Luther Burbank: His Methods and Discoveries 2: 286-295. (1914)
Raspberry x Strawberry Hybrids

One of the most curious hybridizing experiments that I have ever conducted was made in an effort to test the limitations of affinity between the various members of the rose family. I had on my place a bush of the California dewberry, a plant that differs from most other members of the family in that its staminate and pistillate flowers are borne separately.

The particular bush in question had only pistillate flowers, and as it grew in isolation, it ordinarily bore no fruit, as its flowers were seldom fertilized. At most it occasionally developed single drupelets, a result no doubt of partial fertilization from grains of pollen accidentally brought from a distance by wind or insect.

The isolation of the plant, and the fact that it bore unisexual flowers, seemed to offer favorable opportunity for experiment.

SOME HYBRID BERRIES

Upon this plant I applied the pollen of various species of plants of the same family. The list is a striking one, for it included the apple, the mountain-ash, the hawthorn, the quince, the pear, and various kinds of roses.

I worked at these hybridizations attentively during the blooming season of the dewberry the summer of 1886.

The pistils thus fertilized developed an abundant crop of fruit, and in the ensuing season I raised from these berries between five and six thousand seedlings.

Never on earth, perhaps, was there seen a more widely varying lot of seedlings that were the immediate offspring of a single plant. The hybrids took almost every possible form that could be suggested as combining the traits of the various parent plants. Most of them were absolutely thornless. Many grew upright like the apple tree, showing nothing of the drooping tendency even of the raspberry, much less the trailing habit of the dewberry. The leaves were generally quite smooth, some resembling those of the pear, others being partially trifoliate, and most of them assuming strange and unusual forms.

When this motley company came to the time of blooming, there was still another surprise, for the flowers were as varied as the foliage. Some of the blossoms were crimson in color, half as large as an apple blossom; some were pink and quite small; others were white. A large number of plants, however, did not bloom at all, although they were attentively cared for, and were otherwise normal.

From these strange hybrids I not unnaturally expected to raise a remarkable variety of fruits.

I had hopes even of being able to produce something of real value, at any rate from the second generation.

But when it came time for the fruits to ripen, another surprise awaited me; only two plants out of the five thousand produced a single fruit. One of these was a plant somewhat resembling a raspberry bush, and this produced a number of ill-tasting berries of a yellowish-brown. The other bush produced insignificant fruits of orange-yellow color.

Though unpromising in themselves, these fruits were carefully watched and guarded, for I felt convinced that possibilities of strange variation were contained in them, if only I could get from them a few seedlings. But when the fruits were fully matured, I examined the seeds and found all of them hollow. They were nothing but shells containing no kernel.

So by no possibility could I get a single seedling of a succeeding generation.

Some of the most curious of the plants were preserved for another season, but they proved as unproductive as before; and as I needed the ground for other purposes I felt constrained to destroy the entire company of curious hybrids. In all my experience I never destroyed a lot of plants with more sincere regret.

An experiment perhaps even bolder was made at about the time of my experience with the hybrid dewberries. This was the hybridization of the strawberry and the raspberry.


Leaves of Strawberry-Raspberry Hybrids
The strawberry-raspberry hybrids produced by Mr. Burbank had leaves which were uniformly trifoliate, but which varied greatly in size and shape. Characteristic samples of the different forms are here shown. It was peculiarly to be regretted that the hybrids were not fertile, as a new and highly interesting form of fruit would doubtless have resulted had it been possible to establish a permanent race combining the blood of the strawberry and the raspberry.

The attempt to cross plants of such unlike appearance would seem to most experimenters absurd. Yet the cross was successfully effected. The raspberry was selected as the pistillate plant, and pollen was applied from whatever strawberry was at hand. It was impossible to choose as to the latter point, for the strawberry is for the most part out of season when the raspberry blossoms. I had to use such material as I could find.

The pollenation proved effective, and the raspberry plant produced a full crop of fruit.

There is no very marked immediate effect observable from such a hybridization. The pulp of the berry seems not to be affected; but the essential seeds within the berry are enormously modified, as the sequel showed. For when the raspberry seeds were planted in the greenhouse, the young hybrid plants that came up in profusion had all the appearance of ordinary strawberry plants. No one who inspected them casually would suspect their hybrid origin.

The raspberry, the pistillate parent on which the seeds had grown, has leaves with five leaflets. But there was no leaf of this character among all the hybrids; without exception their leaves were trifoliate like the leaf of the strawberry.

In other words, in the matter of foliage, the strawberry plant was entirely prepotent or dominant, and the characteristics of the other parent were latent or recessive.

When the hybrids were old enough, they were carefully set out in rows in the open field. For a month or more after transplanting they showed no inclination to depart from the habit of the strawberry. To close inspection it might appear that the main stem was unusually thick, and that the leaves were a little more wrinkled than is usual with the strawberry, and their edges slightly more serrated. But aside from this, the hybrid plants were seemingly true strawberries.

About the first of June, however, the plants began to throw out underground stolons, whereas strawberry runners are normally on the surface. These stolons suggested roots of the raspberry, yet the new plants that sprang from them here and there were exactly like the strawberry plants. So at this stage it would seem that the influence of the mother parent had been but slight.

But along in July came the transformation. Rather suddenly each main plant sent up two, three, or more strong smooth canes, which grew to the height of from two to five feet. These canes were absolutely thornless, as were all other portions of the plant; they were as smooth as strawberry plants in leaf and stem, but their form and manner of growth now departed strangely from the traditions of the trailing parent.

Obviously the influence of the raspberry parent had at last made itself potent.

Some of the plants were yellowish, indicating that the berries would probably be yellow; others were reddish. There were no blossoms the first season, but the ensuing year panicles of blossoms of great size were put forth, some of the bunches being twelve inches in breadth—far larger than those usually seen on the raspberry. In a single panicle there were sometimes several hundred flowers. The individual blossoms were generally larger than the flowers of the raspberry, but slightly smaller than those of the strawberry.

In the center of each blossom was a miniature berry, which might be said to resemble either a strawberry or a raspberry, being so small that its exact characteristics could hardly be distinguished.

I was quite sure I had a valuable cross, and that at least one might be found among the many that would produce fruit. But in this I was disappointed; not a plant produced a single berry. The miniature fruit remained unchanged in size until it finally dropped from the bush in the fall.

The following season a few of the plants bore one or two fruits having two or three drupelets each, like mere fragments of a normal raspberry. But not a seed was found. So here the experiment ended, and the hybrid strawberry-raspberries followed the hybrid dewberries to the brush heap.