaki (9), self- and cross-incompatibility in cabbage is caused by the slow rate of growth of the pollen tubes. In incompatible pollinations the slow growth is due to the presence of a substance that inhibits the growth of pollen tubes through the stylar tissue and in compatible pollinations the normal growth rate of the pollen tubes is due either to the absence of the inhibiting substance or the presence of an accelerating substance able to prevent the inhibiting action. The inhibiting substance is produced most abundantly when the pistil is in full vigor, and its production declines with the decline of the vigor of the pistil. The pseudofertility of bud pollination of incompatible matings is due mainly to insufficient inhibiting action, owing to immaturity of the style, and to the lower time interval for pollen-tube growth, as well as to the shorter distance to be traversed. Kakizaki’s results are explained by the hypothesis that two contradictory allelomorphic series of genes are concerned.  S1, S2, and S3 constitute the inhibiting series and T1, and T2 the accelerating series. The S series is epistatic to the T, but "T in double dose is more active than S in simple dose."  In order to explain different degrees of fertility it is assumed that the allelomorphs function in different intensities or that one or more factors of minor value are concerned. When selfed, some self-incompatible plants breed true, while others segregate into 1 self-compatible : 3 self-incompatible.  Self-compatible always segregates into 1 self-compatible : 1. self-incompatible. Pearson’s (21) results in most respects agree with those of Kakizaki", and Detjen (4) likewise agrees that—

incompatibility in the common cabbage is governed in the main by a series of multiple allelomorphs which result in the manifestation of very distinct types.  There are, however, other factors outside of such series that may affect the compatibility of plants such as was observed in the complete reversal of the Zinnia Rosette strain from practically complete self- and cross-incompatibility to practically full self-compatibility. Environmental factors such as temperature also may affect seed setting and temporarily mask the genetical factors.

RESISTANCE TO YELLOWS

Walker (32) has clearly demonstrated that resistance to yellows (Fusarium conglutinans Wr.) is a monogenic dominant to susceptibility to the disease in most of the yellows-resistant varieties developed by him and his coworkers. Anderson (2), one of his associates, has shown that the resistance of Wisconsin Hollander is genetically complex, as it cannot be permanently fixed and is influenced by environmental conditions. When grown at 22° to 24° C. all plants of Wisconsin Hollander are susceptible, while resistant types in which the resistance is due to the single dominant factor are fully resistant at these temperatures.

LINKAGE

The results of Pease’s work (25, 26) indicate the existence of three of the possible nine linkage groups. In the first group are found the genes (or one of a multiple series) for (1) petiolate leaves, (2) one of the genes for head formation, (3) entire leaves, (4) wide leaves, (5) possibly one factor for crinkling of the leaf, (6) bulbing of stem, and (7) red (purple) foliage color. A second group contains (1) a second gene for head formation, (2) a gene for tall plant habit, and (3) probably one of the genes for leaf crinkling.

“Asparagodes” malformation of the leaf thus far has not been linked with either of the above groups.

Malinowski (14) and Pease (25) report complete correlation between the degree of head formation and curliness in the F2 of a cabbage x curly kale cross. Malinowski inferred that the heading was due to three pairs of genes, ABC, and that curliness is produced by three other pairs of genes, XYZ, with complete linkage between A and x, B and y, and C and z.

LITERATURE CITED
 (1) Allgayer, H.  1928. GENETISCHE UNTERSUCHUNGEN MIT GARTENKOHL (BRASSICA OLERACEA) NACH KREUZUNGENVERSUCHEN VON RICHARD FREUDENBERG. Ztschr. Induktive Abstam. u. Vererbungslehre. 47: [191]-260, illus.
 (2) Anderson, M. E.  1933. FUSARIUM RESISTANCE IN WISCONSIN HOLLANDER CABBAGE. Jour. Agr. Research 47: 639-661, illus.
 (3) Blank, L. M., and Walker, J. C.  1933. INHERITANCE OF FUSARIUM RESISTANCE IN BRUSSELS SPROUTS AND KOHLRABI. Jour. Agr. Research 46: 1015-1022.
 (4) Detjen, L. R., and McCue, C. A.  1933. CABBAGE CHARACTERS AND THEIR HEREDITY. Del. Agr. Expt. Sta. Bull. 180, 147 pp., illus.
 (5) Graham, R. J. D., and Stewart, L. B.  1930. VEGETATIVE PROPAGATION OF BROCCOLI FROM HEADS. Bot. Soc. Edinb. Trans. 30: 216-217, illus.
 (6) Jones, L. R., and Gilman, J. C.  1915. THE CONTROL OF CABBAGE YELLOWS THROUGH DISEASE RESISTANCE. Wis. Agr. Expt. Sta. Research Bull. 38, 70 pp., illus.
 (7) Walker, J. C., and Monteith, J., JR.  1925. FUSARIUM RESISTANT CABBAGE: PROGRESS WITH SECOND EARLY varieties. Jour. Agr. Research 30: 1027-1034, illus.
 (8) Walker, J. C., and Tisdale, W. B.  1920. FUSARIUM RESISTANT CABBAGE. Wis. Agr. Expt. Sta. Research Bull. 48, 34 pp., illus.
 (9) Kakizaki, Y.  1930. STUDIES ON THE GENETICS AND PHYSIOLOGY OF SELF- AND CROSS-INCOMPATIBILITY IN THE COMMON CABBAGE (BRASSICA OLERACEA L. VAR. CAPITATA L.). Japan. Jour. Bot. 5: [133]-208, illus.
(10) ――1930. A DOMINANT WHITE-FLOWERED MUTANT OF BRASSICA OLERACEA L. Japan. Jour. Genetics 6: [55}-60.
(11) Kwan, C. C.  1934. INHERITANCE OF SOME PLANT CHARACTERS IN CABBAGE, BRASSICA OLERACEAE, VAR. capitata. Jour. Agr. Assoc. China 126-127:81-124, illus. (Thesis, Cornell Univ., privately publ. 1933.  Filed in office of Graduate School, Cornell Univ.)
(12) Kristofferson, K. B.  1924-27. CONTRIBUTIONS TO THE GENETICS OF BRASSICA OLERACEA (I-II). Hereditas 5: [297}+364, illus., 1924; 9: [343]-348, illus., 1927.
(13)  Magruder, R., and Myers, C. H.  1933. THE INHERITANCE OF SOME PLANT COLORS IN CABBAGE, Jour. Agr. Research 47: 233-248.
(14)  Malinowski, E.  1929. GENETICS OF BRAssIcA. Bibliographia Genetica 5: 1-26.
(15)  Melhus, I. E., Erwin, A. T., and Van Haltern.  1926. CABBAGE YELLOWS, CAUSED BY FUSARIUM CONGLUTINANS, IN IOWA.  Iowa Agr. Expt. Sta. Bull. 235, pp. [187]-216, illus.
(16)  Miller, J. C.  1929. A STUDY OF SOME FACTORS AFFECTING SEED-STALK DEVELOPMENT IN cabbaGE. N. Y. (Cornell) Agr. Expt. Sta. Bull. 488, 46 pp.,illus.
(17)  Moldenhawer, C.  1928. RECHERCHES SUR CERTAINS CROISEMENTS INTHRESSANTS DU GENRE Brassica. Bull. Internatl. Acad. Polon. Sci. et Lettres, ‘Cl. Sci. Math. Nat. (B) 1927: 1049-1071, illus.
(18)  Pearson, O. H.  1932. THE INFLUENCE OF INBREEDING UPON THE SEASON OF MATURITY OF CABBAGE. Amer. Soc. Hort. Sci. Proc. (1931) 28: 359-366.
(19) ――1929. A DOMINANT WHITE FLOWER COLOR IN BRASSICA OLERACEA L, Amer. Nat. 63: 561-565.
(20) ――1931. METHODS FOR DETERMINING THE SOLIDITY OF CABBAGE HEADS. Hilgardia 5: 383-398, illus.
(21) ――1931. FURTHER OBSERVATIONS ON THE TYPE OF STERILITY IN BRASSICA OLERACEA, VAR. cApiTaTa. Amer. Soc. Hort. Sci. Proc. (1930) 27: 337-342.
(22) ――1932. BREEDING PLANTS OF THE CABBAGE GROUP. Calif. Agr. Expt. Sta. Bull. 532, 22 pp., illus.
(23) ――1933. INCOMPATIBILITY IN BROCCOLI AND THE PRODUCTION OF SEED UNDER cages. Amer. Soc. Hort. Sci. Proc. (1932) 29: 468-471.
(24) ――1934. DOMINANCE OF CERTAIN QUALITY CHARACTERS IN CABBAGE, Amer. Soc. Hort. Sci. Proc. 31 (Sup.): 169-176, illus.
(25)  Pease, M.S.  1926. GENETIC STUDIES IN BRASSICA OLERACEA. Jour. Genetics 16:[363]-3835, illus.
(26) ――1927. GENETIC STUDIES IN BRASSICA OLERACEA. II. THE KOHL RABI. Jour. Genetics 17: [253]-267, illus.
(27)  Rasmusson, J. M.  1932. RESULTS FROM A CROSS CABBAGE X SAVOY CABBAGE. Hereditas 16:241-248.
(28)  Sutton, E. P. F.  1924, INHERITANCE OF “‘BOLTING” IN CABBAGE. "TENDENCY TO BOLT" PROBABLY A RECESSIVE MENDELIAN CHARACTER. Jour. Heredity 15: 257-260, illus.
(29)  Walker, J.C.  1930. INHERITANCE OF FUSARIUM RESISTANCE IN CABBAGE. Jour. Agr. Research 40: 721-745, illus.
(30) ――1933. YELLOWS-RESISTANT LINES OF JERSEY WAKEFIELD CABBAGE, Jour. Agr. Research 46: 639-648, illus.
(31) ――and Blank, L. M.  1934. FUSARIUM-RESISTANT DANISH BALDHEAD CABBAGE. Jour. Agr. Research 49: 983-989, illus.
(32)  Monteith, J., Jr., and Wellman, F. L.  1927. DEVELOPMENT OF THREE MIDSEASON VARIETIES OF CABBAGE RESISTANT TO YELLOWS (FUSARIUM CONGLUTINANS WOLL.). Jour. Agr. Research 35: 785-809, illus.
(33) ――and Wellman, F. L.  1928. A SURVEY OF THE RESISTANCE OF SUBSPECIES OF BRASSICA OLERACEA TO YELLOWS (FUSARIUM CONGLUTINANS). Jour. Agr. Research 37: 233-241, illus.

[FOOTNOTES]

ψ  The only consistent difference between broccoli and cauliflower is that broccoli will produce marketable curds during the cold winter weather in the Pacific Coast States, whereas cauliflower requires the warmer weather of the spring season. So the “cauliflower” that easterners get in the winter from the west coast is grown from broccoli seed. "Green sprouting broccoli" does not form a dense "head" or curd, as is the case with white-heading broccoli or cauliflower, because the floral branches elongate and are not blanched by protecting leaves.

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