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), [Illustration: 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.