Wheat grain morphology

Wheat grains, botanically, are the fruits (caryopsis) of the wheat plant. Mature wheat grain or kernel is roughly ovate or egg shaped. Wheat structure is a single seeded fruit, 4 to 10 mm long, consisting of three main parts namely the bran, the endosperm and the germ or embryo.

The fruit coat or pericarp (45 to 50 um thick) surrounds the seed and adhere closely to the seed coat.

Wheat grains are bone on a spike, also called an ear, which is a branched structure. The major axis of this spike bears two rows of alternating secondary axes known as rachillas.

The wheat color, depending on the species and other factors is red to white and is due to material present in the seed coat.

Wheat also is classified based on physical characterizes such as red, white soft, hard spring or winter.

Wheat kernel cross section

The grain obtained from the wheat plant is a nut-like fruit called by botanist a caryopsis. It includes a single seed or kernel enclosed within a thin shell derived from the ovary wall of the maternal parent.

The outer pericarp is consisting of the epidermis and hypodermis. The epidermis consists of a single layer of cells that from the outer surface of the kernel.

On the outer walls of the epidermal cells is the water impervious cuticle. Some epidermal cells at the apex of the kernel are modified to form hairs.

The hypodermis is composed of one to two layers of cells. The inner pericarp is composed of intermediate cells and cross –cells inward from hypodermis.

Long and cylindrical tube cells constitute the inner epidermis of the pericarp. A deep groove or crease extends the entire length of the wheat kernel. It cuts almost to the center of the kernel. In the crease, the seed coat joins the pigment strand, and together they form a complete coat about the endosperm and germ.

Three layers can be distinguished in the seed coat: a thick outer cuticle, a “color layer” that contains pigment, and a very thin inner cuticle.

The bran comprises all outer structures of the kernel inward to, and including the aleurone layer. The aleurone, which forms the outer periphery of the endosperm and the innermost layer of the bran, accounts for 3%-4% of the weight of the kernel; it is usually removed with the bran during milling.

The endosperm is composed of peripheral, prismatic and central cells that are different in shape, size and position within the kernel.

The endosperm cells are packed with starch granules, which lie embedded in a protein matrix. Wheat endosperm contains mainly to types of granules a larger type, mostly about 20-35 micrometers (um) across (A-starch), being lenticular in shape, and a smaller spherical type, ranging from 2-8um in diameter (B-starch).
Wheat grain morphology