Methods and Discoveries (1914)

The Stoneless Plum

Luther Burbank


A Typical Stoneless Plum
In this stoneless plum the seed, with its germinal substance, is retained, but the stone has been almost altogether eliminated. The resulting fruit is a plum of such unique character that you may bite through it almost as readily as you would bite through a strawberry. The production of this stoneless plum is one of Mr. Burbank's greatest achievements. It was accomplished through a long series of hybridizing experiments fully explained in the text.

At an early stage of my almost endless series of experiments in the hybridizing of plums, I chanced to hear of a so-called seedless plum that was said to grow in France, where it had been known for a long time as a curiosity. About 1890 I sent to the Transom Freres Nurseries in France and secured twigs of this plum, which was known merely as the Sans Noyau.

These were grafted on one of my plum trees, and in due course produced a crop of fruit, which as expected, proved to be a blue-black, cranberry-sized fruit, extremely sour, soft, and unfit for eating either raw or cooked. The original shrub, as I have been informed, and as it grew here, is a rambling, thorny bush rather than a tree, utterly worthless for any purpose except the one for which I desired it.

The fruit, besides being flavorless and unpalatable, was scanty in yield.

Moreover the fruit was by no means seedless, notwithstanding its French name. It was only partially stoneless, as most specimens produced fair-sized kernels in the fruit, and every kernel had a thick rim of stone around one side partially covering the kernel. While it therefore lacked much of exhibiting the condition of stonelessness that I had hoped to see, it did, nevertheless, show a tendency to abandon the stony covering that has always characterized all the fruits of the plum family.

From the outset I was convinced that by proper hybridizing and selective breeding it could be made valuable.

Next season the blossoms of the freak plum were fertilized with the pollen of the French prune and with that of numerous other plums and prunes.

The seedlings from these crosses were grafted to ensure their earlier bearing. In the first generation I obtained some plums fully twice as large as their seed parent. Most of these had stones, however, and were soft, sour fruits. A very few of them were partially stoneless, and from these the work was continued.

GETTING RESULTS

The next generation showed some general improvement in the growth of the tree and the size and quality of the fruit. All the seedlings of the cross from the Sans Noyau upon the French prune were grafted and fruited, even though many of them showed the thorny, dwarfed, ill-shaped type of tree of the uncultivated ancestor.

After two or three generations there was a marked tendency to improvement.

In a large lot of seedlings, in 1904, I obtained two that seemed to me of favorable appearance—for much can be known from the quality of leaf and stem long before the time of fruiting.

And when, two years later, the grafts thus selected bore fruit, it was delightful to find my predictions verified; the fruit was almost absolutely stoneless, only the faintest splinter of stone occasionally appearing. And combined with this sloneless condition there were qualities of size and flavor that made the fruit practically equal to the French prune. Moreover, as is often the case with hybrids one strain of which is wild stock, the new plum proved to he a very good bearer.

So my ideal of an eatable plum having no stone about its seed was almost achieved.

I say almost achieved because there still remained, in the case of the plums of best quality, a fragment of shell which varied from a small crescent about one side of the kernel to an almost invisible granule. There were some individual plants among the numberless seedlings that bore fruit in which the stone was absolutely eliminated and, in some cases, the seed also.

But it proved extremely difficult to combine this quality of entire stonelessness with the desirable qualities of size and flavor, lacking which the fruit could have no practical value.

Further hybridizing experiments, aimed at the production of an absolutely stoneless plum of fine flavor, are still under way; but in the meantime there are several varieties actually in hand that are of most admirable quality and yet stoneless. In the ordinary French prune, from three to six per cent. of the entire fruit is stone; while in my stoneless prune called the "Conquest" the fragment of stone does not represent more than a thousandth part of the bulk or weight of the fruit.

And among the eight or ten hundred varieties of stoneless plums now growing in my orchard, there are sure to be some that will show still further improvement.

WHY THE TASK WAS DIFFICULT

The task of producing a stoneless plum had proved very difficult chiefly because it had all along been necessary to bear in mind a number of quite different objective points.

It was not sufficient to produce a stoneless plum. From the practical standpoint there would be no object in that unless the fruit about the stoneless kernel was of good size and of palatable quality. And, unfortunately, there appeared to be no tendency to correlate stonelessness with good quality of fruit.

In point of fact the tendency was quite the other way; and, indeed, this was to be expected in view of the fact that the original partially stoneless plum was a small, acid fruit growing on a wild bush.

The problem was to combine two lines of ancestry that were in many respects directly in conflict. It would have been impossible to do this had it not proved that stonelessness and good quality of fruit, although not originally combined, have the attributes of what may be called unit characters, and hence can be assembled in a single fruit in the later generations of a hybrid progeny.


Three Stages of Development
At the left is the original wild French plum, called the San Noyau—of insignificant size and practically inedible. It is almost stoneless. Mr. Burbank improved the plum by hybridizing it with cultivated varieties, retaining the stoneless condition and introducing the qualities that make a commercial fruit. The central figure shows the plum at an intermediate stage of development; at the right the same fruit as represented by its improved stoneless descendant a generation or two later.