Equally efficient in the hands of the cultivator has been another method of producing new forms—namely, hybridization. If the pollen of a plant be transferred to the stigma of a related species, offspring is often produced; and the product is a batch of plants intermediate in characters between the two parents, and generally uniform in appearance. Should these be crossed again, a heterogeneous offspring is the result, displaying a variety of characters inherited from one or other original parent. The crossing of varieties, native or cultivated, has the same result. Hybrids occur in nature, but not very frequently. Insects visiting flowers are well known to confine their attention to a great extent to one species at a time, so as agents of hybridization they are not efficient. Again, many hybrids do not produce fertile seed, so that if they arise by natural means they are not perpetuated. In the garden, hybridizing has been resorted to largely; but its practice is not so ancient as the method of producing improved breeds by selection.
The cultivation of specially selected forms is certainly of remote origin, and probably goes back to the earliest days of agriculture: of early date, too, is the introduction into regions where they do not occur naturally of plants desirable for their use or beauty. The records of the cultivation of the Vine, for instance, go back for five or six thousand years in Egypt. Two thousand years ago Pliny writes that ninety-one principal forms could be reckoned in his day, though “the varieties are very nearly as numberless as the districts in which they grow.” Theophrastus, three hundred years earlier, discourses learnedly of the different kinds of cultivated Figs, etc., and their superiority over the wild kinds. These and other authors make frequent mention of plants introduced into Greece or Italy from the East for their usefulness or their pleasing qualities. Nowadays, the number of species cultivated, the innumerable forms of these which are grown, and the wide distribution which these forms have attained, have resulted in the cultivated flora of a country like England being, so far as the higher plants are concerned, much larger than the native flora, even when all the plants which are grown under glass are left out of consideration.
In the case of plants of economic importance, the usual aim of selection has been increase of size or productiveness of the parts which are useful. In some instances selection has taken several directions inside the limits of a single species, as in the forms of Cabbage, which are all the offspring of Brassica oleracea ([Fig. 25]), a seaside plant of Western and Southern Europe, and are mostly creations of comparatively recent date. The Cauliflower has been produced by increasing the size of the inflorescence; White Cabbage by promoting leaf production; Brussels Sprouts by encouraging the development of axillary shoots; while a form with a tall and woody stem is made into walking sticks. More often we find a species developed along a single line. For instance, the tendency to store food materials in a fleshy taproot has been developed in the case of Turnip, Beet, Carrot; the fleshy scale-leaves which form bulbs have been exploited in the case of the Onion; increased stem-growth is promoted in Asparagus; increased leaf-growth in Spinach and Lettuce; while by the development of seeds and fruits of many kinds artificial selection has supplied us with the foods on which the human race mainly subsists. The most important of all these last are, of course, the different grains, which are the seeds of grasses of various genera—Triticum (Wheat), Hordeum (Barley), Secale (Rye), Avena (Oat), Panicum (Millet), Oryza (Rice),
Fig. 25.—Wild Cabbage (Brassica oleracea). 2/3.
Zea (Maize). The value of these to the human race is incalculable, and some of them have been in cultivation for at least five thousand years. In some of them, indeed, the native form is now unknown, the improved varieties alone having been preserved by the care of man. The Wheats are a case in point. While a wild grass growing in Palestine has been quite recently identified as the probable source of the Hard Wheats, the native parent of the Soft Wheats is unknown. That productiveness has in all cases been much increased by long selection there can be no doubt; it may be pointed out that several species of Triticum, Hordeum, and Avena, allies of the Wheat, Barley, and Oat, are included in the native British flora, but they are useless as producers of grain.
Nowhere is the effect on plants of selection and cultivation seen better than in our native fruit-trees. We have only to compare the size, flavour, and almost endless variety of apples and pears with the fruit of the wild stock of these two species—the Crab (Pyrus Malus) and Wild Pear (Pyrus communis) of our hedgerows—to realize how much has been accomplished. In garden flowers, also, we see most striking results of continuous selection. By taking advantage of the tendency of stamens and carpels to change occasionally into petals, and of petals to increase in number, “double flowers” have been effected. When “doubling” is complete—that is, when the conversion into petals is thorough—no seed can of course be produced, and the plants must be propagated by cuttings. Different other slight natural variations, exaggerated by selection and cultivation, have been the source of innumerable “varieties” in our gardens.
Sometimes the natural variation is by no means slight, but of a striking character which the efforts of gardeners have not succeeded in developing further. Take, for instance, the case of fastigiate trees, such as the Lombardy Poplar (Populus nigra, var. italica) or the Irish Yew (Taxus baccata, var. fastigiata). These are freaks or sports, the character being that all the branches, not only the leader, tend to assume a vertical position. The Irish Yew originated as a wild “female” (pistillate) seedling found on the hills of Co. Fermanagh about 1780 and never rediscovered. It appears to be a juvenile form, preserving throughout life its seedling characters—a kind of Peter Pan among plants. Of the Lombardy Poplar the origin is not known, but it was no doubt similar. Seedlings of the Irish Yew revert to the ordinary type, and all the Irish Yews in cultivation are pieces of the original plant grown as cuttings. Poplars, like the Yew, bear the “male” (staminate) and “female” (pistillate) flowers on different trees, and the original Lombardy Poplar having been a “male” it also can be propagated only by cuttings—probably seedlings would in any case revert to the usual form.
The reverse of this abnormal erect habit is seen in weeping trees, where the branches for unexplained reasons seek to grow downward. In nature this results in a creeping habit. If planted on a height the branches will deliberately grow downwards towards the ground. Cultivators graft such forms on the top of a tall stem of a normal specimen, with the result that we see in the Weeping Ash and similar gardeners’ productions.