The brittle-eared emmer closely resembles Triticum dicoc coides Korn, a wild species of wheat found in the mountainous parts of Syria, and has doubtless descended from it. The other wheats of the emmer group have probably arisen by mutation and crossing within the group.
It is in regard to the origin of the wheats of the bread wheat group that the greatest obscurity and uncertainty exists, and no Plant has yet been discovered which resembles any of these wheats. Some botanists have suggested that the wild ancestor has become extinct, others hope that it may possibly be found by further search in south-western Asia. From a prolonged study of many hundreds of forms Professor Percival concludes that the bread wheat group with its vast number of varieties has arisen by hybridization of a wheat of the emmer group with Aegilops ovata and A. cylindrica, two wild species of grass found in southern and eastern Europe and western Asia. Both these cross readily with emmer and bread wheats, and although the hybrids are usually sterile a few fertile hybrids have been obtained.
From veil early times attempts have been made to improve the wheat crop, either in yield, quality of grain, earliness, dis ease resistance or some other desirable character. Among the ancient Romans mass selection was practised, a number of the best ears or largest grains being chosen annually from the crops and the produce used as seed for the succeeding crop; in this manner a high average yield was maintained.
Some of the world's most famous improved wheats have been selections of single ears or plants exhibiting superior characters, such individuals being picked out of an ordinary crop, or found growing casually on roadsides, in hedges, or among other crops on cultivated ground.
Hybridization has also been practised with a view of obtaining improved wheats. At first crossing was carried out in order to secure a greatly varying offspring, from which it was hoped to select plants of superior merit. Since the discovery by Mendel of the laws of inheritance (see HEREDITY, MENDELISM), interest in hybridization has been greatly stimulated, and in all wheat growing countries the crossing of wheats is being extensively pursued, the aim being to combine in one plant the good qualities found in two or more separate individuals. The most important
character from the point of view of the farmer is total yield of grain, a hereditary quality largely influenced by environment.
Attempts to combine this character with earliness, baking quality, winter hardiness, disease resistance and other desirable qualities have been only partially successful.
Wheat is one of the most widely distributed of the cereals, and is grown in almost all countries of the world, except those in the hot and humid regions of the tropics. The great areas from which the world's supplies of wheat are derived lie in the northern hemisphere between 3o-6o N. latitude, but crops are grown near the equator in the mountainous parts of Africa and South America, and their cultivation extends as far north as the Arctic Circle, or slightly beyond it in certain parts of Europe, Asia and North America.
Wheat may be grown at sea level or at elevations up to io,000 feet or more in Tibet, Abyssinia and the highlands of Ecuador and Colombia.
Some varieties will withstand a temperature of 20 C below freezing point without injury, if the plants are protected by a covering of snow. In countries with severe winters spring kinds are cultivated; the more prolific autumn sown varieties of west ern Europe being only adapted to districts with comparatively mild winters.
Wheat will thrive in almost any kind of soil except dry sands and wet peaty soils; it succeeds best, however, upon deep, well drained loams and clays.
The yield of grain per acre depends on a number of independent factors, such as the variety of wheat, climate and soil, time of sowing, amount of seed and other conditions, the maximum return being only obtained when all the conditions named are favourable and exerting their influence simultaneously—an extremely rare occurrence. In ordinary farm practice the yield of grain varies from about 10-20 bushels (600-720 lb.) in the warm countries of southern Europe, North Africa, Argentina, India and Aus tralia, to 3o or 4o bushels (1,800-2,400 lb.) per acre in the United Kingdom and the adjacent countries of north-western Europe. The average yields of the United States and Canada is about.