Conference on Genetics (1906) 401-404.
THE BREEDING OF COLD-RESISTANT FRUITS.
Prof. N. E. HANSEN
(State Agricultural College and Experiment Station, Brookings, South Dakota, U.S.A.)
IN the great grain- and stock-growing region known as the prairie NorthWest, comprising the northern part of the Mississippi and Missouri river-valleys of the United States of America, considerable trouble has been experience& in the cultivation of the fruits brought originally from the milder and moister regions of Western Europe. The history of our western prairie horticulture records a multitude of failures from this cause. For example, it has cost, at a low estimate, over £20,000,000 to learn that the apples of Western Europe are not adapted to the climatic extremes of this vast continental region. The periodical crucial-test winters, such as 1855-6, 1872-3, 1884-5, 1898-9, show that they lack resistance against severe winter freezing. Many thousands of seedlings have been raised from this West European race in the endeavour to advance the limits of successful apple cultivation northwestward, but without permanent success, the test winters usually making an end to the experiment. The introduction of the Russian race of apples has advanced this limit far north and northwestward of the former limits, and rapid progress is now being made with their American seedlings and with crosses of the Russian with the West European type, the latter being usually known as the American apples.
Plums from many countries have been tested upon this fertile inland plain, but after "the smoke of battle" has cleared away, the native plums of the prairie North-West (Prunus americana) remain in undisputed possession of the field. Great advances have been made in breeding large plums of good table quality from this indigenous race; hybrids with plums from other parts of the world are also coming on. So rapid has been the progress with pure native seedlings that an ingenious western experimenter has ventured the opinion that our native western plums are really descendants from plums brought over in the Mongolian migration from Eastern Asia, especially Japan, by the prehistoric ancestors of the present North American Indians.
The history of raspberry culture is in a measure a repetition of the failure of the apple—an utter lack of success with the West European raspberries. The native prairie race is now being ameliorated.
While working in fruit-breeding since 1895 in South Dakota, and for some years previous to that time in Iowa, my endeavour has been to discover some underlying law in the ever-shifting panorama of phenomena. Two years ago the number of fruit seedlings was fully a quarter of a million; the number has since been augmented by the raising of many thousands of new seedlings, and decreased by the destruction of many thousands of inferior seedlings every year.
The explanation that is to my mind the most satisfactory is the law of De Candolle, given in his "History of Cultivated Plants." De Candolle states that no species of plant has extended 100 miles north of its original limits within historic times, although seeds have been carried far north of the original limits by birds and other agencies; and that changements of form or duration are required, or periods of four or five thousand years are needed, for it to endure a greater degree of cold. In testing a collection of over 500 varieties and species of ornamental trees and shrubs from various parts of the world, their different degrees of hardiness under severe climatic conditions, which meant at one time 40° Fahr, below zero with the ground bare of snow, led me to investigate the geographical range of each species. De Candolle's law of hardiness, just quoted, appeared to me a satisfactory explanation of the behaviour of these many species of plants.
In connection with the law just quoted it will be of interest to give some of the costly experience, in the prairie North-West, of our nurserymen and tree-planters. I will give only a few out of many instances.
The box elder (Acer Negundo or Negundo aceroides) from Virginia, winter kills in the North-West, while the local form of the box elder indistinguishable from it by botanical characters, is perfectly hardy. The tree-planters of Manitoba have found that the box elder from several hundred miles south will not endure Manitoba winters, while the same species from their own locality, known as the Manitoba maple, is perfectly hardy. In Russia the box elder was once considered tender in the north; it was found that their seed was gathered near St. Louis; since then seed from the far north has been tested and found hardy.
The red cedar was formerly brought to the north in large quantities from Tennessee, which is well to the south. Northern nurserymen have learned that they must cultivate only the northern form of the red cedar to avoid total failure.
Robert Douglas, of Illinois, found a great difference in hardiness between the northern and southern forms of the black walnut. The former lived, the southern died, in Northern Illinois,
They have found also that they must be careful as to the source of western conifers. For cultivation in the prairie North-West, the form from the Pacific slope of the Rocky Mountains is tender, while if gathered on the eastern slope of the Rockies it is hardy.
Much more evidence might be given along the same line, all going to show that Nature has done this great work of acclimatising species, but that thousands of years are needed for the work. The converse of the law appears to be true, that a species cannot be extended southward to any great extent beyond its natural limits. Munson, of Texas, has found that the northern americana plums are winter-killed in Texas, because the buds start prematurely at the first warm spell in mid-winter. In Russia a similar tendency has been noticed with the Siberian larch when brought into Southern Russia.
It will be noted that this law applies only to wooded plants that must endure the winter, not to annual plants such as Indian corn or maize.
This species has been advanced many hundreds of miles northward by shortening the season. In what is considered its native home, in the tropical or semi-tropical regions of South and Central America, it takes seven months for maturity, and attains a height of twenty feet, while at its northern limits it is five feet or less in height, and takes less than three months for maturity. This work was done by Indians long before the advent of the white man. It will be noticed that the plant has not lost its need for a high degree of heat during its ripening period, and has not increased its power of resistance against frost. In the winter-time the dry seed, if kept dry, will resist any degree of cold. The plant, in other words, has been shortened in season only, and not changed in its capacity to endure cold.
It is now quite evident, from a survey of the whole field, that hardiness cannot be obtained by selection alone. This is the work for Nature, not man, to undertake. It is unprofitable for him to begin a labour that takes many thousands of years for completion. But hardiness can be obtained by crossing with a hardy species.
As to hardiness being a Mendelian character, I know not. In our work of selection hitherto we have insisted on large size and good quality of the fruit, as well as hardiness, which has compelled the destruction of thousands of inferior-fruited seedlings which were hardy; and in plants propagated by budding and grafting it has not been necessary to fix the type. That hardiness can be transferred, by crossing a tender species, now admits of no question. For example, my hybrids of the wild prairie strawberry with the French ever-bearing type survive, while the French parental type the winter kills. The hybrid of Western sand cherry with a Chinese apricot is hardy, while the Chinese species (Prunus Simoni) is winter-killed.
The question arises—"What is hardiness?" Some fifteen years ago the Iowa State Horticultural Society had an investigation conducted to determine the nature of hardiness in the apple. Chemical examinations were made of the wood of hardy and tender varieties; the cell structure was examined under high powers of the microscope, and the number of palisade cells in the leaf was investigated. All led to negative results. It would be a great advantage to be able to determine by chemical or histological examination as to whether a new variety of apple would prove hardy in our test winters, but we must regard the problem as unsolved. Hardiness can be transmitted; it is something intangible to superficial examination, but inherent in the plant itself.
The United States Department of Agriculture has extended the citrus fruit belt northward, by hybridising the cold-resistant Citrus trifoliata of Japan with choice sweet oranges. This work is of the greatest possible value, and incidentally illustrates this same possibility of hardiness being imparted by crossing with a hardy species.
There is a limit to the northward extension of the cultivated apple (Pyrus Malus) even by the hardiest representatives of the Russian race. This is very likely determined by the cold-resistant capacity of the indigenous race of Pyrus Malus in Russia itself. To extend the apple limit north-westward it will be necessary to hybridise with the pure Siberian crab (Pyrus baccata); this is now being done in many places. The work of Thomas Andrew Knight one hundred years ago in England, would help in this work, were it possible to find the hybrids of this ancestry which he originated at that time. However, from the thousands of seedlings of this parentage so far produced, by design or as chance seedlings, in the United States, we have not secured the winter-keeping capacity which is so greatly desired. To illustrate this need, I may add that the Minnesota State Horticultural Society has offered one thousand dollars reward to anyone who will originate an apple equal in hardiness to the 'Duchess of Oldenburg,' in size and quality to 'Wealthy,' and in winter-keeping capacity to 'Malinda.' So far the prize has not been awarded.
One of my favourite plants is the Western sand cherry (Prunus Besseyi); this bush form of the cherry is a native of the dry plains of the North-West, and is a favourite fruit of the Sioux Indians. In going over a patch of 25,000 plants of the third generation under cultivation last summer, some plants were found bearing fruit fully one inch in diameter, and of good quality; this shows the cumulative effect of selection. Apart from its being a promising species from the fruit standpoint, it is of interest as a possible dwarf stock for peaches. I find that peaches upon this stock bear as potted trees when less than three feet in height.
The converse of the law also appears true, that the southern limits of a species may be extended by crossing with a heat-resistant species. The best example I know is the Kieffer pear. This pear originated as a chance seedling near Philadelphia, and is a hybrid of the Chinese sand pear with the Bartlett or some other choice representative of the West European type. This variety has made possible the extension of the commercial pear belt several hundred miles southward in the United States. We hope now to obtain better quality by infusing a larger percentage of the European species in the Kieffer and its seedlings.
In illustrating the work of plant-breeders in general, I have sometimes said that we are looking for the Shakespeare of the species, and that no minor authors will suffice. The light thrown upon plant heredity by Mendel and De Vries gives us great hope for the future. The De Vries mutation theory itself has been aptly termed a mutation of Darwinism.
The modern plant-breeder rides an automobile upon the highway of evolution, and the theories of De Vries and of Mendel may be two of the wheels. It is a great and inspiriting thought that evolution is, so to speak, a kangaroo and not a snail, and that a new and valuable plant may appear suddenly, as Minerva sprang full-fledged from the head of Jupiter.
We are thankful for all this new and recent light upon heredity, and are endeavouring to apply the principle in as many ways as possible. But we must deal in large numbers. From the ashes of millions of seedlings must arise, phoenix-like, the new creations which will dominate our prairie pomology.