Journal of Heredity 5: 269-272 (1914)
A RADISH-CABBAGE HYBRID
Cross Between Two Genera Shows Extraordinary Vigor but Absolute Sterility—
Pollen Irregular Both in Size and in Shape—Two Extra Stamens Present in Some of the Flowers.*
FLIPPO GRAVATT, Blacksburg, Va.
*This work was carried on during post-graduate studies at Virginia Polytechnic Institute under Prof. H. L. Price and Prof. E. A. Smyth.

IT IS a general rule with organic forms that whenever their systematic differences are very great, they will not hybridize with each other. A large number of species will hybridize with other species of the same genera, though the product obtained is usually sterile. The great majority of species will not hybridize with other species of the same genus. In regard to crosses between different genera a good many have been reported. Each of these successful crosses represents hundreds of unreported failures, owing to the great difficulty in effecting such a cross. These generic crosses have been effected in various different families, both in the Dicotyledons and in the Monocotyledons.

In the Cruciferae, mention was found of one generic cross, that between the garden radish, Raphanus sativus, and the garden cabbage, Brassica oleracea. This cross was made by Segeret, a German, many years ago, although there seems to be doubt about it in the mind of Fockle, who reports Segeret's work. [CybeRose Note: Sageret was French.]

The chief distinction between the genera Raphanus and Brassica are that the pod of the former is one celled or with spongy transverse partitions, while that of Brassica is -longitudinally two celled and dehiscent. Between the two species there are a number of differences, but these will he noted later. The radish and the cabbage have both been developed in cultivation for hundreds of years.

In the greenhouse during 1910 a large number of flowers of three different varieties of radish were emasculated and bagged. Two days later, when most of the stigmas were in receptive condition, they were pollinated with pollen from a cabbage plant which was a first generation hybrid between the varieties "Volga Russian" and "Curled Savoy." Perhaps the fact that the cabbage plant was a hybrid had something to do with the success of this generic cross. The bags were left over the pollinated flowers for two weeks. At the end of that time all had dropped off except two small pods of the variety "Long Scarlet Short Top." A large number of cabbage flowers were emasculated and pollinated with different varieties of radish, but no fruit set.

In September, 1910, the contents of these two pods, consisting of several small, shriveled seeds, were planted. Only one came up and from the first it could be seen that it was a cross. It was very vigorous and grew rapidly, finally being transplanted from a 20 inch pot to a central bed.

CHARACTER OF THE HYBRID

The hybrid produced a great abundance of large leaves. In size and shape they are nearer to the cabbage, but are much larger than those of either parent. The largest leaf of the cross was five feet nine inches from tip to point of attachment, and one foot seven inches broad. The leaf was measured at the time the plant started blooming.

In color the hybrid leaf is a mean between the light green of the radish and the dark or blue green of the cabbage. In taste it is more like the cabbage, but one can still detect some of the radish pungency. The radish leaf is pubescent with stiff hairs, while the cabbage leaf is glabrous. Leaves of the cross are nearly glabrous. The cabbage leaf is much thicker than that of the radish: leaves of the cross are intermediate between the two.


RADISH-CABBAGE HYBRID WHILE YOUNG
The growth habit of both parents is similar to that here shown, when they are young. Later the cabbage "heads up" and the radish does not; the hybrid followed the radish in this respect, but did not develop any tuberous root such as the radish produces. (Fig. 14.)

It would seem likely that the hybrid would inherit some tendency towards tuberous root formation from its radish parent, but such did not seem to be the case. The hybrid does not even show a well developed main tap root, but divides into numerous branches at a short distance below the surface.

The stem of this hybrid is several times larger than the combined stems of the two parents. When the hybrid reached the size shown in figure 15, the circumference of the stem was 13 1/2 inches, measured at a distance of one foot from the ground. The stem holds its size for some distance from the root. The lower part or about the first two feet of the stem is very thickly beset with leaves, thus resembling the cabbage in habit. These leaves drop off on account of shade from above, leaving large and prominent scars. Starting from the surface of the ground, the major portion of the stem is a purplish red color just the same as that of the. radish, but this purplish red color covers the stem of the hybrid much more thoroughly and extends further up than it does on the radish parent.

GROWTH HABIT

When still young the general growth habit of the radish and of the cabbage is quite similar. It is a spreading, diffuse growth. It is not long, however. before the cabbage begins to head up and packs most of its leaves into a solid, bud-like formation. Some time after this head reaches maturity, the bud begins growing in the center, by its development breaks the head open, and then grows upward rapidly, being nourished On the food stored in the thick leaves.

AN EXAMPLE OF HYBRID VIGOR
Hybrid forms usually excel either of their parents in vigor, and agriculturists have long taken advantage of this fact, in the production of both plants and animals. It is seldom, however, that so much extra energy is shown as by this cross between radish and cabbage, neither of which ordinarily reaches great size. The cross continued growing for five months after the above photograph was taken, and at the time it was killed by bacterial rot, it had gone through the ventilator of the greenhouse and was traveling down the roof on both sides. (Fig. 15.)

The radish differs from the cabbage in that it retains its open diffuse habit of growth. Instead of forming a solid head of leaves, it develops a tuberous root and in this stores its surplus food. Then when the radish reaches maturity, a bloom stalk is sent up, being nourished chiefly on stored food from the root.

The hybrid has an open, diffuse growth habit. During the stage in which the radish develops its root and the cabbage its head, the hybrid continues to develop numerous large leaves. These leaves are so thick that a large number of the lower ones die and drop off, due to lack of light.

At first there is a single elongated shoot, but sometime before blooming, numerous side branches start out. In figure 14, the side branches are just starting. The main shoot develops into a bloom stalk just as in the case of the parents. This one is shortly followed by others from the side shoots. These bloom shoots attain a much greater length than those of either of the parents. The side shoots make agrowth of several feet before any buds are developed. Figure 15 shows the enormous growth of this hybrid. It continued to grow for nearly five months after this photograph was taken, finally being killed by a bacterial rot of the root and stem. At the time of its death the hybrid had grown out through the ventilator and for a short way down the roof of the greenhouse on both sides.

FLOWERS

The pediceled flowers of this hybrid are borne on a greatly elongated rachis, which in some cases attains the length of three feet. The flowers of the parents are borne in the same way, though the flowering axis or rachis does not grow to such a length as with the cross. Perhaps the greater length of the rachis was due to the fact that no pods set. and therefore all the food went to the development of the flowers.

The buds of the radish are obovate in shape, while those of the cabbage are oblong-elliptical and much larger and longer. The buds of the cross are a mean between those of the two parents in shape and size. Near the top of the bud of the radish, there are several prominent hairs growing out from the sepals. The cabbage and the radishcabbage cross do not have these hairs.

The cross has four petals, intermediate in size between the two parents. The radish parent was dominant in regard to color of the petals, the background color being white with a purplish tinge and veins. There was no sign of the rich yellow color of the petals of the cabbage parent.

The radish and cabbage flowers each have six stamens. Most of the flowers of the cross had six stamens but a good many had two extra well developed stamens, making eight. There were about 15% of the flowers which showed this variation. These two extra stamens bring up some interesting questions which will be discussed in another article.

It was noticed that the pollen from this cross was not powdery and light as that of the radish and cabbage, but stuck together and had a tendency to ball up. There was often difficulty in spread ing the pollen out on the stigma. On examination it was found that the pollen from the radish and that from the cabbage were very regular in size and shape, while that of the hybrid was very irregular in both respects. Attempts to germinate the pollen from the hybrid in various solutions all failed.

Most of the ovaries of the hybrid appear to be two-celled like those of the cabbage parent. One slide of a hybrid pistil showed sections of an ovary with three cells.

STERILITY OF THE HYBRID

About crosses between species De Vries says: "Whenever their systematic differences are too great, the crosses will be infertile, even with the pollen of their parents. Repeated crosses are impossible and no practical results can be obtained." Generic crosses are nearly always sterile. Most of them do not even produce flowers and if they do, the flowers are usually aborted in some way.

In March, 1911, this hybrid started blooming in great profusion. Nearly every day during March, April and May, pollen from different varieties of radish and cabbage, including the parents, and from cauliflower, collards, Brussels sprouts and their crosses, was tried on the flowers of the radish-cabbage hybrid, but nothing resulted. The hybrid was also sterile to its own pollen. Flowers of the radish, cabbage and various Brassica crosses were emasculated. bagged and pollinated with the hybrid pollen, without results. The same process of pollination was gone through in the winter of 1912 with a root cutting of the hybrid and again resulted in failure.