26 FLOURS. 



(20) Wheat Flour. 



A transverse section of a grain of wheat, examined under the 

 microscope, exhibits the following layers : 



(1) Outer epidermis of the pericarp, composed of tabular cells 

 which, in surface view, are polygonal, elongated, and have thickened, 

 pitted walls. In the upper part of the grain it bears simple, uni- 

 cellular, conical hairs, the lumen of which is somewhat abruptly 

 enlarged at the base. 



(2) Hypoderma, consisting of cells which, towards the exterior, 

 closely resemble those of the outer epidermis, but in the inner part 

 vary in form and often lignify. 



(3) A layer of transverse cells ; these are tangentially elongated 

 and have thickened, pitted walls. 



(4) Inner epidermis of the pericarp, consisting of .small cells 

 with rounded section, but elongated in surface view ; their tube- 

 like appearance has gained for them the name of tubular cells. 

 These four layers constitute the pericarp of the grain. 



. (5) Seed-coat or brown layer composed of two layers of cells 

 closely applied to one another, and of a yellow or yellowish-brown 

 colour. 



(6) Hyaline layer; a layer of rectangular cells with small, 

 narrow lumen. 



(7) Proteid or aleurone layer; a single layer of cubical cells 

 with very thick walls, and filled with a granular substance. 



(8) Endosperm, consisting of polygonal cells filled with starch, 

 the characters of which have already been described. 



To whatever degree of fineness the flour may have been reduced 

 it always contains portions of the pericarp and seed-coats in addition 

 to the starch, although the latter, of course, constitutes by far the 

 greater portion. 



The diagnostic characters of wheat flour are : 



(a) The shape and size of the large starch grains. 



(b) The hairs, with lumen enlarged at the base, but in the upper 

 part rather narrower than the wall. 



(c) The thick-walled, pitted cells of the hypoderma. 



(d) The thick walled, pitted transverse cells. 



