Imágenes de páginas
PDF
[graphic][merged small]

grows about an inch beneath the soil, irrespective of the depth to which the grain was planted. From the crown are thrown out whorls of coronal or permanent roots. Any node of the wheat stalk under or near the soil may also throw out a whorl of permanent roots, somewhat similar to those of corn. There are four or five whorls with three to five roots each. The roots from the base of the crown strike directly downward, while those from the later whorls run at an angle for a few inches before taking a vertical direction. Most of the main roots penetrate to a depth of over 4 feet, perhaps 5 or 6 feet, provided the water-line is not closer to the surface than that distance, for below this the roots will not enter to any appreciable extent. The roots of wheat have been traced to a depth of 7 feet, and it has been found that if those of one plant were placed end to end they would reach 1,704 feet. The deep roots are all fine threads of practically uniform diameter throughout their entire length. They branch and rebranch freely to a depth of 18 or 20 inches, about eight branch roots occurring to an inch length of a main root. At a greater depth, branches are few or absent, and it is supposed that the deep roots are for securing moisture. The roots do not branch or feed much in the region just below that stirred by the plow, if that region is hard and gummy, as is often the case. The upper whorls give forth roots that are larger and coarser, and which resemble the brace roots in corn. It is said that the roots extend chiefly at their extremities, while the stem elongates equally, or nearly so, in all of its contiguous parts. The root development seems to be greatest in durum wheats. Early spring and summer rains cause shallow rooting. In the absence of these rains in the far west, a deeper root system, capable of resisting superficial droughts, is developed. Poor soil causes the roots to age rapidly.

Culms.—The culms of wheat are usually hollow, but in some varieties they are quite filled with pith. The length varies greatly in different varieties, soils and seasons, a fact which results in greater variation in size and yield of straw than of grain. Common wheat averages from three to five feet in height. The liability of lodging depends greatly on the culm, the length of which is also important in harvesting.

1 Hunt. Cereals In Amer. (1904), p. 27.

Leaves.—When the internodes lengthen and the spike pushes upward, the wheat is said to shoot. Previous to this, the nodes are so close together that the plant seems to consist almost entirely of leaves. There are four principal parts to the leaf: The blade; the sheath, which clasps the stem and is split down the side opposite the blade; the ligule, also clasping the culm, and located where the blade and sheath join; and the leaf auricle, thin projections growing from the base of the blade. The first leaves of the wheat plant and the germ whorl of roots do not live through the winter in some varieties.

The Flower of Wheat is constituted collectively of the organs of reproduction, together with the two inclosing chaffy parts. The inner of these two parts is known as a palea, while the outer and lower one is the flowering glume. The latter often bears a long appendage, characteristic of bearded wheat. These awns or beards vary greatly in length even in the same spike, and in some varieties are deciduous upon ripening. Their color varies from light yellow to black, wheat; ^4, ovary, o, and stig- The Spikelets.—Each consists of from

WSKraSMK**" t0 flve flowers encased within two

ering;C, flower before open-hard oval chaffy coverings called outer

ing, a, anthers, /, filament, J b

I, todicule; D, flower about glumes. In common wheat each

toopen' 6pikelet generally matures two, and

sometimes three, grains. The glumes vary greatly in form, color and size. The stem or rachis of the spike is of a zigzag form. On each of its joints or shoulders sits a single spikelet, attached by an exceedingly short rachilla. Arranged alternately on the stem, with flat sides toward the center, the spikelets usually give the head of wheat a square appearance when viewed endwise. Viewed from the side, the spike may be straight or curved; it may have uniform sides, or taper toward both ends, or only toward base or apex; or it may be clubbed at either end. The filling of the spikelets has much to do with the appearance of the spike, which varies much in different varieties. There is also great variation in compactness. Fifteen to twenty fertile spikelets, containing from 30 to 50 grains, are usually formed on a spike of wheat, the average length of which is between 3 and 4 inches. Humboldt said that in Mexico each spike of wheat averaged 90 grains, though some had as many as 160. Mummy wheat has been observed with ears containing nearly a dozen branches. There are 150 grains in one ear, and as many as 60 ears from one seed. Wheat has the advantage of extreme diminution of the number of seeds to each flower, giving richness in starch and gluten, combined with the advantage of numerous flowers on each plant, giving many seeds.

[graphic]

The Wheat Kernel is a dry, indehiscent, unilocular caryopsis. It is oval in shape, and has the appearance of being folded upon itself from two sides. A ventral crease marks the coming together of the two folds. At the base of the berry opposite the crease is found the embryo, germ, or chit. At the apex is a collection of minute hairs. The entire grain fills from 20 to 30 cubic millimeters of space, of which at least thirteen-fourteenths are occupied with the starchy endosperm. The latter almost surrounds the embryo, and its cells are very irregularly shaped. The embryo is composed of the absorbent organ (scutellum), and the miniature first leaves and roots. It forms about 6 per cent of the wheat kernel.

The endosperm and embryo are com

* *l W, f,i mi, f, L-iniv u wil,', in

rletely enclosed by a single layer of gIuten cellsi a, inner coat of 1' f' ° f Bran; e, coloring matter of

aleurone or gluten cells. The weight of bran;/ and g, outer coats of this layer is 8 per cent of that of the bTMn; A, epidermis of kernel.

whole grain. The next covering is a single layer of collapsed cells, known as the tegmen. This is again surrounded by a third envelope, the testa, or episperm, which contains the greater part of the coloring matter of the grain. This coloring matter is of two kinds, one a palish yellow, and the other an orange yellow, and the degree in which one or the other predominates determines whether the wheat is known as white, yellow or red. The three layers just described constitute the envelope of the seed proper. They in turn are again inclosed by the pericarp, which is also composed of three layers, all colorless. The exterior of these three membranes, the cuticle, is easily removed by rubbing. Then come two layers of cellular tissue, the epicarp (from which spring the hairs above mentioned) and the endocarp. The tegmen and testa form about 2 per cent of the weight of the grain, and the pericarp forms fully 3 per cent. Thus the bran forms at least 13 per cent of the grain.

[graphic]
[ocr errors]

Germination.—The three conditions essential to the germination of wheat are moisture, warmth and oxygen. In the absence of any one of these the process will not begin, or if it has begun it will cease. Johnson defines the period of germination as lasting from the time when the

rootlet becomes Cross section of grain of wheat on the left. (From ,,;„:i,i0 until +>./» nicro-photograph by Tolman.) Transverse section, vibioie unui me

of the seed

[graphic]

are

on the right, of an unripe grain enlarged about WO stores times from drawing by Bessey. 1, ovary wall or pericarp; 2, outer integument; 3, inner integument; 4, mother remains of nucellus; 5, aleurone cells; G, starch cells. , , , , ,,

exhausted and the

young plant is wholly cast upon its own resources. At 41° F., the time required for the rootlet to appear in wheat is about six days, which time corresponds to the more general idea of the period of germination. At 51° this time is shortened about one-half. The time required for the completion of germination is 40 to 45 days at 41 to 55° and 10 to 12 days at 95 to 100°. The lowest temperature at which wheat will germinate is 41°, the highest 104°, and that of most rapid germination, 84°. This is according to Johnson. Other authorities claim that wheat will germinate and grow on melting ice. It has also been said that it does not germinate successfully at a high temperature, and consequently should not be sown until cool weather in southern climates. Dissolved salts seem to aid germination under ordinary field conditions. In germinating, wheat absorbs from five to six times its

« AnteriorContinuar »