Grain develops from the ovary and its ovule after fertilization by pollen. The flower structures of various cereal plants are different, hence these differences reflect in the structure of the individual grain in kernel. In maize, the male and female structures are found in separate flower structures on the plant. The male flower structure commonly referred to as the tassel, is located at the top of the corn stalk. Pollen is shed from tassels and subsequently comes into contact with the female flower structure thereby
producing the grain. The female structure contains a central rachis commonly referred to as cob. The cob contains a series of rows of sessile spikelet and is enclosed by overlapping bracts (husks). The silks which are found on the corncob structure are the stigmas, the pollen receiving organs. Each spikelet contains two flowers one fertile and another sterile.
The grain of barley, oats, wheat, rice and sorghum develops from flowers which contain the ovary, three stamens and two scale like lodicules. These structures are surrounded by a pair of bracts called the lemma and the palea. In rye and wheat, the lemma and palea are loosely attached to the grain.
During threshing these particles are separated from
grain and constituted what is known as chaff.
Barley, rice and oats retain their lemma and palea during threshing, thus giving rise to structures called husks or hulls. In barley, the lemma and palea fuse with the grain. In oats, the lemma and palea do not fuse with the kernel but enclose and adhere tightly to the entire grain. This hull structure can be removed during processing, resulting in dehulled oats called groats. Rice hulls are removed during processing.
Individual kernels of grain are called caryopses. Grains which contain husks (oats, barley and rice) are called covered caryopses; whereas, grain lacking husks (maize, wheat, rye and sorghum) are referred as naked caryopses. Each kernel exclusive of the husk is composed of two main parts - pericarp and seed.
The pericarp consists of two layers. The outer layer contains the epidermis and hydroderm collectively referred to as the beeswing. The inner layer of the pericarp contains cross cells and tube cells. Throughout the ripening process of grain, the inner most layer of pericarp becomes distorted and torn and thus giving a tube like appearance called tube cells.
The seed portion of grain can be divided into four parts: 1) seed coat 2) hyaline layer (nucellar layer), 3) endosperm and 4) germ (embryo). When grain is processed in such a way that the germ and starch endosperm are removed the composite of the remaining parts of the seed and the pericarp is called the bran. The seed coat is either one or two layers thick. There is very little cellular structure in the region. Likewise, the hyalinase layer lacks any cellular structure, but rather acts as an embryo sac.
The endosperm of grain can be divided into two parts called the aleurone and the starchy endosperm.
The aleurone surrounds the starchy endosperm of grain, but does not encampass the scutellam of the embryo. The cells of aleurone are thick walled, cuboidal, and rich in oil, niacin and mineral matter. Phytic acid is also produced in larger quantities in this region. The number of layers of cells in this region varies according to type of grain. Wheat, rye, oats and sorghum have generally one layer of cells in the aleurone. Depending on the particular variety, maize may contain from 1 to 6, barley 2 to 4 and rice 2 to 6 layers.
The starchy endosperm portion of grains contains thin walled cells that are highly variable in shape, size and contents. Pentosans are found in large amounts in the cell walls of this region, but starch and protein take up most of the cell contents. Starch is found primarily in the form of granules, with protein filling the intergranular spaces. In wheat, the cells adjacent to the aleurone are relatively higher in protein and lower in starch than the rest of the starchy endosperm. The concentration of starch in maize depends on type of the maize and the area of the kernel being analysed. The endosperm of the maize is divided in to two regions - the crown and the horny region. The crown contains loosely packed starch granules with little protein. The protein content of horny region is much higher than crown region.
Upon germination, food reserves in the endosperm are mobilized and passed on top of the embryo by scutellum. The plumule of the embryo gives rise to the growing bud and the radical to root system.