After fertilization the grain begins to develop, its volume increasing day by day. The pericarp expands as the seed within enlarges, and reaches its maximum size in four or five weeks, when the water con tent is about 7o% of the weight of the grain. Later the dry weight increases, the water content gradually decreasing to 12-14% when the grain is ripe. At the time of maximum vol ume the grain is green, the embryo almost completely formed and the cells of the endosperm contain a large amount of water. At this stage the grain is said to be "milk ripe ;" on squeezing it yields a white liquid in which float vast numbers of starch grains. In the "yellow ripe" stage which follows, the grain has assumed its creamy or reddish tint,—the chlorophyll of the cells of the peri carp having disappeared,—and the endosperm now kneads like wax or dough. In two or three days the "ripe" stage is reached when the characteristic colour of the grain is clearly marked, its endosperm becomes harder and the flinty or mealy character is established.
The time taken in the ripening process depends on the variety of the wheat, and the climate of the locality in which it is grown; in the south of England the time elapsing between the date of appearance of the ear from the leaf-sheath and the production of ripe grain is from 8-9 weeks for most wheats.
Species and Races. — The classification of the vast number of forms of wheat presents num erous difficulties, and many at tempts have been made by bot anists since the time of Linnaeus (1753) who recognized five species. Most of these schemes are based upon differences in the morphology of the ear, and the majority refer to a comparatively few of the known forms and these chiefly herbarium speci mens. The most comprehensive classification based upon the study of living plants of prac tically all known kinds growing under .similar conditions is that given in Professoi Percival's monograph, The Wheat Plant. In this work are recognized eleven cultivated species or races, which fall into three groups as follows :—Group I. Einkorn (Triticum monococcum L.). Group II. Emmer (T. dicoccurn Schub.) ; Mac aroni wheat (T. durum Desf.) ; Rivet wheat (T. turgidurn L.) ; Egyptian Cone wheat (T. pyramidale Perciv.) ; Khorasan wheat (T. orientate Perciv.) ; Polish wheat (T. polonicurn L.). Group III. Bread wheat (T. vulgare Host.) ; Club wheat (T. compactum Host.) ; Indian Dwarf wheat (T. sphaerococcum Perciv.) ; Spelt or Dinkel (T. Spelta L.). The chromosome number of the wheats of group I. is 14, that of group II. is 28, while that of
group III. is 42.
Einkorn (German, one grain) ; Engrain (French; Small spelt). The wheats of this race have bearded, thin flat ears from 5-9 cm. long. The spikelets generally contain only a single grain. The rachis of the ear when ripe is brittle and disarticulates at each node or joint when thrashed. The grains are small, laterally com pressed and pointed at each end. It is an ancient race cultivated only by primitive peoples in Europe, Asia Minor and Mo rocco, and chiefly used as food for cattle and horses.
Emmer.—The ears of this race are bearded and flattened, the spikelets containing two grains which are narrow and generally pointed at each end. In the typical Emmer the rachis is brittle, and the grain firmly held between the sharply keeled glumes can only be obtained free from the latter with difficulty. Archaeologi cal evidence shows that Emmer is one of the most ancient of the wheats; it was extensively grown by Neolithic peoples of Central Europe and was the only wheat cultivated in Egypt in pre dynastic and early dynastic times. At present it is grown for human food by primitive races in India, Persia, the Caucasus, Abyssinia and Morocco, and a small quantity in South Germany and the Basque districts of Spain. Small amounts are also grown in other parts of the world for horse and cattle food.
Macaroni wheats are tall solid-strawed kinds with bearded ears and sharply keeled glumes. The grain is narrow, pointed at each end, with hard translucent endosperm specially suitable for the manufacture of macaroni and other similar pastes. These wheats have glabrous leaves and resist drought well, giving good crops of grain in districts with a low rainfall. They are cultivated throughout the Mediterranean region, in Central Asia, India, South Africa and the warmer regions of North and South America.
Rivet or Cone wheats are tall solid-strawed forms with heavy, bearded ears, sharply keeled glumes and large, plump, blunt tipped grains with a characteristic dorsal hump. The endosperm of of the grain is soft and starchy, more suited to the biscuit-maker than the baker of bread. Some varieties of rivet, often named mummy, miracle or seven-headed wheats, have branched ears. The rivet wheats have soft, velvety leaves and require a warm climate for full development. They are grown chiefly in countries along the northern side of the Mediterranean from Portugal to the Caucasus, though small amounts of some varieties are met with in the warm parts of other countries.