i5i THE STRUCTURE OF ECONOMIC PLANTS 



The starchy endosperm constitutes the main bulk of the 

 grain, and its cells are thin-walled, with their long axes at right 

 angles to the surface of the grain. They are filled with starch 

 and contain all of the gluten found in the endosperm. The 

 lenticular starch grains are intermediate in size between the 

 larger ones found in rye and the smaller ones which occur in 

 barley. The endosperm, including the aleurone layer, constitutes 

 approximately 85 to 90 per cent of the entire bulk of the ma- 

 ture grain, the embryo about 6 per cent, and the bran layer the 

 remainder. 



The Embryo. — The mature embryo lies embedded in the endo- 

 sperm slightly to one side and at the base of the grain opposite the 

 groove. The embryonic axis consists of the primary root, enclosed 

 by the root sheath or coleorhiza; the epicotyledonary portion of 

 the axis, enclosed by the coleoptile; and two lateral divergences, 

 the scutellum and the epiblast. (Fig. 64.) The scutellum is the 

 large, lateral, shield-shaped cotyledon which slightly exceeds the 

 embryonic axis in length. Its convex, abaxial surface lies in close 

 contact with the starchy cells of the endosperm; and the epidermal 

 cells are elongated at right angles to the surface of the scutellum, 

 constituting an epithelial layer which is secretory in function. In 

 some varieties, invaginations occur in the surface, but this is not 

 as common as in corn. The scutellum partially surrounds the 

 coleoptile and extends slightly beyond it so that a projection is 

 formed, known as the ventral scale, which overhangs the coleop- 

 tile. The epiblast is a small non-vascular outgrowth which 

 arises in the region of the cotyledonary node on the side of the axis 

 distal to the scutellum. 



The conical coleoptile is entirely closed except just below the 

 tip, where there is a slit-like pore away from the scutellum; and 

 enclosed by it are the primordia of two or three foliage leaves. 

 The first foliage leaf is diverged from the axis on the side opposite 

 the scutellum, and succeeding leaves have a similar angular 

 divergence of 180°. A lateral bud is developed in the axil of the 

 coleoptile on the side adjacent to the scutellum. 



The Vascular System of the Embryo. — The vascular strands 

 of the embryo consist of elongated thin-walled conducting ele- 

 ments, and the scutellum is supplied with a broad strand which is 

 directed towards its tip, branching frequently at its upper limits 

 so that a fan-like system is formed. The small branches curve to 



