Man has cultivated wild plants to meet his many needs, and through selective breeding has modified these plants profundly. The cabbage (Brassica oleracea) provides a good illustration of this process.
Energy reserves and growth cycle
The wild cabbage is a biennial plant: its reproductive cycle normally stretches over a two-year period. In the first growing season, a very short stem with a compact head of leaves appears. These leaves are an active center of photosynthesis; the resulting substances are stockpiled in either the leaves themselves or the base of the stem. When winter comes, this activity ceases almost entirley. In the second spring, photosynthesis resumes, but the winter cold has completely modified the cabbage's growth pattern: the stem lenghtens quickly, bearing fewer and smaller leaves than it did at the base, and culminates in a flowered tip which will enable the plant to reproduce. Finally, when the cabbage reaches a height of few dozen centimeters, it produces several hundred seeds and dies. Thanks to the abundant store of energy reserves accumulated in the first year, this second-year phase of rapid growth, flowering, and seed productino lasts only a few weeks.
Selection: kohlrabi, broccoli, caulifrower, and more.
The cabbage's energy reserves make it a perfect food for herbivorous and onmivorous animals. Man cultivated it for its nutritional value. Later, growers selected those plants that seemed to offer what they considered the finest in terms of food quality. From generation to generation, they have increased the cabbage's nutritive value -in others words its reserves, and improved the distribution of these reserves in the plant. By selecting plants with the most abundant reserves possible, regardless of their distribution, growers produced the fodder cabbage. Its heavy stem lengthens markedly even during the first year and bears thick leaves. Although both the stems and leaves of the fodder cabbage are extremely high in nutrientes, they are not very tender, so this type of cabbage is used exclusively for animal feed. The common garden cabbage was developed by selecting plants which produce a head of numerous tight leaves in the first year and which store their reserves in the leaves rather than the stem. The leaves are especially tender and are consumed primariyly in autumn or winter, but in any case before the growth phase in the second year.
By taking the opposite approach and selecting plants which accumulate their reserves in the base of the stem rather than the leaves, growers obtained the kohlrabi, or turmp cabbage.
As for the brussels sprout, it is characterized by important lenghening of the lower portion of the stem, which bears large tender buds rich in nutrientes, during the first year. The stem is tipped by a cluster of leaves without much food value.
Broccoli was obtained by exploiting the fact that early in the second year, the growing stem is tender and pleasant tasting. Therefore, breeders selected cabbages with particularly fleshy stems that can be eaten just as the first flowers are forming.
The cauliflower is the result of selecting cabbages in which the stem grows very slowly in length, but thickens considerably because flower formatino is particularly late. There are numerous varieties of cauliflower. Somo of these, like the wild cabbage, require cold winter temperatures to stimulate the development of the so-called flower bearing stem. These varieties are seeded in summer and not harvested until the following spring. However, other varieties have also been selected in which the cauliflower-like stem develops spontaneously when the plant reaches a certain age. These varieties are seeded in spring and harvested in summer or autumn, depending on their growth rate. These examples illustrate how growers have modified characteristics of the reproductive cycle, allowing us to farm cauliflower at almost any period of the year by selecting the appropriate varieties and planting schedules.
All these vegetables are very different from the wild cabbage, not only in their shapes and sizes, but also in their reproductive cycle. They have been obtained by manipulating the wild cabbage's spontaneous variability. But if this is true, why aren't cabbages resembling cauliflowers or kohlrabi ever found in nature? The answer is simple: these types are less well-adapted than wild cabbage to surveve and reproduce in natural conditions. They only survive on farms because of fertilizers, weeding, and other care. It is remarkable to note that all these extremely different members of the cabbage family can crossbreed, producing vigorous and fertile hybrids (resembling wild cabbage more than either of their parents, and so useless to man). They all belong to the same species, Brassica oleracea, and probably only differ from wild forms in a relatively small number of genes-nonetheless, these genetic differences yield spectacular effects.
Similarly, many plants cultivated by man are nothing more than particular, grower-selected varieties of species we are familiar with in nature. This statement is even more true for animals, since it clearly seems that efforts to breed new races of domesticated animals have never produced a new species.
Prof. Jean Génermont
University of Paris XI
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