TECHNOLOGY OF CEREALS distinguished. The majority, a central mass des- At maturity the starchy endosperm dies but cribed as starchy endosperm, consists of cells aleurone cells continue to respire, albeit at a packed with nutrients that can be mobilized to very slow rate, for long periods. The aleurone support growth of the embryonic axis at the tissue covers the outer surface of the embryo but onset of germination. Nutrients are stored in its cells in this area may become separated and insoluble form, the major component being the degenerate. Their ability to respire and to pro- carbohydrate starch. Next in order of abundance duce enzymes on germination is in some doubt is protein. In all cereals there is an inverse Briggs, 1978) gradient involving these two components, the protein percentage per unit mass of endosperm tissue increasing towards the periphery (Fig. Seed coats 2.17 Surrounding the endosperm and embryo lie the Cell size also diminishes towards the outside remains of the nucellus, the body within the ovule nd this is accompanied by increasing cell wall in which the cavity known as the embryo sac thickness. The walls of the starchy endosperm of develops. Following fertilization the embryo and wheat are composed mainly of arabinoxylans endosperm expand at the expense of the nucellus while in barley and oats(1-3)and(1-4)B-D which is broken down except for a few remnants glucans predominate. Cellulose contributes little of tissue and a single layer of squashed empty to cereal endosperm walls except in the case of cells from the nucellar epidermis. epidermal cells rice(see p. 64). in many higher plants secrete a cuticle and a Surrounding the starchy endosperm is the cuticle is present on the outer surface of the other endosperm tissue, the aleurone, consisting nucellar epidermis of many cereals of one to three layers of thick-walled cells with The outermost tissue of the seed is the testa or dense contents and prominent nuclei seed coat(the nucellar epidermis is also regarded The number of layers present is characteristic as a seed coat but its origin is different from that of the cereal species, wheat, rye, oats, maize and of the testa which develops from the integuments) sorghum having one and barley and rice having The testa may consist of one or two cellular layers three. Unlike the tissue they surround, aleurone In some varieties of sorghum a testa is absent cells contain no starch but they have a high altogether. Where two layers are present the long protein content and they are rich in lipid They axes of their elongated cells lie at approximately are extremely important in both grain deve- 90 to each other. Frequently the testa accumulates lopment, during which they divide to produce corky substances in its cells during grain ripening starchy endosperm cells, and germination, when and this may confer colour on the grain and in most species they are a site of synthesis of certainly reduces the permeability of the testa. a hydrolytic enzymes responsible for solubilizing cuticle, thicker than that of the nucellar epidermis the reserves is typical, and this also plays a role in regulating The balance between aleurone and scutellum water and gaseous exchange in the latter role varies among species. both Both testa and nucellus are tissues which once tissues synthesize the enzymes in response to formed part of the ovule of the mother plant hormones including giberellic acid, transmitted They are thus of an earlier generation than the from the embryonic axis(via the scutellum in the endosperm and embryo which they surround and case of the aleurone). A further function of to which they closely adhere aleurone cells in some millets and sorghum is the transfer of metabolites into the starchy endo- Pericarp sperm during grain maturation. This activity is deduced from the knobbly, irregular thickenings The pericarp (or fruitcoat) is a multilayered on their walls often associated with such transfers structure consisting of several complete and in- Rost and Lersten, 1970 complete layers. In all cereal grains the pericarp38 TECHNOLOGY OF CEREALS distinguished. The majority, a central mass des￾cribed as starchy endosperm, consists of cells packed with nutrients that can be mobilized to support growth of the embryonic axis at the onset of germination. Nutrients are stored in insoluble form, the major component being the carbohydrate starch. Next in order of abundance is protein. In all cereals there is an inverse gradient involving these two components, the protein percentage per unit mass of endosperm tissue increasing towards the periphery (Fig. 2.17). Cell size also diminishes towards the outside and this is accompanied by increasing cell wall thickness. The walls of the starchy endosperm of wheat are composed mainly of arabinoxylans, while in barley and oats (1-3) and (1-4) p-D glucans predominate. Cellulose contributes little to cereal endosperm walls except in the case of rice (see p. 64). Surrounding the starchy endosperm is the other endosperm tissue, the aleurone, consisting of one to three layers of thick-walled cells with dense contents and prominent nuclei. The number of layers present is characteristic of the cereal species, wheat, rye, oats, maize and sorghum having one and barley and rice having three. Unlike the tissue they surround, aleurone cells contain no starch but they have a high protein content and they are rich in lipid. They are extremely important in both grain deve￾lopment, during which they divide to produce starchy endosperm cells, and germination, when in most species they are a site of synthesis of hydrolytic enzymes responsible for solubilizing the reserves. The balance between aleurone and scutellum in the latter role varies among species. Both tissues synthesize the enzymes in response to hormones including giberellic acid, transmitted from the embryonic axis (via the scutellum in the case of the aleurone). A further function of aleurone cells in some millets and sorghum is the transfer of metabolites into the starchy endo￾sperm during grain maturation. This activity is deduced from the knobbly, irregular thickenings on their walls often associated with such transfers (Rost and Lersten, 1970). At maturity the starchy endosperm dies but aleurone cells continue to respire, albeit at a very slow rate, for long periods. The aleurone tissue covers the outer surface of the embryo but its cells in this area may become separated and degenerate. Their ability to respire and to pro￾duce enzymes on germination is in some doubt (Briggs, 1978). Seed coats Surrounding the endosperm and embryo lie the remains of the nucellus, the body within the ovule in which the cavity known as the embryo sac develops. Following fertilization the embryo and endosperm expand at the expense of the nucellus, which is broken down except for a few remnants of tissue and a single layer of squashed empty cells from the nucellar epidermis. Epidermal cells in many higher plants secrete a cuticle and a cuticle is present on the outer surface of the nucellar epidermis of many cereals. The outermost tissue of the seed is the testa or seed coat (the nucellar epidermis is also regarded as a seed coat but its origin is different from that of the testa which develops from the integuments). The testa may consist of one or two cellular layers. In some varieties of sorghum a testa is absent altogether. Where two layers are present the long axes of their elongated cells lie at approximately 90" to each other. Frequently the testa accumulates corky substances in its cells during grain ripening and this may confer colour on the grain and certainly reduces the permeability of the testa. A cuticle, thicker than that of the nucellar epidermis, is typical, and this also plays a role in regulating water and gaseous exchange. Both testa and nucellus are tissues which once formed part of the ovule of the mother plant. They are thus of an earlier generation than the endosperm and embryo which they surround and to which they closely adhere. Pericarp The pericarp (or fruitcoat) is a multilayered structure consisting of several complete and in￾complete layers. In all cereal grains the pericarp