SEED STRUCTURE 207 



second type, of which mustard seed is a fair example, 

 the nourishing material or endosperm is stored within 

 the embryo. 



WHEAT 



The structural elements present in wheat are found 

 with but slight variations in most of the grains. The 

 wheat grain is a fruit, possessing exocarp, mesocarp 

 and endocarp layers. The seed is in close proximity 

 to the endocarp, and some of the tissues of the latter 

 layer may be present in wheat flour. 



Tissues of the Testa and Tegmen. These elements 

 are usually closely adherent and each consists of a 

 single layer of cells (Plate 68). On transverse view, 

 the individual elements appear as small, elongated, 

 rectangular, thick-walled cells in close contact with 

 a thin layer of tissue which rarely shows definite cell 

 structure, and which represents the tegmen. In 

 powdered materials these tissues are seen on sur- 

 face view and consist of long, thin-walled, slightly 

 colored angled cells of irregular shape. The testa 

 and tegmen cells usually extend at right angles to each 

 other as may be plainly seen in surface preparations. 



Endosperm Tissues. Remnants of a perisperm 

 layer usually occur in the seeds of grains; but the' 

 tissues are so reduced that they appear as a thin 

 membrane between the tegmen and the endosperm. 

 The cellular structure of this perisperm can only be 

 distinguished in surface preparations. In transverse 

 sections the outermost layer of the endosperm will 

 be apparent as regularly arranged, square or rect- 

 angular, thick- walled cells (Plate 68), containing darker 

 contents than the cells more deeply located. These 



