following extract is given to explain the manner in which Mr. Knight conducted his celebrated experiments on fruits, which rewarded him with some varieties that were highly esteemed:—"Many varieties of the apple were collected which had been proved to afford, in mixtures with each other, the finest cider. A tree of each was then obtained by grafting upon a Paradise stock, and these trees were trained to a south wall, or if grafted on Siberian crab, to a west wall, till they afforded blossoms, and the soil in which they were planted was made of the most rich and favorable kind. Each blossom of this species of fruit contains about twenty chives or males (stamens,) and generally five pointals or females (pistils,) which spring from the center of the cup or cavity of the blossom. The males stand in a circle just within the bases of the petals, and are formed of slender threads, each of which terminates in an anther. It is necessary in these experiments that both the fruit and seed should attain as large a size and as much perfection as possible, and therefore a few blossoms only were suffered to remain on each tree. As soon as the blossoms were nearly full-grown, every male in each was carefully extracted, proper care being taken not to injure the pointals; and the blossoms, thus prepared, were closed again, and suffered to remain till they opened spontaneously. The blossoms of the tree which it was proposed to make the male parent of the future variety, were accelerated by being brought into contact with the wall, or retarded by being detached from it, so that they were made to unfold at the required period; and a portion of their pollen, when ready to fall from the mature anthers, was during three or four successive mornings deposited upon the pointals of the blossoms, which consequently afforded seeds. It is necessary in this experiment that one variety of apple only should bear unmutilated blossoms; for, where other varieties are in flower at the same time, the pollen of these will often be conveyed by bees to the prepared blossoms, and the result of the experiment will in consequence be uncertain and unsatisfactory." * * *
In his Pomona Herefordiensis, he says:—"It is necessary to contrive that the two trees from which you intend to raise the new kind, shall blossom at the same time; therefore, if one is an earlier sort than the other, it must be retarded by shading or brought into a cooler situation, and the latest forwarded by a warm wall or a sunny position, so as to procure the desired result."
We must distinguish between hybrids proper and crosses, as it were between races or between what may have been erroneously designated species, for there has been a great deal of looseness in the manner of using these terms by some writers. A true hybrid9 is produced only when the pollen of one species has been used to fertilize the ovules of another, and as a general rule these can only be produced between plants which are very nearly allied, as between species of the same genus. Even such as these, however, cannot always be hybridized, for we have never found a mule or hybrid between the apple and pear, the currant and gooseberry, nor between the raspberry and blackberry, though each of these, respectively, appear to be very nearly related, and they are all of the order Rosaceæ.
In hybrids there appears to be a mixture of the elements of each, and the characters of the mule or cross will depend upon one or the other, which it will more nearly resemble. True hybrids are mules or infertile, and cannot be continued by seed, but must be propagated by cuttings, or layers, or grafting. If not absolutely sterile at first, they become so in the course of the second or third generation. This is proved by several of our flowering plants that have been wonderfully varied by ingenious crossing of different species. But it has been found that the hybrid may be fertilized by pollen taken from one of its parents, and that then the offspring assumes the characters of that parent.10
Natural hybrids do not often occur, though in diœcious plants, this seems to have been the case with willows that present such an intricate puzzle to botanists in their classification, so that it has become almost impossible to say what are the limits and bounds of some of the species. Hybrids are, however, very frequently produced by art, and particularly among our flowering plants, under the hands of ingenious gardeners. Herbert thinks, from his observations, "that the flowers and organs of reproduction partake of the characters of the female parent, while the foliage and habit, or the organs of vegetation, resemble the male."
Simply crossing different members of the same species, like the crossing of races in animal life, is not always easily accomplished; but we here find much less difficulty, and we do not produce a mule progeny. In these experiments the same precautions must be taken to avoid the interference of natural agents in the transportation of pollen from flower to flower; but this process is now so familiar to horticulturists, that it scarcely needs a mention. In our efforts with the strawberry, some very curious results have occurred, and we have learned that some of the recognized species appear under this severe test to be well founded, as the results have been infertile. Where the perfection of the fruit depends upon the development of the seed, this is a very important matter to the fruit-grower; but fortunately this is not always the case, for certain fruits swell and ripen perfectly, though containing not a single well developed seed. It would be an interesting study to trace out those plants which do furnish a well developed fleshy substance or sarcocarp, without the true seeds. Such may be found occasionally in the native persimmon, in certain grapes, and in many apples; but in the strawberry, blackberry, and raspberry, the berry which constitutes our desirable fruit, never swells unless the germs have been impregnated and the seeds perfect. In the stone-fruits the stone or pit is always developed, but the enclosed seed is often imperfect from want of impregnation or other cause—and yet the fleshy covering will sometimes swell and ripen.
One of the most successful experimenters in this country is Doctor J.P. Kirtland, near Cleveland, Ohio, whose efforts at crossing certain favorite cherries, were crowned with the most happy results, and all are familiar with the fruits that have been derived from his crosses. The details of his applying the pollen of one flower to the pistils of another are familiar to all intelligent readers, and have been so often set forth, that they need not be repeated in this case—great care is necessary to secure the desired object, and to guard against interference from causes that would endanger or impair the value of the results.
Van Mons' theory was based upon certain assumptions and observations, some of which are well founded, others are not so firmly established. He claimed correctly that all our best fruits were artificial products, because the essential elements for the preservation of the species in their natural condition, are vigor of the plant and perfect seeds for the perpetuation of the race. It has been the object of culture to diminish the extreme vigor of the tree so as to produce early fruitage, and at the same time to enlarge and to refine the pulpy portion of the fruit. He claimed, as a principle, that our plants of culture had always a tendency to run back toward the original or wild type, when they were grown from seeds. This tendency is admitted to exist in many cases, but it is also claimed, that when a break is once made from the normal type, the tendency to improve may be established. Van Mons asserted that the seeds from old trees would be still more apt to run back toward the original type, and that "the older the tree, the nearer will the seedlings raised from it approach the wild state," though he says they will not quite reach it. But the seeds from a young tree, having itself the tendency to melioration, are more likely to produce improved sorts.
He thinks there is a limit to perfection, and that, when this is reached, the next generation will more probably produce bad fruit than those grown from an inferior sort, which is on the upward road of progression. He claims that the seeds of the oldest varieties of good fruit yield inferior kinds, whereas those taken from new varieties of bad fruit, and reproduced for several generations, will certainly give satisfactory results in good fruit.
He began with seeds from a young seedling tree, not grafted upon another stock; he cared nothing for the quality of the fruit, but preferred that the variety was showing a tendency to improvement or variation. These were sowed, and from the plants produced, he selected such as appeared to him to have evidence of improvement, (it is supposed by their less wild appearance), and transplanted them to stations where they could develop themselves. When they fruited, even if indifferent, if they continued to give evidence of variation, the first seeds were saved and planted and treated in the same way.
Balfour's Manual.
Balfour's Manual.
