134 SEEDS. 



(68) White Mustard Seeds. 



The seeds of Brassica alba, Linn. (N.O. Cruciferse). 

 The seeds are yellow in colour, and so minutely pitted that 

 they appear smooth to the naked eye. 



In the seed-coats the following layers can be distinguished : 



(1) An epidermis (am) made up of large cells containing 

 mucilage which swells very rapidly and very considerably in 

 contact with water. 



(2) A collenchymatous layer (col) composed of two rows of 

 polygonal cells, the walls of which are thickened, particularly in 

 the angles 



(3) A sclerenchymatous layer (sc) consisting of a single row of 

 cells, the lateral and inner walls of which are thickened and 

 pale yellow in colour. The cells are tolerably uniform in size 

 and arrangement. 



These three layers represent the outer seed-coat. 



(4) A membranous layer (cm) composed of two or three rows 

 of strongly flattened cells. These are free trom the dark brown 

 pigment which is contained in the corresponding cells of black 

 mustard seed. 



Within the seed-coats is the embryo surrounded by the 

 aleurone layer (ap) ; the latter consists of a single row of isodia- 

 metric polygonal cells which have uniformly thickened walls 

 and granular contents. The epidermis of the cotyledons (ec) is 

 somposed of irregular empty cells amongst which groups of two 

 or three smaller ones may be observed ; these are stomata in 

 process of formation. The cells of the cotyledons themselves 

 (co) are filled with aleurone grains and fixed oil. The aleurone 

 grains are small and irregular in shape ; they contain numerous 

 minute globoids but no crystalloids. 



The powder consists, like that of black mustard, largely of 

 fragments of the cotyledons and radicle. The portions of the 

 seed-coat are pale yellow in colour, and hence easily dis- 

 tinguished from the red-brown fragments of black mustard. 

 The polygonal network that is so easily seen on the upper sur- 

 face of the sclerenchymatous layer of black mustard is mush 

 less conspicuous. 



The diagnostic characters of powdered white mustard are : 



(a) The pale yellow sclerenchymatous layer. 



(b) The epidermis cells with striated mucilage. 



(c) The collenchymatous hypodermal layer. 



(d) The small, irregular, aleurone grains containing 

 numerous minute globoids, 



