SEED STRUCTURE 211 



lar forms with slightly thickened walls. The sub- 

 epidermal cells are large, thin-walled and very irregular 

 in form. The stone cells are small and of varying 

 lengths, arranged so that the short side of the cell is 

 toward the epidermal layers. These stone cells are 

 termed the palisade layer. The pigment cells are 

 long and narrow, and are so. arranged that their long 

 sides are toward the palisade layer. In powdered 

 mustard these tissues are usually seen on surface 

 view, although small fragments of seed coat tissue 

 may be apparent on sectional view. The epidermis 

 (Plate 69) is brown and composed of large, polygonal 

 cells with beaded walls. The subepidermal tissue 

 is usually adherent to the palisade layer and is visible 

 as large polygonal cells of indistinct form in fragments 

 of the palisade tissue. The palisade tissue is composed 

 of small, thick-walled, angled stone cells. The pig- 

 ment layer consists of large, angled cells of dark color, 

 usually adherent to the palisade tissue. 



Endosperm Tissues. — The endosperm of mustard 

 is contained within the cotyledons or miniature 

 leaves of the embryo. The cotyledons are covered 

 by a thin epidermal layer of cutinized cells. The 

 endosperm cells (Plate 69), are angled or irregularly 

 circular in form with slightly thickened white walls. 

 In powdered mustard the endosperm cells usually 

 appear in masses composed of irregularly circular, 

 white-walled cells containing a grayish aleurone. 



Embryo Tissues. — The cotyledons of the mustard 

 embryo may show distinct signs of tissue differentiation. 

 A rudimentary leaf epidermis and traces of palisade 

 cells are present; and the fibro-vascular elements are 

 represented in the stem portion of the embryo. 



