SEED STRUCTURE 209 



cells contain aleurone and are known as aleurone or 

 gluten cells. The remaining endosperm tissues consist 

 of large, thin-walled parenchyma cells filled with 

 starch. In powdered materials the perisperm layer 

 appears as a thin membrane composed of large, thin, 

 wavy-walled cells. The aleurone cells are apparent 

 in sectional and surface views. On sectional view 

 they are square and usually occur in strips of five 

 to ten adhering cells (Plate 68). On surface view 

 they appear as circular or polygonal forms, with thick 

 white walls and gray contents. The endosperm par- 

 enchyma is apparent as irregularly polygonal and 

 circular thin-walled cells, filled with starch and seldom 

 present in unbroken condition. 



Embryo Tissues. — The embryo, or germ, of wheat 

 is a small body at one end of the grain. It consists 

 of a large cotyledon, or seed leaf, which encircles the 

 other parts of the embryo. The embryo is composed 

 of small parenchymatic cells with rudiments of fibro- 

 vascular tissue. In the milling of flour the embryo 

 is removed from the grain, as, owing to the fat 

 contained, it imparts an unpleasant taste to the 

 product. 



BLACK MUSTARD 



The structural elements present in this seed are 

 found with greater or less variation in many other 

 seeds. 



Tissues of the Testa and Tegmen. — These tissues 

 are arranged in four layers in the following order, 

 epidermal mucilage cells, subepidermal cells, stone 

 cells and pigment cells. Seen in sectional view (Plate 

 69), the outer epidermal cells appear as long rectangu- 



