WHEAT. Among the three or f our most important cereals utilized as food by man wheat (Triticum) occupies the first place. It is the cereal above all others from which good bread can be made and in this form is consumed by the most highly civilized nations of the world.
The grain from which the plant is grown is a caryopsis or nut-like fruit containing a single seed. The thin shell or pericarp of the fruit and the coat of the seed are so closely united when ripe that they cannot be separated. The colour of the grain is usually a pale creamy tint or some shade of red but in certain Abyssinian wheats it is purple. The surface is smooth except at the tip which is covered with hairs ; the dorsal side is convex, the ventral side hav ing a longitudinal furrow. At the base of the grain on the dorsal side is an oval wrinkled patch which covers the embryo of the seed. The embryo possesses several rudimentary rootlets and a terminal bud, or plumule, from which the stem and leaves of the future plant develop. Its position, relative size and parts, as well as the structure of the rest of the grain are shown in the longitudinal section given in fig. 2. Beneath the epidermis are a few layers of cells belong ing to the pericarp; within these is the seed coat which sur rounds the embryo and the endosperm or floury part of the seed. The outer layer of the endosperm consists of cubical cells containing minute aleuron grains composed of protems.
The rest, which makes up the bulk of the endosperm, is a mass of thin-walled cells within which are vast numbers of starch grains imbedded in a matrix of proteins from which the gluten so essential to the manufacture of good porous bread is derived.
Life-history.—Germination of the grain occurs in a few days when sown in warm soil; the rootlets, breaking out first, are soon followed by the plumule leaves. The main axis or stem extends a little and the lateral buds upon it grow into short stems upon which buds also arise, and these in turn produce short stems and buds ; such branching of the young plant, termed "tillering," which occurs close to the ground, continues for a variable length of time dependent on the variety of the wheat, the time of sowing, con dition of the soil and other factors. The many short stems pro duced are at first hidden by the leaves; later they rapidly elongate or "shoot" upwards forming the straws of the crop. The much branched axis of a
strongly "tillered" plant may thus give rise to 5, 1o, 20 or even ioo straws and ears, all of which have come from the embryo of a single grain.
The inflorescence or ear of wheat con sists of a notched axis or rachis which bears on alternate sides from 18-25 flattened spikelets, one at each notch. Each spikelet has a thin central axis—the rachilla. At the base are two boat-shaped chaffy scales, the empty glumes ; then follow a number of flowers arranged alternately along the rachilla, each enclosed between a flowering glume and palea, the former of which may or may not terminate in a long beard or awn. The flower is very simple, consisting of three stamens and a feathery styled ovary, at the base of which are two minute membranous scales termed lodicules. The number of flowers in a spikelet varies in different races of wheat, but in ordinary bread wheat is 4-6 or more, of which usually not more than two or three develop into grain, the rest being abortive.
After the ear escapes from the upper leaf sheath flowering takes place in five or six days, when the glumes surrounding the flowers are pushed apart by the lodicules which swell and become turgid at this period. The filaments of the flower lengthen rapidly and the anthers dehisce at their tips. Some of the abundant pollen falls on the feathery stigmas, often before the flower opens, the rest being shed into the air. Normally the glumes remain separated about 15-2o minutes and then close; self-pollination and self-fertilization is the rule, but crossing from pollen brought to the flower by air currents before the glumes close is common in warm climates and not infrequent in Britain in some seasons. Flowering goes on throughout the day. The lowest flower of each spikelet opens first, the rest following in succession upwards. The first spikelet to flower lies in the middle third of the ear, and flowering progresses upwards and downwards from this point, the last to open being those of the terminal and basal spikelets respec tively. The whole ear completes its flower ing in five or six days in warm weather, but is prolonged to six or eight days when the sky is overcast.